[PATCH 2/67] aufs manual - Kernel

This is a discussion on [PATCH 2/67] aufs manual - Kernel ; From: Junjiro Okajima initial commit aufs manual Signed-off-by: Junjiro Okajima --- Documentation/filesystems/aufs/aufs.5 | 1608 +++++++++++++++++++++++++++++++++ 1 files changed, 1608 insertions(+), 0 deletions(-) diff --git a/Documentation/filesystems/aufs/aufs.5 b/Documentation/filesystems/aufs/aufs.5 new file mode 100644 index 0000000..7335e14 --- /dev/null +++ b/Documentation/filesystems/aufs/aufs.5 @@ -0,0 +1,1608 @@ ...

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  1. [PATCH 2/67] aufs manual

    From: Junjiro Okajima

    initial commit
    aufs manual

    Signed-off-by: Junjiro Okajima
    ---
    Documentation/filesystems/aufs/aufs.5 | 1608 +++++++++++++++++++++++++++++++++
    1 files changed, 1608 insertions(+), 0 deletions(-)

    diff --git a/Documentation/filesystems/aufs/aufs.5 b/Documentation/filesystems/aufs/aufs.5
    new file mode 100644
    index 0000000..7335e14
    --- /dev/null
    +++ b/Documentation/filesystems/aufs/aufs.5
    @@ -0,0 +1,1608 @@
    +.ds AUFS_VERSION 20080516-mm
    +.ds AUFS_XINO_FNAME .aufs.xino
    +.ds AUFS_XINO_DEFPATH /tmp/.aufs.xino
    +.ds AUFS_DIRWH_DEF 3
    +.ds AUFS_WH_PFX .wh.
    +.ds AUFS_WH_PFX_LEN 4
    +.ds AUFS_WKQ_NAME aufsd
    +.ds AUFS_NWKQ_DEF 4
    +.ds AUFS_WH_DIROPQ .wh..wh..opq
    +.ds AUFS_WH_BASENAME .wh.aufs
    +.ds AUFS_WH_PLINKDIR .wh.plink
    +.ds AUFS_BRANCH_MAX 127
    +.ds AUFS_MFS_SECOND_DEF 30
    +.\".so aufs.tmac
    +.
    +.eo
    +.de TQ
    +.br
    +.ns
    +.TP \$1
    +..
    +.de Bu
    +.IP \(bu 4
    +..
    +.ec
    +.\" end of macro definitions
    +.
    +.\" ----------------------------------------------------------------------
    +.TH aufs 5 \*[AUFS_VERSION] Linux "Linux Aufs User\[aq]s Manual"
    +.SH NAME
    +aufs \- another unionfs. version \*[AUFS_VERSION]
    +
    +.\" ----------------------------------------------------------------------
    +.SH DESCRIPTION
    +Aufs is a stackable unification filesystem such as Unionfs, which unifies
    +several directories and provides a merged single directory.
    +In the early days, aufs was entirely re-designed and re-implemented
    +Unionfs Version 1.x series. After
    +many original ideas, approaches and improvements, it
    +becomes totally different from Unionfs while keeping the basic features.
    +See Unionfs Version 1.x series for the basic features.
    +Recently, Unionfs Version 2.x series begin taking some of same
    +approaches to aufs\[aq]s.
    +
    +.\" ----------------------------------------------------------------------
    +.SH MOUNT OPTIONS
    +At mount-time, the order of interpreting options is,
    +.RS
    +.Bu
    +simple flags, except xino/noxino, udba=inotify and dlgt
    +.Bu
    +branches
    +.Bu
    +xino/noxino
    +.Bu
    +udba=inotify
    +.Bu
    +dlgt
    +.RE
    +
    +At remount-time,
    +the options are interpreted in the given order,
    +e.g. left to right, except dlgt. The \[oq]dlgt\[cq] option is
    +disabled in interpreting.
    +.RS
    +.Bu
    +create or remove
    +whiteout-base(\*[AUFS_WH_PFX]\*[AUFS_WH_BASENAME]) and
    +whplink-dir(\*[AUFS_WH_PFX]\*[AUFS_WH_PLINKDIR]) if necessary
    +.Bu
    +re-enable dlgt if necessary
    +.RE
    +.
    +.TP
    +.B br:BRANCH[:BRANCH ...] (dirs=BRANCH[:BRANCH ...])
    +Adds new branches.
    +(cf. Branch Syntax).
    +
    +Aufs rejects the branch which is an ancestor or a descendant of anther
    +branch. It is called overlapped. When the branch is loopback-mounted
    +directory, aufs also checks the source fs-image file of loopback
    +device. If the source file is a descendant of another branch, it will
    +be rejected too.
    +
    +After mounting aufs or adding a branch, if you move a branch under
    +another branch and make it descendant of anther branch, aufs will not
    +work correctly.
    +.
    +.TP
    +.B [ add | ins ]:index:BRANCH
    +Adds a new branch.
    +The index begins with 0.
    +Aufs creates
    +whiteout-base(\*[AUFS_WH_PFX]\*[AUFS_WH_BASENAME]) and
    +whplink-dir(\*[AUFS_WH_PFX]\*[AUFS_WH_PLINKDIR]) if necessary.
    +
    +If there is the same named file on the lower branch (larger index),
    +aufs will hide the lower file.
    +You can only see the highest file.
    +You will be confused if the added branch has whiteouts (including
    +diropq), they may or may not hide the lower entries.
    +.\" It is recommended to make sure that the added branch has no whiteout.
    +
    +If a process have once mapped a file by mmap(2) with MAP_SHARED
    +and the same named file exists on the lower branch,
    +the process still refers the file on the lower(hidden)
    +branch after adding the branch.
    +If you want to update the contents of a process address space after
    +adding, you need to restart your process or open/mmap the file again.
    +.\" Usually, such files are executables or shared libraries.
    +(cf. Branch Syntax).
    +.
    +.TP
    +.B del:dir
    +Removes a branch.
    +Aufs does not remove
    +whiteout-base(\*[AUFS_WH_PFX]\*[AUFS_WH_BASENAME]) and
    +whplink-dir(\*[AUFS_WH_PFX]\*[AUFS_WH_PLINKDIR]) automatically.
    +For example, when you add a RO branch which was unified as RW, you
    +will see whiteout-base or whplink-dir on the added RO branch.
    +
    +If a process is referencing the file/directory on the deleting branch
    +(by open, mmap, current working directory, etc.), aufs will return an
    +error EBUSY.
    +.
    +.TP
    +.B mod:BRANCH
    +Modifies the permission flags of the branch.
    +Aufs creates or removes
    +whiteout-base(\*[AUFS_WH_PFX]\*[AUFS_WH_BASENAME]) and/or
    +whplink-dir(\*[AUFS_WH_PFX]\*[AUFS_WH_PLINKDIR]) if necessary.
    +
    +If the branch permission is been changing \[oq]rw\[cq] to \[oq]ro\[cq], and a process
    +is mapping a file by mmap(2)
    +.\" with MAP_SHARED
    +on the branch, the process may or may not
    +be able to modify its mapped memory region after modifying branch
    +permission flags.
    +(cf. Branch Syntax).
    +.
    +.TP
    +.B append:BRANCH
    +equivalent to \[oq]addlast index + 1):BRANCH\[cq].
    +(cf. Branch Syntax).
    +.
    +.TP
    +.B prepend:BRANCH
    +equivalent to \[oq]add:0:BRANCH.\[cq]
    +(cf. Branch Syntax).
    +.
    +.TP
    +.B xino=filename
    +Use external inode number bitmap and translation table. It is set to
    +/\*[AUFS_XINO_FNAME] by default, or
    +\*[AUFS_XINO_DEFPATH].
    +Comma character in filename is not allowed.
    +
    +The files are created per an aufs and per a branch filesystem, and
    +unlinked. So you
    +cannot find this file, but it exists and is read/written frequently by
    +aufs.
    +(cf. External Inode Number Bitmap and Translation Table).
    +.
    +.TP
    +.B noxino
    +Stop using external inode number bitmap and translation table.
    +
    +If you use this option,
    +Some applications will not work correctly.
    +.\" And pseudo link feature will not work after the inode cache is
    +.\" shrunk.
    +(cf. External Inode Number Bitmap and Translation Table).
    +.
    +.TP
    +.B trunc_xib
    +Truncate the external inode number bitmap file. The truncation is done
    +automatically when you delete a branch unless you do not specify
    +\[oq]notrunc_xib\[cq] option.
    +(cf. External Inode Number Bitmap and Translation Table).
    +.
    +.TP
    +.B notrunc_xib
    +Stop truncating the external inode number bitmap file when you delete
    +a branch.
    +(cf. External Inode Number Bitmap and Translation Table).
    +.
    +.TP
    +.B create_policy | create=CREATE_POLICY
    +.TQ
    +.B copyup_policy | copyup | cpup=COPYUP_POLICY
    +Policies to select one among multiple writable branches. The default
    +values are \[oq]create=tdp\[cq] and \[oq]cpup=tdp\[cq].
    +link(2) and rename(2) systemcalls have an exception. In aufs, they
    +try keeping their operations in the branch where the source exists.
    +(cf. Policies to Select One among Multiple Writable Branches).
    +.
    +.TP
    +.B verbose | v
    +Print some information.
    +Currently, it is only busy file (or inode) at deleting a branch.
    +.
    +.TP
    +.B noverbose | quiet | q | silent
    +Disable \[oq]verbose\[cq] option.
    +This is default value.
    +.
    +.TP
    +.B dirwh=N
    +Watermark to remove a dir actually at rmdir(2) and rename(2).
    +
    +If the target dir which is being removed or renamed (destination dir)
    +has a huge number of whiteouts, i.e. the dir is empty logically but
    +physically, the cost to remove/rename the single
    +dir may be very high.
    +It is
    +required to unlink all of whiteouts internally before issuing
    +rmdir/rename to the branch.
    +To reduce the cost of single systemcall,
    +aufs renames the target dir to a whiteout-ed temporary name and
    +invokes a pre-created
    +kernel thread to remove whiteout-ed children and the target dir.
    +The rmdir/rename systemcall returns just after kicking the thread.
    +
    +When the number of whiteout-ed children is less than the value of
    +dirwh, aufs remove them in a single systemcall instead of passing
    +another thread.
    +This value is ignored when the branch is NFS.
    +The default value is \*[AUFS_DIRWH_DEF].
    +.
    +.TP
    +.B plink
    +.TQ
    +.B noplink
    +Specifies to use \[oq]pseudo link\[cq] feature or not.
    +The default is \[oq]plink\[cq] which means use this feature.
    +(cf. Pseudo Link)
    +.
    +.TP
    +.B clean_plink
    +Removes all pseudo-links in memory.
    +In order to make pseudo-link permanent, use
    +\[oq]auplink\[cq] script just before one of these operations,
    +unmounting aufs,
    +using \[oq]ro\[cq] or \[oq]noplink\[cq] mount option,
    +deleting a branch from aufs,
    +adding a branch into aufs,
    +or changing your writable branch as readonly.
    +If you installed both of /sbin/mount.aufs and /sbin/umount.aufs, and your
    +mount(8) and umount(8) support them, and /etc/default/auplink is configured,
    +\[oq]auplink\[cq] script will be executed automatically and flush pseudo-links.
    +(cf. Pseudo Link)
    +.
    +.TP
    +.B udba=none | reval | inotify
    +Specifies the level of UDBA (User\[aq]s Direct Branch Access) test.
    +(cf. User\[aq]s Direct Branch Access and Inotify Limitation).
    +.
    +.TP
    +.B diropq=whiteouted | w | always | a
    +Specifies whether mkdir(2) and rename(2) dir case make the created directory
    +\[oq]opaque\[cq] or not.
