rsync efficiency - Tools

This is a discussion on rsync efficiency - Tools ; Hello all, I have a doubt that i think you hackers of rsync has the answer. ;-) I have make this post on my blog: http://www.posix.brte.com.br/blog/?p=312 to start a serie about the copy-on-write semantics of ZFS. In my test "VI" ...

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  1. rsync efficiency

    Hello all,
    I have a doubt that i think you hackers of rsync has the answer. ;-)
    I have make this post on my blog:
    http://www.posix.brte.com.br/blog/?p=312
    to start a serie about the copy-on-write semantics of ZFS. In my test
    "VI" did rewrite the whole file just for change 3 bytes, so the whole
    file was reallocated.
    What i want to know from you is about the techniques used by rsync
    (and about other softwares that you know), for change a few bytes in
    the middle of a big file. Can be a simple question for you, but i
    really think how rsync can change 18k inside a 1gb file, without
    rewrite the whole file (or a lot of indirect blocks).
    If we are talking about a SO without copy-on-write filesystem, maybe
    we can rewrite just that block (??), but in ZFS for example, if we
    have a 128K block, and we need to add 10k, that change will propagate
    to the whole tree of blocks, right?
    And i think rsync like many softwares, create a temporary file on the
    destination, and the whole file is rewriten locally, just the changes
    over the wire. Is that right?
    The question is: There is a efficient/safe way to change 10k of data
    in a 1gb file, whithout a lot of rewrites? rsync uses some technique
    for that, or is totally dependent on the filesystem?

    Thanks a lot!

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  2. Re: rsync efficiency

    Hi there,

    there are various challenges with this: rsync would typically use the
    mtime to check if a file has changed and if so, it would scan the source
    file and the target file and compage checksums. Thus, both files need to
    be read in whole - which affects performance. This is especially the
    case if you transfer database files or virtual machines, both typically
    existing of large files that are changed on block level.
    You can speed this up (if you have a fast network connection) using the
    -w option. This will not scan both files but instead start a new
    transfer if a change is detected. This results in a linear read on the
    source file and a linear write on the destination. Thus, the transfer
    takes as long as a linear write takes - and not a read and parallel
    write on the destination side (as indicated above).
    In theory, you could speed this up of you transfer the journal from the
    source to the destination. This is what is known as logshipping on
    databases. However, it would require changes to the underlying block
    device driver, would be limited to specific platforms and it would
    resemble existing technologies that could be used in certain cases. I
    think this is not the intention of rsync.
    Last, if you need a block-level sync, you could take a look at drbd
    which implements some of the features that I've mentioned above.


    Greetings
    Benjamin

    On Fri, Sep 19, 2008 at 04:40:05PM -0300, Marcelo Leal wrote:
    > Hello all,
    > I have a doubt that i think you hackers of rsync has the answer. ;-)
    > I have make this post on my blog:
    > http://www.posix.brte.com.br/blog/?p=312
    > to start a serie about the copy-on-write semantics of ZFS. In my test
    > "VI" did rewrite the whole file just for change 3 bytes, so the whole
    > file was reallocated.
    > What i want to know from you is about the techniques used by rsync
    > (and about other softwares that you know), for change a few bytes in
    > the middle of a big file. Can be a simple question for you, but i
    > really think how rsync can change 18k inside a 1gb file, without
    > rewrite the whole file (or a lot of indirect blocks).
    > If we are talking about a SO without copy-on-write filesystem, maybe
    > we can rewrite just that block (??), but in ZFS for example, if we
    > have a 128K block, and we need to add 10k, that change will propagate
    > to the whole tree of blocks, right?
    > And i think rsync like many softwares, create a temporary file on the
    > destination, and the whole file is rewriten locally, just the changes
    > over the wire. Is that right?
    > The question is: There is a efficient/safe way to change 10k of data
    > in a 1gb file, whithout a lot of rewrites? rsync uses some technique
    > for that, or is totally dependent on the filesystem?
    >
    > Thanks a lot!
    >
    > --
    >
    > [http://www.posix.brte.com.br/blog]
    > --------==== pOSix rules ====-------
    > --
    > Please use reply-all for most replies to avoid omitting the mailing list.
    > To unsubscribe or change options: https://lists.samba.org/mailman/listinfo/rsync
    > Before posting, read: http://www.catb.org/~esr/faqs/smart-questions.html


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