    +In other words, to create \[oq]\*[AUFS_WH_DIROPQ]\[cq] under the created or renamed
    +directory, or not to create.
    +When you specify diropq=w or diropq=whiteouted, aufs will not create
    +it if the
    +directory was not whiteouted or opaqued. If the directory was whiteouted
    +or opaqued, the created or renamed directory will be opaque.
    +When you specify diropq=a or diropq==always, aufs will always create
    +it regardless
    +the directory was whiteouted/opaqued or not.
    +The default value is diropq=w, it means not to create when it is unnecessary.
    +If you define CONFIG_AUFS_COMPAT at aufs compiling time, the default will be
    +diropq=a.
    +You need to consider this option if you are planning to add a branch later
    +since \[oq]diropq\[cq] affects the same named directory on the added branch.
    +.
    +.TP
    +.B warn_perm
    +.TQ
    +.B nowarn_perm
    +Adding a branch, aufs will issue a warning about uid/gid/permission of
    +the adding branch directory,
    +when they differ from the existing branch\[aq]s. This difference may or
    +may not impose a security risk.
    +If you are sure that there is no problem and want to stop the warning,
    +use \[oq]nowarn_perm\[cq] option.
    +The default is \[oq]warn_perm\[cq] (cf. DIAGNOSTICS).
    +.
    +.TP
    +.B coo=none | leaf | all
    +Specifies copyup-on-open level.
    +When you open a file which is on readonly branch, aufs opens the file after
    +copying-up it to the writable branch following this level.
    +When the keyword \[oq]all\[cq] is specified, aufs copies-up the opening object even if
    +it is a directory. In this case, simple \[oq]ls\[cq] or \[oq]find\[cq] cause the copyup and
    +your writable branch will have a lot of empty directories.
    +When the keyword \[oq]leaf\[cq] is specified, aufs copies-up the opening object except
    +directory.
    +The keyword \[oq]none\[cq] disables copyup-on-open.
    +The default is \[oq]coo=none\[cq].
    +.
    +.TP
    +.B dlgt
    +.TQ
    +.B nodlgt
    +If you do not want your application to access branches though aufs or
    +to be traced strictly by task I/O accounting, you can
    +use the kernel threads in aufs. If you enable CONFIG_AUFS_DLGT and
    +specify \[oq]dlgt\[cq] mount option, then
    +aufs delegates its internal
    +access to the branches to the kernel threads.
    +
    +When you define CONFIG_SECURITY and use any type of Linux Security Module
    +(LSM), for example SUSE AppArmor, you may meet some errors or
    +warnings from your security module. Because aufs access its branches
    +internally, your security module may detect, report, or prohibit it.
    +The behaviour is highly depending upon your security module and its
    +configuration.
    +In this case, you can use \[oq]dlgt\[cq] mount option, too.
    +Your LSM will see the
    +aufs kernel threads access to the branch, instead of your
    +application.
    +
    +The delegation may damage the performance since it includes
    +task-switch (scheduling) and waits for the thread to complete the
    +delegated access. You should consider increasing the number of the
    +kernel thread specifying the aufs module parameter \[oq]nwkq.\[cq]
    +
    +Currently, aufs does NOT delegate it at mount and remount time.
    +The default is nodlgt which means aufs does not delegate the internal
    +access.
    +.\" .
    +.\" .TP
    +.\" .B dirperm1
    +.\" .TQ
    +.\" .B nodirperm1
    +.\" By default (nodirperm1), aufs checks the permission bits of target
    +.\" directory on all branches. If any of them refused the requested
    +.\" access, then aufs returns negative even if the topmost permission bits
    +.\" of the directory allowed the access.
    +.\" If you enable CONFIG_AUFS_DLGT and specify \[oq]dirperm1\[cq] option, aufs
    +.\" doesn\[aq]t check the directories on all lower branches but the topmost
    +.\" one.
    +.
    +.TP
    +.B shwh
    +.TQ
    +.B noshwh
    +By default (noshwh), aufs doesn\[aq]t show the whiteouts and
    +they just hide the same named entries in the lower branches. The
    +whiteout itself also never be appeared.
    +If you enable CONFIG_AUFS_SHWH and specify \[oq]shwh\[cq] option, aufs
    +will show you the name of whiteouts
    +with keeping its feature to hide the lowers.
    +Honestly speaking, I am rather confused with this \[oq]visible whiteouts.\[cq]
    +But a user who originally requested this feature wrote a nice how-to
    +document about this feature. See Tips file in the aufs CVS tree.
    +
    +.\" ----------------------------------------------------------------------
    +.SH Module Parameters
    +.TP
    +.B nwkq=N
    +The number of kernel thread named \*[AUFS_WKQ_NAME].
    +
    +Those threads stay in the system while the aufs module is loaded,
    +and handle the special I/O requests from aufs.
    +The default value is \*[AUFS_NWKQ_DEF].
    +
    +The special I/O requests from aufs include a part of copy-up, lookup,
    +directory handling, pseudo-link, xino file operations and the
    +delegated access to branches.
    +For example, Unix filesystems allow you to rmdir(2) which has no write
    +permission bit, if its parent directory has write permission bit. In aufs, the
    +removing directory may or may not have whiteout or \[oq]dir opaque\[cq] mark as its
    +child. And aufs needs to unlink(2) them before rmdir(2).
    +Therefore aufs delegates the actual unlink(2) and rmdir(2) to another kernel
    +thread which has been created already and has a superuser privilege.
    +
    +If you enable CONFIG_SYSFS, you can check this value through
    +/module/aufs/parameters/nwkq.
    +
    +So how many threads is enough? You can check it by
    +/fs/aufs/stat, if you enable CONFIG_AUFS_SYSAUFS (for
    +linux\-2.6.24 and earlier) or CONFIG_AUFS_STAT (for linux\-2.6.25 and
    +later) too.
    +It shows the maximum number of the enqueued work
    +at a time per a thread. Usually they are all small numbers or
    +0. If your workload is heavy
    +and you feel the response is low, then check these values. If there
    +are no zero and any of them is larger than 2 or 3, you should set \[oq]nwkq\[cq]
    +module parameter greater then the default value.
    +But the reason of the bad response is in your branch filesystem, to
    +increase the number of aufs thread will not help you.
    +
    +The last number in /fs/aufs/stat after comma is the maximum
    +number of the \[oq]no-wait\[cq] enqueued work at a time. Aufs enqueues such
    +work to the system global workqueue called \[oq]events\[cq], but does not wait
    +for its completion. Usually they does no harm the time-performance of
    +aufs.
    +.
    +.TP
    +.B brs=1 | 0
    +Specifies to use the branch path data file under sysfs or not.
    +
    +If the number of your branches is large or their path is long
    +and you meet the limitation of mount(8) ro /etc/mtab, you need to
    +enable CONFIG_SYSFS and set aufs module parameter brs=1.
    +If your linux version is linux\-2.6.24 and earlier, you need to enable
    +CONFIG_AUFS_SYSAUFS too.
    +
    +When this parameter is set as 1, aufs does not show \[oq]br:\[cq] (or dirs=)
    +mount option through /proc/mounts, and /sbin/mount.aufs does not put it
    +to /etc/mtab. So you can keep yourself from the page limitation of
    +mount(8) or /etc/mtab.
    +Aufs shows branch paths through /fs/aufs/si_XXX/brNNN.
    +Actually the file under sysfs has also a size limitation, but I don\[aq]t
    +think it is harmful.
    +
    +The default is brs=0, which means /fs/aufs/si_XXX/brNNN does not exist
    +and \[oq]br:\[cq] option will appear in /proc/mounts, and /etc/mtab if you
    +install /sbin/mount.aufs.
    +If you did not enable CONFIG_AUFS_SYSAUFS (for
    +linux\-2.6.24 and earlier), this parameter will be
    +ignored.
    +
    +There is one more side effect in setting 1 to this parameter.
    +If you rename your branch, the branch path written in /etc/mtab will be
    +obsoleted and the future remount will meet some error due to the
    +unmatched parameters (Remember that mount(8) may take the options from
    +/etc/mtab and pass them to the systemcall).
    +If you set 1, /etc/mtab will not hold the branch path and you will not
    +meet such trouble. On the other hand, /proc/mounts which holds the
    +branch path is updated dynamically. So it must not be obsoleted.
    +.
    +.TP
    +.B sysrq=key
    +Specifies MagicSysRq key for debugging aufs.
    +You need to enable both of CONFIG_MAGIC_SYSRQ and CONFIG_AUFS_DEBUG.
    +If your linux version is linux\-2.6.24 and earlier, you need to enable
    +CONFIG_AUFS_SYSAUFS too.
    +Currently this is for developers only.
    +The default is \[oq]a\[cq].
    +
    +.\" ----------------------------------------------------------------------
    +.SH Branch Syntax
    +.TP
    +.B dir_path[ =permission [ + attribute ] ]
    +.TQ
    +.B permission := rw | ro | rr
    +.TQ
    +.B attribute := wh | nolwh
    +dir_path is a directory path.
    +The keyword after \[oq]dir_path=\[cq] is a
    +permission flags for that branch.
    +Comma, colon and the permission flags string (including \[oq]=\[cq])in the path
    +are not allowed.
    +Any filesystem can be a branch, but aufs and unionfs.
    +If you specify aufs or unionfs as a branch, aufs will return an error
    +saying it is overlapped or nested.
    +If you enable CONFIG_AUFS_ROBR, you can use aufs as a non-writable
    +branch of another aufs.
    +
    +Cramfs in linux stable release has strange inodes and it makes aufs
    +confused. For example,
    +.nf
    +$ mkdir -p w/d1 w/d2
    +$ > w/z1
    +$ > w/z2
    +$ mkcramfs w cramfs
    +$ sudo mount -t cramfs -o ro,loop cramfs /mnt
    +$ find /mnt -ls
    + 76 1 drwxr-xr-x 1 jro 232 64 Jan 1 1970 /mnt
    + 1 1 drwxr-xr-x 1 jro 232 0 Jan 1 1970 /mnt/d1
    + 1 1 drwxr-xr-x 1 jro 232 0 Jan 1 1970 /mnt/d2
    + 1 1 -rw-r--r-- 1 jro 232 0 Jan 1 1970 /mnt/z1
    + 1 1 -rw-r--r-- 1 jro 232 0 Jan 1 1970 /mnt/z2
    +.fi
    +
    +All these two directories and two files have the same inode with one
    +as their link count. Aufs cannot handle such inode correctly.
    +Currently, aufs involves a tiny workaround for such inodes. But some
    +applications may not work correctly since aufs inode number for such
    +inode will change silently.
    +If you do not have any empty files, empty directories or special files,
    +inodes on cramfs will be all fine.
    +
    +A branch should not be shared as the writable branch between multiple
    +aufs. A readonly branch can be shared.
    +
    +The maximum number of branches is configurable at compile time.
    +The current value is \*[AUFS_BRANCH_MAX] which depends upon
    +configuration.
    +
    +When an unknown permission or attribute is given, aufs sets ro to that
    +branch silently.
    +
    +.SS Permission
    +.
    +.TP
    +.B rw
    +Readable and writable branch. Set as default for the first branch.
    +If the branch filesystem is mounted as readonly, you cannot set it \[oq]rw.\[cq]
    +.\" A filesystem which does not support link(2) and i_op\->setattr(), for
    +.\" example FAT, will not be used as the writable branch.
    +.
    +.TP
    +.B ro
    +Readonly branch and it has no whiteouts on it.
    +Set as default for all branches except the first one. Aufs never issue
    +both of write operation and lookup operation for whiteout to this branch.
    +.
    +.TP
    +.B rr
    +Real readonly branch, special case of \[oq]ro\[cq], for natively readonly
    +branch. Assuming the branch is natively readonly, aufs can optimize
    +some internal operation. For example, if you specify \[oq]udba=inotify\[cq]
    +option, aufs does not set inotify for the things on rr branch.
    +Set by default for a branch whose fs-type is either \[oq]iso9660\[cq],
    +\[oq]cramfs\[cq], \[oq]romfs\[cq] or \[oq]squashfs.\[cq]
    +
    +.SS Attribute
    +.
    +.TP
    +.B wh
    +Readonly branch and it has/might have whiteouts on it.
    +Aufs never issue write operation to this branch, but lookup for whiteout.
    +Use this as \[oq]=ro+wh\[cq].
    +.
    +.TP
    +.B nolwh
    +Usually, aufs creates a whiteout as a hardlink on a writable
    +branch. This attributes prohibits aufs to create the hardlinked
    +whiteout, including the source file of all hardlinked whiteout
    +(\*[AUFS_WH_PFX]\*[AUFS_WH_BASENAME].)
    +If you do not like a hardlink, or your writable branch does not support
    +link(2), then use this attribute.
    +But I am afraid a filesystem which does not support link(2) natively
    +will fail in other place such as copy-up.
    +Use this as \[oq]=rw+nolwh\[cq].
    +Also you may want to try \[oq]noplink\[cq] mount option, while it is not recommended.
    +
    +.\" ----------------------------------------------------------------------
    +.SH External Inode Number Bitmap and Translation Table (xino)
    +Aufs uses one external bitmap file and one external inode number
    +translation table files per an aufs and per a branch
    +filesystem by
    +default. The bitmap is for recycling aufs inode number and the others
    +are a table for converting an inode number on a branch to
    +an aufs inode number. The default path
    +is \[oq]first writable branch\[cq]/\*[AUFS_XINO_FNAME].
    +If there is no writable branch, the
    +default path
    +will be \*[AUFS_XINO_DEFPATH].
    +.\" A user who executes mount(8) needs the privilege to create xino
    +.\" file.
    +
    +Those files are always opened and read/write by aufs frequently.
    +If your writable branch is on flash memory device, it is recommended
    +to put xino files on other than flash memory by specifying \[oq]xino=\[cq]
    +mount option.
    +
    +The
    +maximum file size of the bitmap is, basically, the amount of the
    +number of all the files on all branches divided by 8 (the number of
    +bits in a byte).
    +For example, on a 4KB page size system, if you have 32,768 (or
    +2,599,968) files in aufs world,
    +then the maximum file size of the bitmap is 4KB (or 320KB).
    +
    +The
    +maximum file size of the table will
    +be \[oq]max inode number on the branch x size of an inode number\[cq].
    +For example in 32bit environment,
    +
    +.nf
    +$ df -i /branch_fs
    +/dev/hda14 2599968 203127 2396841 8% /branch_fs
    +.fi
    +
    +and /branch_fs is an branch of the aufs. When the inode number is
    +assigned contiguously (without \[oq]hole\[cq]), the maximum xino file size for
    +/branch_fs will be 2,599,968 x 4 bytes = about 10 MB. But it might not be
    +allocated all of disk blocks.
    +When the inode number is assigned discontinuously, the maximum size of
    +xino file will be the largest inode number on a branch x 4 bytes.
    +Additionally, the file size is limited to LLONG_MAX or the s_maxbytes
    +in filesystem\[aq]s superblock (s_maxbytes may be smaller than
    +LLONG_MAX). So the
    +support-able largest inode number on a branch is less than
    +2305843009213693950 (LLONG_MAX/4\-1).
    +This is the current limitation of aufs.
    +On 64bit environment, this limitation becomes more strict and the
    +supported largest inode number is less than LLONG_MAX/8\-1.
    +
    +The xino files are always hidden, i.e. removed. So you cannot
    +do \[oq]ls \-l xino_file\[cq].
    +If you enable CONFIG_SYSFS, you can check these information through
    +/fs/aufs//xino (for linux\-2.6.24 and earlier, you
    +need to enable CONFIG_AUFS_SYSAUFS too).
    +The first line in /fs/aufs//xino shows the information
    +of the bitmap file, in the format of,
    +
    +.nf
    +x
    +.fi
    +
    +Note that a filesystem usually has a
    +feature called pre-allocation, which means a number of
    +blocks are allocated automatically, and then deallocated
    +silently when the filesystem thinks they are unnecessary.
    +You do not have to be surprised the sudden changes of the number of
    +blocks, when your filesystem which xino files are placed supports the
    +pre-allocation feature.
    +
    +The rests are hidden xino file information in the format of,
    +
    +.nf
    +: , x
    +.fi
    +
    +If the file count is larger than 1, it means some of your branches are
    +on the same filesystem and the xino file is shared by them.
    +Note that the file size may not be equal to the actual consuming blocks
    +since xino file is a sparse file, i.e. a hole in a file which does not
    +consume any disk blocks.
    +
    +Once you unmount aufs, the xino files for that aufs are totally gone.
    +It means that the inode number is not permanent.
    +
    +The xino files should be created on the filesystem except NFS.
    +If your first writable branch is NFS, you will need to specify xino
    +file path other than NFS.
    +Also if you are going to remove the branch where xino files exist or
    +change the branch permission to readonly, you need to use xino option
    +before del/mod the branch.
    +
    +The bitmap file can be truncated.
    +For example, if you delete a branch which has huge number of files,
    +many inode numbers will be recycled and the bitmap will be truncated
    +to smaller size. Aufs does this automatically when a branch is
    +deleted.
    +You can truncate it anytime you like if you specify \[oq]trunc_xib\[cq] mount
    +option. But when the accessed inode number was not deleted, nothing
    +will be truncated.
    +If you do not want to truncate it (it may be slow) when you delete a
    +branch, specify \[oq]notrunc_xib\[cq] after \[oq]del\[cq] mount option.
    +
    +If you do not want to use xino, use noxino mount option. Use this
    +option with care, since the inode number may be changed silently and
    +unexpectedly anytime.
    +For example,
    +rmdir failure, recursive chmod/chown/etc to a large and deep directory
    +or anything else.
    +And some applications will not work correctly.
    +.\" When the inode number has been changed, your system
    +.\" can be crazy.
    +If you want to change the xino default path, use xino mount option.
    +
    +After you add branches, the persistence of inode number may not be
    +guaranteed.
    +At remount time, cached but unused inodes are discarded.
    +And the newly appeared inode may have different inode number at the
    +next access time. The inodes in use have the persistent inode number.
    +
    +When aufs assigned an inode number to a file, and if you create the
    +same named file on the upper branch directly, then the next time you
    +access the file, aufs may assign another inode number to the file even
    +if you use xino option.
    +Some applications may treat the file whose inode number has been
    +changed as totally different file.
    +
    +.\" ----------------------------------------------------------------------
    +.SH Pseudo Link (hardlink over branches)
    +Aufs supports \[oq]pseudo link\[cq] which is a logical hard-link over
    +branches (cf. ln(1) and link(2)).
    +In other words, a copied-up file by link(2) and a copied-up file which was
    +hard-linked on a readonly branch filesystem.
    +
    +When you have files named fileA and fileB which are
    +hardlinked on a readonly branch, if you write something into fileA,
    +aufs copies-up fileA to a writable branch, and write(2) the originally
    +requested thing to the copied-up fileA. On the writable branch,
    +fileA is not hardlinked.
    +But aufs remembers it was hardlinked, and handles fileB as if it existed
    +on the writable branch, by referencing fileA\[aq]s inode on the writable
    +branch as fileB\[aq]s inode.
    +
    +Once you unmount aufs, the plink info for that aufs kept in memory are totally
    +gone.
    +It means that the pseudo-link is not permanent.
    +If you want to make plink permanent, try \[oq]auplink\[cq] script just before
    +one of these operations,
    +unmounting your aufs,
    +using \[oq]ro\[cq] or \[oq]noplink\[cq] mount option,
    +deleting a branch from aufs,
    +adding a branch into aufs,
    +or changing your writable branch to readonly.
    +
    +This script will reproduces all real hardlinks on a writable branch by linking
    +them, and removes pseudo-link info in memory and temporary link on the
    +writable branch.
    +Since this script access your branches directly, you cannot hide them by
    +\[oq]mount \-\-bind /tmp /branch\[cq] or something.
    +
    +If you are willing to rebuild your aufs with the same branches later, you
    +should use auplink script before you umount your aufs.
    +If you installed both of /sbin/mount.aufs and /sbin/umount.aufs, and your
    +mount(8) and umount(8) support them, and /etc/default/auplink is configured,
    +\[oq]auplink\[cq] script will be executed automatically and flush pseudo-links.
    +
    +The /etc/default/auplink is a simple shell script which does nothing but defines
    +$FLUSH. If your aufs mount point is set in $FLUSH, \[oq]auplink\[cq] flushes
    +the pseudo-links on that mount point.
    +If $FLUSH is set to \[oq]ALL\[cq], \[oq]auplink\[cq] will be executed for every aufs.
    +
    +The \[oq]auplink\[cq] script uses \[oq]aulchown\[cq] binary, you need to install it too.
    +The \[oq]auplink\[cq] script executes \[oq]find\[cq] and \[oq]mount \-o remount\[cq], they may take a
    +long time and impact the later system performance.
    +If you did not install /sbin/mount.aufs, /sbin/umount.aufs or /sbin/auplink,
    +but you want to flush pseudo-links, then you need to execute \[oq]auplink\[cq] manually.
    +If you installed and configured them, but do not want to execute \[oq]auplink\[cq] at
    +umount time, then use \[oq]\-i\[cq] option for umount(8).
    +
    +.nf
    +# auplink /your/aufs/root flush
    +# umount /your/aufs/root
    +or
    +# auplink /your/aufs/root flush
    +# mount -o remount,mod:/your/writable/branch=ro /your/aufs/root
    +or
    +# auplink /your/aufs/root flush
    +# mount -o remount,noplink /your/aufs/root
    +or
    +# auplink /your/aufs/root flush
    +# mount -o remount,del:/your/aufs/branch /your/aufs/root
    +or
    +# auplink /your/aufs/root flush
    +# mount -o remount,append:/your/aufs/branch /your/aufs/root
    +.fi
    +
    +The plinks are kept both in memory and on disk. When they consumes too much
    +resources on your system, you can use the \[oq]auplink\[cq] script at anytime and
    +throw away the unnecessary pseudo-links in safe.
    +
    +Additionally, the \[oq]auplink\[cq] script is very useful for some security reasons.
    +For example, when you have a directory whose permission flags
    +are 0700, and a file who is 0644 under the 0700 directory. Usually,
    +all files under the 0700 directory are private and no one else can see
    +the file. But when the directory is 0711 and someone else knows the 0644
    +filename, he can read the file.
    +
    +Basically, aufs pseudo-link feature creates a temporary link under the
    +directory whose owner is root and the permission flags are 0700.
    +But when the writable branch is NFS, aufs sets 0711 to the directory.
    +When the 0644 file is pseudo-linked, the temporary link, of course the
    +contents of the file is totally equivalent, will be created under the
    +0711 directory. The filename will be generated by its inode number.
    +While it is hard to know the generated filename, someone else may try peeping
    +the temporary pseudo-linked file by his software tool which may try the name
    +from one to MAX_INT or something.
    +In this case, the 0644 file will be read unexpectedly.
    +I am afraid that leaving the temporary pseudo-links can be a security hole.
    +It makes sense to execute \[oq]auplink /your/aufs/root flush\[cq]
    +periodically, when your writable branch is NFS.
    +
    +When your writable branch is not NFS, or all users are careful enough to set 0600
    +to their private files, you do not have to worry about this issue.
    +
    +If you do not want this feature, use \[oq]noplink\[cq] mount option and you do
    +not need
    +to install \[oq]auplink\[cq] script and \[oq]aulchown\[cq] binary.
    +
    +.SS The behaviours of plink and noplink
    +This sample shows that the \[oq]f_src_linked2\[cq] with \[oq]noplink\[cq] option cannot follow
    +the link.
    +
    +.nf
    +none on /dev/shm/u type aufs (rw,xino=/dev/shm/rw/.aufs.xino,br:/dev/shm/rw=rw:/dev/shm/ro=ro)
    +$ ls -li ../r?/f_src_linked* ./f_src_linked* ./copied
    +ls: ./copied: No such file or directory
    +15 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ../ro/f_src_linked
    +15 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ../ro/f_src_linked2
    +22 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ./f_src_linked
    +22 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ./f_src_linked2
    +$ echo abc >> f_src_linked
    +$ cp f_src_linked copied
    +$ ls -li ../r?/f_src_linked* ./f_src_linked* ./copied
    +15 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ../ro/f_src_linked
    +15 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ../ro/f_src_linked2
    +36 -rw-r--r-- 2 jro jro 6 Dec 22 11:03 ../rw/f_src_linked
    +53 -rw-r--r-- 1 jro jro 6 Dec 22 11:03 ./copied
    +22 -rw-r--r-- 2 jro jro 6 Dec 22 11:03 ./f_src_linked
    +22 -rw-r--r-- 2 jro jro 6 Dec 22 11:03 ./f_src_linked2
    +$ cmp copied f_src_linked2
    +$
    +
    +none on /dev/shm/u type aufs (rw,xino=/dev/shm/rw/.aufs.xino,noplink,br:/dev/shm/rw=rw:/dev/shm/ro=ro)
    +$ ls -li ../r?/f_src_linked* ./f_src_linked* ./copied
    +ls: ./copied: No such file or directory
    +17 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ../ro/f_src_linked
    +17 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ../ro/f_src_linked2
    +23 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ./f_src_linked
    +23 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ./f_src_linked2
    +$ echo abc >> f_src_linked
    +$ cp f_src_linked copied
    +$ ls -li ../r?/f_src_linked* ./f_src_linked* ./copied
    +17 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ../ro/f_src_linked
    +17 -rw-r--r-- 2 jro jro 2 Dec 22 11:03 ../ro/f_src_linked2
    +36 -rw-r--r-- 1 jro jro 6 Dec 22 11:03 ../rw/f_src_linked
    +53 -rw-r--r-- 1 jro jro 6 Dec 22 11:03 ./copied
    +23 -rw-r--r-- 2 jro jro 6 Dec 22 11:03 ./f_src_linked
    +23 -rw-r--r-- 2 jro jro 6 Dec 22 11:03 ./f_src_linked2
    +$ cmp copied f_src_linked2
    +cmp: EOF on f_src_linked2
    +$
    +.fi
    +
    +.\"
    +.\" If you add/del a branch, or link/unlink the pseudo-linked
    +.\" file on a branch
    +.\" directly, aufs cannot keep the correct link count, but the status of
    +.\" \[oq]pseudo-linked.\[cq]
    +.\" Those files may or may not keep the file data after you unlink the
    +.\" file on the branch directly, especially the case of your branch is
    +.\" NFS.
    +
    +If you add a branch which has fileA or fileB, aufs does not follow the
    +pseudo link. The file on the added branch has no relation to the same
    +named file(s) on the lower branch(es).
    +If you use noxino mount option, pseudo link will not work after the
    +kernel shrinks the inode cache.
    +
    +This feature will not work for squashfs before version 3.2 since its
    +inode is tricky.
    +When the inode is hardlinked, squashfs inodes has the same inode
    +number and correct link count, but the inode memory object is
    +different. Squashfs inodes (before v3.2) are generated for each, even
    +they are hardlinked.
    +
    +.\" ----------------------------------------------------------------------
    +.SH User\[aq]s Direct Branch Access (UDBA)
    +UDBA means a modification to a branch filesystem manually or directly,
    +e.g. bypassing aufs.
    +While aufs is designed and implemented to be safe after UDBA,
    +it can make yourself and your aufs confused. And some information like
    +aufs inode will be incorrect.
    +For example, if you rename a file on a branch directly, the file on
    +aufs may
    +or may not be accessible through both of old and new name.
    +Because aufs caches various information about the files on
    +branches. And the cache still remains after UDBA.
    +
    +Aufs has a mount option named \[oq]udba\[cq] which specifies the test level at
    +access time whether UDBA was happened or not.
    +.
    +.TP
    +.B udba=none
    +Aufs trusts the dentry and the inode cache on the system, and never
    +test about UDBA. With this option, aufs runs fastest, but it may show
    +you incorrect data.
    +Additionally, if you often modify a branch
    +directly, aufs will not be able to trace the changes of inodes on the
    +branch. It can be a cause of wrong behaviour, deadlock or anything else.
    +
    +It is recommended to use this option only when you are sure that
    +nobody access a file on a branch.
    +It might be difficult for you to achieve real \[oq]no UDBA\[cq] world when you
    +cannot stop your users doing \[oq]find / \-ls\[cq] or something.
    +If you really want to forbid all of your users to UDBA, here is a trick
    +for it.
    +With this trick, users cannot see the
    +branches directly and aufs runs with no problem, except \[oq]auplink\[cq] script.
    +But if you are not familiar with aufs, this trick may make
    +yourself confused.
    +
    +.nf
    +# d=/tmp/.aufs.hide
    +# mkdir $d
    +# for i in $branches_you_want_to_hide
    +> do
    +> mount -n --bind $d $i
    +> done
    +.fi
    +
    +When you unmount the aufs, delete/modify the branch by remount, or you
    +want to show the hidden branches again, unmount the bound
    +/tmp/.aufs.hide.
    +
    +.nf
    +# umount -n $branches_you_want_to_unbound
    +.fi
    +
    +If you use FUSE filesystem as an aufs branch which supports hardlink,
    +you should not set this option, since FUSE makes inode objects for
    +each hardlinks (at least in linux\-2.6.23). When your FUSE filesystem
    +maintains them at link/unlinking, it is equivalent
    +to \[oq]direct branch access\[cq] for aufs.
    +
    +.
    +.TP
    +.B udba=reval
    +Aufs tests only the existence of the file which existed. If
    +the existed file was removed on the branch directly, aufs
    +discard the cache about the file and
    +re-lookup it. So the data will be updated.
    +This test is at minimum level to keep the performance and ensure the
    +existence of a file.
    +This is default and aufs runs still fast.
    +
    +This rule leads to some unexpected situation, but I hope it is
    +harmless. Those are totally depends upon cache. Here are just a few
    +examples.
    +.
    +.RS
    +.Bu
    +If the file is cached as negative or
    +not-existed, aufs does not test it. And the file is still handled as
    +negative after a user created the file on a branch directly. If the
    +file is not cached, aufs will lookup normally and find the file.
    +.
    +.Bu
    +When the file is cached as positive or existed, and a user created the
    +same named file directly on the upper branch. Aufs detects the cached
    +inode of the file is still existing and will show you the old (cached)
    +file which is on the lower branch.
    +.
    +.Bu
    +When the file is cached as positive or existed, and a user renamed the
    +file by rename(2) directly. Aufs detects the inode of the file is
    +still existing. You may or may not see both of the old and new files.
    +Todo: If aufs also tests the name, we can detect this case.
    +.RE
    +
    +If your outer modification (UDBA) is rare and you can ignore the
    +temporary and minor differences between virtual aufs world and real
    +branch filesystem, then try this mount option.
    +.\" And when you modify a branch directly, set udba=inotify temporary
    +.\" before the modification and set udba=reval again after that.
    +.
    +.TP
    +.B udba=inotify
    +Aufs sets \[oq]inotify\[cq] to all the accessed directories on its branches
    +and receives the event about the dir and its children. It consumes
    +resources, cpu and memory. And I am afraid that the performance will be
    +damaged, but it is most strict test level.
    +There are some limitations of linux inotify, see also Inotify
    +Limitation.
    +So it is recommended to leave udba default option usually, and set it
    +to inotify by remount when you need it.
    +
    +When a user accesses the file which was notified UDBA before, the cached data
    +about the file will be discarded and aufs re-lookup it. So the data will
    +be updated.
    +When an error condition occurs between UDBA and aufs operation, aufs
    +will return an error, including EIO.
    +To use this option, you need linux\-2.6.18 and later, and need to
    +enable CONFIG_INOTIFY and CONFIG_AUFS_UDBA_INOTIFY.
    +
    +To rename/rmdir a directory on a branch directory may reveal the same named
    +directory on the lower branch. Aufs tries re-lookuping the renamed
    +directory and the revealed directory and assigning different inode
    +number to them. But the inode number including their children can be a
    +problem. The inode numbers will be changed silently, and
    +aufs may produce a warning. If you rename a directory repeatedly and
    +reveal/hide the lower directory, then aufs may confuse their inode
    +numbers too. It depends upon the system cache.
    +
    +When you make a directory in aufs and mount other filesystem on it,
    +the directory in aufs cannot be removed expectedly because it is a
    +mount point. But the same named directory on the writable branch can
    +be removed, if someone wants. It is just an empty directory, instead
    +of a mount point.
    +Aufs cannot stop such direct rmdir, but produces a warning about it.
    +
    +
    +.\" ----------------------------------------------------------------------
    +.SH Linux Inotify Limitation
    +Unfortunately, current inotify (linux\-2.6.18) has some limitations,
    +and aufs must derive it. I am going to address some harmful cases.
    +
    +.SS IN_ATTRIB, updating atime
    +When a file/dir on a branch is accessed directly, the inode atime (access
    +time, cf. stat(2)) may or may not be updated. In some cases, inotify
    +does not fire this event. So the aufs inode atime may remain old.
    +
    +.SS IN_ATTRIB, updating nlink
    +When the link count of a file on a branch is incremented by link(2)
    +directly,
    +inotify fires IN_CREATE to the parent
    +directory, but IN_ATTRIB to the file. So the aufs inode nlink may
    +remain old.
    +
    +.SS IN_DELETE, removing file on NFS
    +When a file on a NFS branch is deleted directly, inotify may or may
    +not fire
    +IN_DELETE event. It depends upon the status of dentry
    +(DCACHE_NFSFS_RENAMED flag).
    +In this case, the file on aufs seems still exists. Aufs and any user can see
    +the file.
    +
    +.SS IN_IGNORED, deleted rename target
    +When a file/dir on a branch is unlinked by rename(2) directly, inotify
    +fires IN_IGNORED which means the inode is deleted. Actually, in some
    +cases, the inode survives. For example, the rename target is linked or
    +opened. In this case, inotify watch set by aufs is removed by VFS and
    +inotify.
    +And aufs cannot receive the events anymore. So aufs may show you
    +incorrect data about the file/dir.
    +
    +.\" ----------------------------------------------------------------------
    +.SH Policies to Select One among Multiple Writable Branches
    +Aufs has some policies to select one among multiple writable branches
    +when you are going to write/modify something. There are two kinds of
    +policies, one is for newly create something and the other is for
    +internal copy-up.
    +You can select them by specifying mount option \[oq]create=CREATE_POLICY\[cq]
    +or \[oq]cpup=COPYUP_POLICY.\[cq]
    +These policies have no meaning when you have only one writable
    +branch. If there is some meaning, it must be damaging the performance.
    +
    +.SS Exceptions for Policies
    +In every cases below, even if the policy says that the branch where a
    +new file should be created is /rw2, the file will be created on /rw1.
    +.
    +.Bu
    +If there is a readonly branch with \[oq]wh\[cq] attribute above the
    +policy-selected branch and the parent dir is marked as opaque,
    +or the target (creating) file is whiteouted on the ro+wh branch, then
    +the policy will be ignored and the target file will be created on the
    +nearest upper writable branch than the ro+wh branch.
    +.RS
    +.nf
    +/aufs = /rw1 + /ro+wh/diropq + /rw2
    +/aufs = /rw1 + /ro+wh/wh.tgt + /rw2
    +.fi
    +.RE
    +.
    +.Bu
    +If there is a writable branch above the policy-selected branch and the
    +parent dir is marked as opaque or the target file is whiteouted on the
    +branch, then the policy will be ignored and the target file will be
    +created on the highest one among the upper writable branches who has
    +diropq or whiteout. In case of whiteout, aufs removes it as usual.
    +.RS
    +.nf
    +/aufs = /rw1/diropq + /rw2
    +/aufs = /rw1/wh.tgt + /rw2
    +.fi
    +.RE
    +.
    +.Bu
    +link(2) and rename(2) systemcalls are exceptions in every policy.
    +They try selecting the branch where the source exists as possible since
    +copyup a large file will take long time. If it can\[aq]t be, ie. the
    +branch where the source exists is readonly, then they will follow the
    +copyup policy.
    +.
    +.Bu
    +There is an exception for rename(2) when the target exists.
    +If the rename target exists, aufs compares the index of the branches
    +where the source and the target are existing and selects the higher
    +one. If the selected branch is readonly, then aufs follows the copyup
    +policy.
    +
    +.SS Policies for Creating
    +.
    +.TP
    +.B create=tdp | top\-down\-parent
    +Selects the highest writable branch where the parent dir exists. If
    +the parent dir does not exist on a writable branch, then the internal
    +copyup will happen. The policy for this copyup is always \[oq]bottom-up.\[cq]
    +This is the default policy.
    +.
    +.TP
    +.B create=rr | round\-robin
    +Selects a writable branch in round robin. When you have two writable
    +branches and creates 10 new files, 5 files will be created for each
    +branch.
    +mkdir(2) systemcall is an exception. When you create 10 new directories,
    +all are created on the same branch.
    +.
    +.TP
    +.B create=mfs[:second] | most\-free\-space[:second]
    +Selects a writable branch which has most free space. In order to keep
    +the performance, you can specify the duration (\[oq]second\[cq]) which makes
    +aufs hold the index of last selected writable branch until the
    +specified seconds expires. The first time you create something in aufs
    +after the specified seconds expired, aufs checks the amount of free
    +space of all writable branches by internal statfs call
    +and the held branch index will be updated.
    +The default value is \*[AUFS_MFS_SECOND_DEF] seconds.
    +
    +In this mode, a FUSE branch needs special attention.
    +The struct fuse_operations has a statfs operation. It is OK, but the
    +parameter is struct statvfs* instead of struct statfs*. So almost
    +all user\-space implementaion will call statvfs(3)/fstatvfs(3) instead of
    +statfs(2)/fstatfs(2).
    +In glibc, [f]statvfs(3) issues [f]statfs(2), open(2)/read(2) for
    +/proc/mounts,
    +and stat(2) for the mountpoint. With this situation, a FUSE branch will
    +cause a deadlock in creating something in aufs. Here is a sample
    +scenario,
    +.\" .RS
    +.\" .IN -10
    +.Bu
    +create a file just under the aufs root dir.
    +.Bu
    +aufs will aquire a write-lock for the parent directory.
    +.Bu
    +aufs may call statfs internally for each writable branches to decide the
    +branch which has most free space.
    +.Bu
    +FUSE in kernel\-space converts and redirects the statfs request to the
    +user\-space.
    +.Bu
    +the user-space statfs handler will call [f]statvfs(3).
    +.Bu
    +the [f]statvfs(3) in glibc will access /proc/mounts and issue
    +stat(2) for the mountpoint. But those require a read-lock for the aufs
    +root directory.
    +.Bu
    +Then a deadlock occurs.
    +.\" .RE 1
    +.\" .IN
    +
    +In order to avoid this deadlock, I would suggest not to call
    +[f]statvfs(3). Here is a sample code to do this.
    +.nf
    +struct statvfs stvfs;
    +
    +main()
    +{
    + [f]statvfs(..., &stvfs)
    +}
    +
    +statfs_handler(const char *path, struct statvfs *arg)
    +{
    + struct statfs stfs;
    + [f]statfs(..., &stfs);
    + memcpy(arg, &stvfs, sizeof(stvfs));
    + arg->f_bfree = stfs.f_bfree;
    + arg->f_bavail = stfs.f_bavail;
    + arg->f_ffree = stfs.f_ffree;
    + arg->f_favail = /* any value */;
    +}
    +.fi
    +.
    +.TP
    +.B create=mfsrr:low[:second]
    +Selects a writable branch in most-free-space mode first, and then
    +round-robin mode. If the selected branch has less free space than the
    +specified value \[oq]low\[cq] in bytes, then aufs re-tries in round-robin mode.
    +Try an arithmetic expansion of shell which is defined by POSIX.
    +For example, $((10 * 1024 * 1024)) for 10M.
    +You can also specify the duration (\[oq]second\[cq]) which is equivalent to
    +the \[oq]mfs\[cq] mode.
    +.
    +.TP
    +.B create=pmfs[:second]
    +Selects a writable branch where the parent dir exists, such as tdp
    +mode. When the parent dir exists on multiple writable branches, aufs
    +selects the one which has most free space, such as mfs mode.
    +
    +.SS Policies for Copy-Up
    +.
    +.TP
    +.B cpup=tdp | top\-down\-parent
    +Equivalent to the same named policy for create.
    +This is the default policy.
    +.
    +.TP
    +.B cpup=bup | bottom\-up\-parent
    +Selects the writable branch where the parent dir exists and the branch
    +is nearest upper one from the copyup-source.
    +.
    +.TP
    +.B cpup=bu | bottom\-up
    +Selects the nearest upper writable branch from the copyup-source,
    +regardless the existence of the parent dir.
    +
    +.\" ----------------------------------------------------------------------
    +.SH Exporting Aufs via NFS
    +Aufs is supporting NFS-exporting in linux\-2.6.18 and later.
    +Since aufs has no actual block device, you need to add NFS \[oq]fsid\[cq] option at
    +exporting. Refer to the manual of NFS about the detail of this option.
    +
    +In linux\-2.6.23 and earlier,
    +it is recommended to export your branch filesystems once before
    +exporting aufs. By exporting once, the branch filesystem internal
    +pointer named find_exported_dentry is initialized. After this
    +initialization, you may unexport them.
    +Additionally, this initialization should be done per the
    +filesystem type. If your branches are all the same filesystem
    +type, you need to export just one of them once.
    +If you have never export a filesystem which is used in your
    +branches, aufs will initialize the internal pointer by the default
    +value, and produce a
    +warning. While it will work correctly, I am afraid it will be unsafe
    +in the future.
    +In linux\-2.6.24 and later, this exporting is unnecessary.
    +
    +Additionally, there are several limitations or requirements.
    +.RS
    +.Bu
    +The version of linux kernel must be linux\-2.6.18 or later.
    +.Bu
    +You need to enable CONFIG_AUFS_EXPORT.
    +.Bu
    +The branch filesystem must support NFS-exporting. For example, tmpfs in
    +linux\-2.6.18 (or earlier) does not support it.
    +.Bu
    +NFSv2 is not supported. When you mount the exported aufs from your NFS
    +client, you will need to some NFS options like v3 or nfsvers=v3,
    +especially if it is nfsroot.
    +.Bu
    +If the size of the NFS file handle on your branch filesystem is large,
    +aufs will
    +not be able to handle it. The maximum size of NFSv3 file
    +handle for a filesystem is 64 bytes. Aufs uses 24 bytes for 32bit
    +system, plus 12 bytes for 64bit system. The rest is a room for a file
    +handle of a branch filesystem.
    +.Bu
    +The External Inode Number Bitmap and Translation Table (xino) is
    +required since NFS file
    +handle is based upon inode number. The mount option \[oq]xino\[cq] is enabled
    +by default.
    +.Bu
    +The branch filesystems must be accessible, which means \[oq]not hidden.\[cq]
    +It means you need to \[oq]mount \-\-move\[cq] when you use initramfs and
    +switch_root(8), or chroot(8).
    +.\" .Bu
    +.\" The \[oq]noplink\[cq] option is recommended, currently.
    +.\" .Bu
    +.\" If you add/del branches many times between the accesses to the same file
    +.\" from the same NFS client,
    +.\" and the number of the add/del operation is greater than the maximum
    +.\" number of branches, then aufs may not handle the request from the NFS
    +.\" client correctly.
    +.RE
    +
    +.\" ----------------------------------------------------------------------
    +.SH Dentry and Inode Caches
    +If you want to clear caches on your system, there are several tricks
    +for that. If your system ram is low,
    +try \[oq]find /large/dir \-ls > /dev/null\[cq].
    +It will read many inodes and dentries and cache them. Then old caches will be
    +discarded.
    +But when you have large ram or you do not have such large
    +directory, it is not effective.
    +
    +If you want to discard cache within a certain filesystem,
    +try \[oq]mount \-o remount /your/mntpnt\[cq]. Some filesystem may return an error of
    +EINVAL or something, but VFS discards the unused dentry/inode caches on the
    +specified filesystem.
    +
    +.\" ----------------------------------------------------------------------
    +.SH Compatible/Incompatible with Unionfs Version 1.x Series
    +If you compile aufs with \-DCONFIG_AUFS_COMPAT, dirs= option and =nfsro
    +branch permission flag are available. They are interpreted as
    +br: option and =ro flags respectively.
    + \[oq]debug\[cq], \[oq]delete\[cq], \[oq]imap\[cq] options are ignored silently. When you
    +compile aufs without \-DCONFIG_AUFS_COMPAT, these three options are
    +also ignored, but a warning message is issued.
    +
    +Ignoring \[oq]delete\[cq] option, and to keep filesystem consistency, aufs tries
    +writing something to only one branch in a single systemcall. It means
    +aufs may copyup even if the copyup-src branch is specified as writable.
    +For example, you have two writable branches and a large regular file
    +on the lower writable branch. When you issue rename(2) to the file on aufs,
    +aufs may copyup it to the upper writable branch.
    +If this behaviour is not what you want, then you should rename(2) it
    +on the lower branch directly.
    +
    +And there is a simple shell
    +script \[oq]unionctl\[cq] under sample subdirectory, which is compatible with
    +unionctl(8) in
    +Unionfs Version 1.x series, except \-\-query action.
    +This script executes mount(8) with \[oq]remount\[cq] option and uses
    +add/del/mod aufs mount options.
    +If you are familiar with Unionfs Version 1.x series and want to use unionctl(8), you can
    +try this script instead of using mount \-o remount,... directly.
    +Aufs does not support ioctl(2) interface.
    +This script is highly depending upon mount(8) in
    +util\-linux\-2.12p package, and you need to mount /proc to use this script.
    +If your mount(8) version differs, you can try modifying this
    +script. It is very easy.
    +The unionctl script is just for a sample usage of aufs remount
    +interface.
    +
    +Aufs uses the external inode number bitmap and translation table by
    +default.
    +
    +The default branch permission for the first branch is \[oq]rw\[cq], and the
    +rest is \[oq]ro.\[cq]
    +
    +The whiteout is for hiding files on lower branches. Also it is applied
    +to stop readdir going lower branches.
    +The latter case is called \[oq]opaque directory.\[cq] Any
    +whiteout is an empty file, it means whiteout is just an mark.
    +In the case of hiding lower files, the name of whiteout is
    +\[oq]\*[AUFS_WH_PFX].\[cq]
    +And in the case of stopping readdir, the name is
    +\[oq]\*[AUFS_WH_PFX]\*[AUFS_WH_PFX].opq\[cq] or
    +\[oq]\*[AUFS_WH_PFX]__dir_opaque.\[cq] The name depends upon your compile
    +configuration
    +CONFIG_AUFS_COMPAT.
    +.\" All of newly created or renamed directory will be opaque.
    +All whiteouts are hardlinked,
    +including \[oq]/\*[AUFS_WH_PFX]\*[AUFS_WH_BASENAME].\[cq]
    +
    +The hardlink on an ordinary (disk based) filesystem does not
    +consume inode resource newly. But in linux tmpfs, the number of free
    +inodes will be decremented by link(2). It is recommended to specify
    +nr_inodes option to your tmpfs if you meet ENOSPC. Use this option
    +after checking by \[oq]df \-i.\[cq]
    +
    +When you rmdir or rename-to the dir who has a number of whiteouts,
    +aufs rename the dir to the temporary whiteouted-name like
    +\[oq]\*[AUFS_WH_PFX]..\[cq] Then remove it after actual operation.
    +cf. mount option \[oq]dirwh.\[cq]
    +
    +.\" ----------------------------------------------------------------------
    +.SH Incompatible with an Ordinary Filesystem
    +stat(2) returns the inode info from the first existence inode among
    +the branches, except the directory link count.
    +Aufs computes the directory link count larger than the exact value usually, in
    +order to keep UNIX filesystem semantics, or in order to shut find(1) mouth up.
    +The size of a directory may be wrong too, but it has to do no harm.
    +The timestamp of a directory will not be updated when a file is
    +created or removed under it, and it was done on a lower branch.
    +
    +The test for permission bits has two cases. One is for a directory,
    +and the other is for a non-directory. In the case of a directory, aufs
    +checks the permission bits of all existing directories. It means you
    +need the correct privilege for the directories including the lower
    +branches.
    +.\" You can change this behaviour with \[oq]dirperm1\[cq] mount option.
    +The test for a non-directory is more simple. It checks only the
    +topmost inode.
    +
    +statfs(2) returns the first branch info except namelen. The namelen is
    +decreased by the whiteout prefix length.
    +
    +Remember, seekdir(3) and telldir(3) are not defined in POSIX. They may
    +not work as you expect. Try rewinddir(3) or re-open the dir.
    +
    +The whiteout prefix (\*[AUFS_WH_PFX]) is reserved on all branches. Users should
    +not handle the filename begins with this prefix.
    +In order to future whiteout, the maxmum filename length is limited by
    +the longest value \- \*[AUFS_WH_PFX_LEN]. It may be a violation of POSIX.
    +
    +If you dislike the difference between the aufs entries in /etc/mtab
    +and /proc/mounts, and if you are using mount(8) in util\-linux package,
    +then try ./mount.aufs script. Copy the script to /sbin/mount.aufs.
    +This simple script tries updating
    +/etc/mtab. If you do not care about /etc/mtab, you can ignore this
    +script.
    +Remember this script is highly depending upon mount(8) in
    +util\-linux\-2.12p package, and you need to mount /proc.
    +
    +Since aufs uses its own inode and dentry, your system may cache huge
    +number of inodes and dentries. It can be as twice as all of the files
    +in your union.
    +It means that unmounting or remounting readonly at shutdown time may
    +take a long time, since mount(2) in VFS tries freeing all of the cache
    +on the target filesystem.
    +.\" In this case, you had better try \[oq]echo 2 > /proc/sys/vm/drop_caches\[cq]
    +.\" just before unmounting in shutdown procedure.
    +.\" It frees unused inodes and dentries quickly.
    +.\" If your system cache is not so large, you do not need this trick.
    +
    +When you open a directory, aufs will open several directories
    +internally.
    +It means you may reach the limit of the number of file descriptor.
    +And when the lower directory cannot be opened, aufs will close all the
    +opened upper directories and return an error.
    +
    +The sub-mount under the branch
    +of local filesystem
    +is ignored.
    +For example, if you have mount another filesystem on
    +/branch/another/mntpnt, the files under \[oq]mntpnt\[cq] will be ignored by aufs.
    +It is recommended to mount the sub-mount under the mounted aufs.
    +For example,
    +
    +.nf
    +# sudo mount /dev/sdaXX /ro_branch
    +# d=another/mntpnt
    +# sudo mount /dev/sdbXX /ro_branch/$d
    +# mkdir -p /rw_branch/$d
    +# sudo mount -t aufs -o br:/rw_branch:/ro_branch none /aufs
    +# sudo mount -t aufs -o br:/rw_branch/${d}:/ro_branch/${d} none /aufs/another/$d
    +.fi
    +
    +There are several characters which are not allowed to use in a branch
    +directory path and xino filename. See detail in Branch Syntax and Mount
    +Option.
    +
    +The file-lock which means fcntl(2) with F_SETLK, F_SETLKW or F_GETLK, flock(2)
    +and lockf(3), is applied to virtual aufs file only, not to the file on a
    +branch. It means you can break the lock by accessing a branch directly.
    +TODO: check \[oq]security\[cq] to hook locks, as inotify does.
    +
    +The fsync(2) and fdatasync(2) systemcalls return 0 which means success, even
    +if the given file descriptor is not opened for writing.
    +I am afraid this behaviour may violate some standards. Checking the
    +behaviour of fsync(2) on ext2, aufs decided to return success.
    +
    +If you want to use disk-quota, you should set it up to your writable
    +branch since aufs does not have its own block device.
    +
    +When your aufs is the root directory of your system, and your system
    +tells you some of the filesystem were not unmounted cleanly, try these
    +procedure when you shutdown your system.
    +.nf
    +# mount -no remount,ro /
    +# for i in $writable_branches
    +# do mount -no remount,ro $i
    +# done
    +.fi
    +If your xino file is on a hard drive, you also need to specify
    +\[oq]noxino\[cq] option or \[oq]xino=/your/tmpfs/xino\[cq] at remounting root
    +directory.
    +
    +To rename(2) directory may return EXDEV even if both of src and tgt
    +are on the same aufs. When the rename-src dir exists on multiple
    +branches and the lower dir has child(ren), aufs has to copyup all his
    +children. It can be recursive copyup. Current aufs does not support
    +such huge copyup operation at one time in kernel space, instead
    +produces a warning and returns EXDEV.
    +Generally, mv(1) detects this error and tries mkdir(2) and
    +rename(2) or copy/unlink recursively. So the result is harmless.
    +If your application which issues rename(2) for a directory does not
    +support EXDEV, it will not work on aufs.
    +Also this specification is applied to the case when the src directroy
    +exists on the lower readonly branch and it has child(ren).
    +
    +.\" ----------------------------------------------------------------------
    +.SH EXAMPLES
    +The mount options are interpreted from left to right at remount-time.
    +These examples
    +shows how the options are handled. (assuming /sbin/mount.aufs was
    +installed)
    +
    +.nf
    +# mount -v -t aufs br:/day0:/base none /u
    +none on /u type aufs (rw,xino=/day0/.aufs.xino,br:/day0=rw:/base=ro)
    +# mount -v -o remount,\\
    + prepend:/day1,\\
    + xino=/day1/xino,\\
    + mod:/day0=ro,\\
    + del:/day0 \\
    + /u
    +none on /u type aufs (rw,xino=/day1/xino,br:/day1=rw:/base=ro)
    +.fi
    +
    +.nf
    +# mount -t aufs br:/rw none /u
    +# mount -o remount,append:/ro /u
    +different uid/gid/permission, /ro
    +# mount -o remount,del:/ro /u
    +# mount -o remount,nowarn_perm,append:/ro /u
    +#
    +(there is no warning)
    +.fi
    +
    +.\" If you want to expand your filesystem size, aufs may help you by
    +.\" adding an writable branch. Since aufs supports multiple writable
    +.\" branches, the old writable branch can be being writable, if you want.
    +.\" In this example, any modifications to the files under /ro branch will
    +.\" be copied-up to /new, but modifications to the files under /rw branch
    +.\" will not.
    +.\" And the next example shows the modifications to the files under /rw branch
    +.\" will be copied-up to /new/a.
    +.\"
    +.\" Todo: test multiple writable branches policy. cpup=nearest, cpup=exist_parent.
    +.\"
    +.\" .nf
    +.\" # mount -v -t aufs br:/rw:/ro none /u
    +.\" none on /u type aufs (rw,xino=/rw/.aufs.xino,br:/rw=rw:/ro=ro)
    +.\" # mkfs /new
    +.\" # mount -v -o remount,add:1:/new=rw /u
    +.\" none on /u type aufs (rw,xino=/rw/.aufs.xino,br:/rw=rw:/new=rw:/ro=ro)
    +.\" .fi
    +.\"
    +.\" .nf
    +.\" # mount -v -t aufs br:/rw:/ro none /u
    +.\" none on /u type aufs (rw,xino=/rw/.aufs.xino,br:/rw=rw:/ro=ro)
    +.\" # mkfs /new
    +.\" # mkdir /new/a new/b
    +.\" # mount -v -o remount,add:1:/new/b=rw,prepend:/new/a,mod:/rw=ro /u
    +.\" none on /u type aufs (rw,xino=/rw/.aufs.xino,br:/new/a=rw:/rw=ro:/new/b=rw:/ro=ro)
    +.\" .fi
    +
    +When you use aufs as root filesystem, it is recommended to consider to
    +exclude some directories. For example, /tmp and /var/log are not need
    +to stack in many cases. They do not usually need to copyup or to whiteout.
    +Also the swapfile on aufs (a regular file, not a block device) is not
    +supported.
    +
    +And there is a good sample which is for network booted diskless machines. See
    +sample/ in detail.
    +
    +.\" ----------------------------------------------------------------------
    +.SH DIAGNOSTICS
    +When you add an branch to your union, aufs may warn you about the
    +privilege or security of the branch, which is the permission bits,
    +owner and group of the top directory of the branch.
    +For example, when your upper writable branch has a world writable top
    +directory,
    +a malicious user can create any files on the writable branch directly,
    +like copyup and modify manually. I am afraid it can be a security
    +issue.
    +
    +When you mount or remount your union without \-o ro common mount option
    +and without writable branch, aufs will warn you that the first branch
    +should be writable.
    +
    +.\" It is discouraged to set both of \[oq]udba\[cq] and \[oq]noxino\[cq] mount options. In
    +.\" this case the inode number under aufs will always be changed and may
    +.\" reach the end of inode number which is a maximum of unsigned long. If
    +.\" the inode number reaches the end, aufs will return EIO repeatedly.
    +
    +When you set udba other than inotify and change something on your
    +branch filesystem directly, later aufs may detect some mismatches to
    +its cache. If it is a critical mismatch, aufs returns EIO and issues a
    +warning saying \[oq]try udba=inotify.\[cq]
    +
    +When an error occurs in aufs, aufs prints the kernel message with
    +\[oq]errno.\[cq] The priority of the message (log level) is ERR or WARNING which
    +depends upon the message itself.
    +You can convert the \[oq]errno\[cq] into the error message by perror(3),
    +strerror(3) or something.
    +For example, the \[oq]errno\[cq] in the message \[oq]I/O Error, write failed (\-28)\[cq]
    +is 28 which means ENOSPC or \[oq]No space left on device.\[cq]
    +
    +.\" .SH Current Limitation
    +.
    +.\" ----------------------------------------------------------------------
    +.\" SYNOPSIS
    +.\" briefly describes the command or function\[aq]s interface. For commands, this
    +.\" shows the syntax of the command and its arguments (including options); bold-
    +.\" face is used for as-is text and italics are used to indicate replaceable
    +.\" arguments. Brackets ([]) surround optional arguments, vertical bars (|) sep-
    +.\" arate choices, and ellipses (...) can be repeated. For functions, it shows
    +.\" any required data declarations or #include directives, followed by the func-
    +.\" tion declaration.
    +.
    +.\" DESCRIPTION
    +.\" gives an explanation of what the command, function, or format does. Discuss
    +.\" how it interacts with files and standard input, and what it produces on
    +.\" standard output or standard error. Omit internals and implementation
    +.\" details unless they\[aq]re critical for understanding the interface. Describe
    +.\" the usual case; for information on options use the OPTIONS section. If
    +.\" there is some kind of input grammar or complex set of subcommands, consider
    +.\" describing them in a separate USAGE section (and just place an overview in
    +.\" the DESCRIPTION section).
    +.
    +.\" RETURN VALUE
    +.\" gives a list of the values the library routine will return to the caller and
    +.\" the conditions that cause these values to be returned.
    +.
    +.\" EXIT STATUS
    +.\" lists the possible exit status values or a program and the conditions that
    +.\" cause these values to be returned.
    +.
    +.\" USAGE
    +.\" describes the grammar of any sublanguage this implements.
    +.
    +.\" FILES
    +.\" lists the files the program or function uses, such as configuration files,
    +.\" startup files, and files the program directly operates on. Give the full
    +.\" pathname of these files, and use the installation process to modify the
    +.\" directory part to match user preferences. For many programs, the default
    +.\" installation location is in /usr/local, so your base manual page should use
    +.\" /usr/local as the base.
    +.
    +.\" ENVIRONMENT
    +.\" lists all environment variables that affect your program or function and how
    +.\" they affect it.
    +.
    +.\" SECURITY
    +.\" discusses security issues and implications. Warn about configurations or
    +.\" environments that should be avoided, commands that may have security impli-
    +.\" cations, and so on, especially if they aren\[aq]t obvious. Discussing security
    +.\" in a separate section isn\[aq]t necessary; if it\[aq]s easier to understand, place
    +.\" security information in the other sections (such as the DESCRIPTION or USAGE
    +.\" section). However, please include security information somewhere!
    +.
    +.\" CONFORMING TO
    +.\" describes any standards or conventions this implements.
    +.
    +.\" NOTES
    +.\" provides miscellaneous notes.
    +.
    +.\" BUGS
    +.\" lists limitations, known defects or inconveniences, and other questionable
    +.\" activities.
    +
    +.SH COPYRIGHT
    +Copyright \(co 2005, 2006, 2007, 2008 Junjiro Okajima
    +
    +.SH AUTHOR
    +Junjiro Okajima
    +
    +.\" SEE ALSO
    +.\" lists related man pages in alphabetical order, possibly followed by other
    +.\" related pages or documents. Conventionally this is the last section.
    --
    1.4.4.4

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  2. Re: [PATCH 4/67] aufs configuration


    On Friday 2008-05-16 16:32, hooanon05@yahoo.co.jp wrote:
    >+
    >+if AUFS
    >+config AUFS_FAKE_DM
    >+ bool
    >+ depends on AUFS
    >+ default n


    When you have "if AUFS" you do not need all the "depends on AUFS".

    >+config AUFS_ROBR
    >+ bool "Aufs as an readonly branch of another aufs"
    >+ depends on AUFS
    >+ default n
    >+ help
    >+ If you want make your aufs to be a part of another aufs, then
    >+ enable this option. In other words, you can specify your aufs
    >+ path in 'br:' mount option for another aufs, but cannot
    >+ specify 'rw' as the branch permission.
    >+ It will damage the performance.
    >+ See detail in aufs.5.


    IIRC, unionfs did not need such an option, so why does AUFS?

    >+config AUFS_DLGT
    >+ bool "Delegate the internal branch access the kernel thread"
    >+ depends on AUFS
    >+ default n


    I do not think you need "default n" either as that should be the
    implicit default.

    >+config AUFS_SPLICE_PATCH
    >+ bool
    >+ depends on AUFS
    >+ default y


    This should just go away if it is always selected anyway.

    >+config AUFS_BR_NFS
    >+ bool
    >+ depends on AUFS
    >+ depends on NFS_FS
    >+ depends on !AUFS_FAKE_DM
    >+ default n if (!AUFS_LHASH_PATCH || AUFS_FAKE_DM)
    >+ default y


    One AUFS_FAKE_DM is redundant here. It should probably be just

    depends on NFS_FS && !AUFS_FAKE_DM && AUFS_LHASH_PATCH

    >+config AUFS_BR_XFS
    >+ bool
    >+ depends on AUFS
    >+ depends on XFS_FS
    >+ default y


    Same here, just depends on XFS_FS.

    >+config AUFS_WORKAROUND_FUSE
    >+ bool "Special handling for FUSE-based filesystem"
    >+ depends on AUFS && FUSE_FS
    >+ default n
    >+ help
    >+ A FUSE-based filesystem may not initialize its inode
    >+ attributes and the FUSE developer thinks the inode attributes
    >+ in a positive dentry which is returned by VFS lookup operation
    >+ are not reliable.
    >+ If you use a FUSE-based filesystem as an aufs branch, and it
    >+ customizes the inode attribute on it without overriding
    >+ fuse_lowlevel_ops.lookup, probably you need to enable this
    >+ configuration.
    >+ If you enable this configuration, aufs calls getattr operation
    >+ in every lookup and revalidate operation for the FUSE-based
    >+ filesystem branch.
    >+ It will damage the performance even if you don't use a
    >+ FUSE-based filesystem branch.


    Performance is not "damaged" but "hurt" :-) [or just "will negatively
    impact performance"]

    >+config AUFS_MAGIC_SYSRQ
    >+ bool
    >+ depends on AUFS
    >+ depends on AUFS_DEBUG
    >+ depends on MAGIC_SYSRQ
    >+ default y


    Since I see this for the first time, a bit of help text would not be
    bad, even if it is not visible in menuconfig.

    >+config AUFS_COMPAT
    >+ bool "Compatibility with Unionfs (obsolete)"


    If it is obsolete, it should not be merged.
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  3. Re: [PATCH 8/67] aufs module global variables and operations


    On Friday 2008-05-16 16:32, hooanon05@yahoo.co.jp wrote:
    >+/* ---------------------------------------------------------------------- */
    >+
    >+/* fake Kconfig */


    I doubt this is needed anymore:

    >+#if 1
    >+
    >+#ifdef CONFIG_AUFS_HINOTIFY
    >+#ifndef CONFIG_INOTIFY
    >+#error enable CONFIG_INOTIFY to use CONFIG_AUFS_HINOTIFY.
    >+#endif
    >+#endif /* CONFIG_AUFS_HINOTIFY */
    >+
    >+#if AUFS_BRANCH_MAX > 511 && PAGE_SIZE > 4096
    >+#warning pagesize is larger than 4kb, \
    >+ CONFIG_AUFS_BRANCH_MAX_511 or smaller is recommended.
    >+#endif
    >+
    >+#ifdef CONFIG_AUFS_STAT
    >+#ifndef CONFIG_SYSFS
    >+#error CONFIG_AUFS_STAT requires CONFIG_SYSFS.
    >+#endif
    >+#endif /* CONFIG_AUFS_STAT */
    >+
    >+#ifdef CONFIG_AUFS_SYSAUFS
    >+#warning CONFIG_AUFS_SYSAUFS is unnecessary for linux-2.6.25 and later.
    >+#endif
    >+
    >+#ifdef CONFIG_AUFS_EXPORT
    >+#if !defined(CONFIG_EXPORTFS) && !defined(CONFIG_EXPORTFS_MODULE)
    >+#error CONFIG_AUFS_EXPORT requires CONFIG_EXPORTFS
    >+#endif
    >+#if defined(CONFIG_EXPORTFS_MODULE) && defined(CONFIG_AUFS)
    >+#error need CONFIG_EXPORTFS = y to link aufs statically with CONFIG_AUFS_EXPORT
    >+#endif
    >+#endif /* CONFIG_AUFS_EXPORT */
    >+

    ....

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  4. Re: [PATCH 6/67] aufs main header file


    On Friday 2008-05-16 16:32, hooanon05@yahoo.co.jp wrote:
    >+
    >+#include
    >+#if LINUX_VERSION_CODE < KERNEL_VERSION(2, 6, 25)
    >+#error you got wrong version
    >+#endif


    Kill this.


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  5. Re: [PATCH 5/67] aufs Makefile

    Hi Junjiro.

    A few comments to your Makefile.

    On Fri, May 16, 2008 at 11:32:19PM +0900, hooanon05@yahoo.co.jp wrote:
    > From: Junjiro Okajima
    >
    > initial commit
    > aufs Makefile
    >
    > Signed-off-by: Junjiro Okajima
    > ---
    > fs/aufs/Makefile | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++ ++++
    > 1 files changed, 57 insertions(+), 0 deletions(-)
    >
    > diff --git a/fs/aufs/Makefile b/fs/aufs/Makefile
    > new file mode 100644
    > index 0000000..05e0864
    > --- /dev/null
    > +++ b/fs/aufs/Makefile
    > @@ -0,0 +1,57 @@
    > +# AUFS Makefile for the Linux 2.6.25 and later
    > +# $Id: Makefile,v 1.3 2008/04/28 03:03:10 sfjro Exp $

    Please drop these two lines

    > +
    > +# the environment variables are not inherited since 2.6.23
    > +ifdef AUFS_EXTRA_CFLAGS
    > +EXTRA_CFLAGS += ${AUFS_EXTRA_CFLAGS}
    > +endif

    And this as well. If this is needed do like the others do.

    > +
    > +########################################
    > +
    > +ifdef CONFIG_AUFS_WORKAROUND_FUSE
    > +# defined in ${srctree}/fs/fuse/inode.c
    > +EXTRA_CFLAGS += -DFUSE_SUPER_MAGIC=0x65735546
    > +endif

    Use:
    ccflags-$(CONFIG_AUFS_WORKAROUND_FUSE) += -DFUSE_SUPER_MAGIC=0x65735546

    > +
    > +ifdef CONFIG_XFS_FS
    > +# defined in ${srctree}/fs/xfs/xfs_sb.h
    > +EXTRA_CFLAGS += -DXFS_SB_MAGIC=0x58465342
    > +endif

    ccflags-$(CONFIG_XFS_FS) += -DXFS_SB_MAGIC=0x58465342

    > +
    > +ifdef CONFIG_TMPFS
    > +# defined in ${srctree}/mm/shmem.c
    > +EXTRA_CFLAGS += -DTMPFS_MAGIC=0x01021994
    > +endif

    ccflags-$(CONFIG_TMPFS) += -DTMPFS_MAGIC=0x01021994

    > +
    > +-include $(dir $(lastword ${MAKEFILE_LIST}))priv.mk


    Please drop this.

    > +#$(warning ${EXTRA_CFLAGS})

    Not used - drop it.

    > +
    > +########################################
    > +
    > +obj-$(CONFIG_AUFS) += aufs.o
    > +aufs-y := module.o super.o sbinfo.o branch.o xino.o sysaufs.o opts.o \
    > + wkq.o vfsub.o dcsub.o \
    > + cpup.o whout.o plink.o wbr_policy.o \
    > + dentry.o dinfo.o \
    > + file.o f_op.o finfo.o \
    > + dir.o vdir.o \
    > + inode.o i_op.o i_op_add.o i_op_del.o i_op_ren.o iinfo.o \
    > + misc.o
    > +
    > +#xattr.o
    > +aufs-$(CONFIG_SYSFS) += sysfs.o
    > +aufs-$(CONFIG_AUFS_BR_NFS) += br_nfs.o
    > +aufs-$(CONFIG_AUFS_BR_XFS) += br_xfs.o
    > +aufs-$(CONFIG_AUFS_WORKAROUND_FUSE) += br_fuse.o
    > +
    > +aufs-$(CONFIG_AUFS_DLGT) += dlgt.o
    > +aufs-$(CONFIG_AUFS_HINOTIFY) += hinotify.o hin_or_dlgt.o


    > +# dirty
    > +ifndef CONFIG_AUFS_HINOTIFY
    > +aufs-$(CONFIG_AUFS_DLGT) += hin_or_dlgt.o
    > +endif

    Can we have this dependency expressed in your Kconfig file?

    > +
    > +aufs-$(CONFIG_AUFS_EXPORT) += export.o
    > +aufs-$(CONFIG_AUFS_ROBR) += robr.o
    > +aufs-$(CONFIG_AUFS_DEBUG) += debug.o
    > +aufs-$(CONFIG_AUFS_MAGIC_SYSRQ) += sysrq.o



    Sam
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  6. Re: [PATCH 5/67] aufs Makefile


    Hi Sam,
    Thank you for your review and comments.

    I will fix as you pointed out.

    Sam Ravnborg:
    > Use:
    > ccflags-$(CONFIG_AUFS_WORKAROUND_FUSE) += -DFUSE_SUPER_MAGIC=0x65735546


    I didn't know that. It is much better.


    > > +# dirty
    > > +ifndef CONFIG_AUFS_HINOTIFY
    > > +aufs-$(CONFIG_AUFS_DLGT) += hin_or_dlgt.o
    > > +endif

    > Can we have this dependency expressed in your Kconfig file?


    I will add a new automatic configuration CONFIG_AUFS_HIN_OR_DLGT.


    Junjiro Okajima
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  7. Re: [PATCH 4/67] aufs configuration


    Hi,
    Thank you for your review and comments.

    I will fix as you pointed out.

    Jan Engelhardt:
    > >+config AUFS_ROBR
    > >+ bool "Aufs as an readonly branch of another aufs"
    > >+ depends on AUFS
    > >+ default n
    > >+ help
    > >+ If you want make your aufs to be a part of another aufs, then
    > >+ enable this option. In other words, you can specify your aufs
    > >+ path in 'br:' mount option for another aufs, but cannot
    > >+ specify 'rw' as the branch permission.
    > >+ It will damage the performance.
    > >+ See detail in aufs.5.

    >
    > IIRC, unionfs did not need such an option, so why does AUFS?


    The *current* unionfs doesn't support such nested mount, ie. unionfs as
    a branch of another unionfs mount.
    Aufs supports it only readonly mode (if you enable this configuration).


    > Performance is not "damaged" but "hurt" :-) [or just "will negatively
    > impact performance"]


    English language is much more difficult than C language for me...


    Junjiro Okajima
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