What is present situation with SAS bus drives? - Hardware

This is a discussion on What is present situation with SAS bus drives? - Hardware ; Periodically I find a need to up grade hardware. I've always run SCSI bus hard disks, but at this point it looks as though the proper course for a hardware upgrade would be to migrate from SCSI to SAS (Serial ...

+ Reply to Thread
Results 1 to 6 of 6

Thread: What is present situation with SAS bus drives?

  1. What is present situation with SAS bus drives?

    Periodically I find a need to up grade hardware. I've always run SCSI
    bus hard disks, but at this point it looks as though the proper course
    for a hardware upgrade would be to migrate from SCSI to SAS (Serial
    Attached SCSI). I've culled some information on line, but certain
    things still puzzle me.

    One is that people originally said that one advantage of SAS over SCSI
    was that it would be cheaper. Why is it that most SAS drives seem to
    a lot more expensive than compatable SCSI disks? Although SAS has been
    out for well over a year, there are surprisingly few SAS drives.

    I get the impression that one can't expect a driver for SAS disks in
    the linux kernel for some time. Did the RH and Suse compile one in for
    their distributions? I assume that for other distributions to use a
    SAS disk at this point, one would have to recompile the kernel with a
    suitable driver. Is that so?

    The Hitachi SAS drive comes in different models having different
    bus connectors. One is the SAS, which comes with a standard 29-pin SAS
    connector. But Hitachi says this connector is designed to be used with
    a backplane into which drives are plugged. I assume though that one
    can use the drive with a SAS cable connected to a SAS controller, such
    as the LSI Logic.

    Anyone in hearing range have any experience using a SAS drive and
    controller under Linux?

    --

    Haines Brown, KB1GRM




  2. Re: What is present situation with SAS bus drives?

    Haines Brown wrote:

    >
    > Anyone in hearing range have any experience using a SAS drive and
    > controller under Linux?
    >


    Yes - Sun's SunFire Opteron servers (4xxx series, can't remember exact model
    number, probably 4100) has an LSI SAS controller - which (and this was a
    year ago at a place I no longer work, so I cannot check) needed the LSI
    drivers updated in the 2.6 kernel of the day - or at least a kernel upgrade
    to the precise kernel of the day - again, sorry for flaky recollection.


    I do recall though, that once that was done they ran fine and were rock
    solid, so by now I doubt there'll be any fundamental problems - as usual it
    will be a case of checking the chipset for support and bugs, or (better)
    evaluate a box and test it.

    HTH

    Tim

  3. Re: What is present situation with SAS bus drives?

    Haines Brown wrote:
    > Periodically I find a need to up grade hardware. I've always run SCSI
    > bus hard disks, but at this point it looks as though the proper course
    > for a hardware upgrade would be to migrate from SCSI to SAS (Serial
    > Attached SCSI). I've culled some information on line, but certain
    > things still puzzle me.
    >
    > One is that people originally said that one advantage of SAS over SCSI
    > was that it would be cheaper. Why is it that most SAS drives seem to
    > a lot more expensive than compatable SCSI disks? Although SAS has been
    > out for well over a year, there are surprisingly few SAS drives.


    I don't know where you're looking, but many vendors carry SAS drives for
    less than a 10% price premium over SCSI. Seagate's current SCSI drives
    all come in SAS variants as well as 68- and 80-pin SCSI. The larger the
    drive, the lower the price premium, AFAICT, from a quick check of a
    couple of vendors.

    > I get the impression that one can't expect a driver for SAS disks in
    > the linux kernel for some time.


    ??? SAS drives are accessed through the kernel's SCSI subsystem. You
    don't need a SAS disk driver, the kernel's sd (SCSI disk) driver works.

    What you do need is a driver for the SAS controller you have.

    The LSI Logic LSIAS1064/LSIAS1068 controllers are listed as supported by
    the Fusion MPT driver in the stock kernel, and have been for quite a
    while by the looks of it (comments in the mpi_sas.h file go back to
    2004). Other LSI Logic SAS controllers (of the MegaRAID family) are
    supported by the megraid driver in the stock kernel. Similarly,
    Adaptec's 4000 and 4800 series SAS RAID controllers are supported by the
    aacraid driver in the stock kernel.

    [snip]
    > The Hitachi SAS drive comes in different models having different
    > bus connectors. One is the SAS, which comes with a standard 29-pin SAS
    > connector. But Hitachi says this connector is designed to be used with
    > a backplane into which drives are plugged. I assume though that one
    > can use the drive with a SAS cable connected to a SAS controller, such
    > as the LSI Logic.


    I've never seen a SAS drive cabled directly. They've all been in hot
    swap SAS enclosures with appropriate backplanes. If you cable directly,
    you loose that hot swap capability. It may work without the backplane,
    but I'm not sure why you'd want to. (I'd stick with 68-pin SCSI in that
    case, personally.)

  4. Re: What is present situation with SAS bus drives?

    John-Paul Stewart writes:

    > I've never seen a SAS drive cabled directly. They've all been in hot
    > swap SAS enclosures with appropriate backplanes. If you cable
    > directly, you loose that hot swap capability. It may work without the
    > backplane, but I'm not sure why you'd want to. (I'd stick with 68-pin
    > SCSI in that case, personally.)


    I thank you and Tim for the information that cleared some things up
    nicely. I gather that while Adaptec and LSI Logic offer Linux drivers
    for their contollers, the drivers needed by these controllers are
    already in the stock kernel.

    On a workstation, since hot swap capability is of little importance,
    the question becomes, why would a person who has been using SCSI over
    the years want to jump ship and go to SAS? Here are some of the
    arguments. I'm not trying to defend a move to SAS, but wonder why
    these arguments shouldn't encourage one do it automatically.

    1. I get the impression, perhaps incorrect, that the future lies with
    SATA and, at the high end, Fiber Channel, and SAS will replace
    parallel SCSI for the mid range. I.e., parallel SCSI is a sinking
    ship. Someone says, for example: "SAS is displacing U320 faster
    than SATA replaced PATA".

    2. The obvious issue of cabling that does not impede air flow in the
    box and allows longer runs and has a lower voltage. Ribbon cabling
    I have found hard to work with in confined spaces and would be glad
    to get away from it.

    3. Gets away from SCSI termination issues. I've found parallel SCSI
    terminations sometimes painful to deal with, and recently with U320
    they have become very expensive.

    4. I gather it has much higher speed data transfers than parallel
    SCSI, although that speed may not be all that important for most
    work stations, just as its support for many more attached
    devices and longer cables.

    5. Use of serial transfer protocol to interface multiple devices means
    reduced signal overhead. I assume that even with just a few
    devices, that would improve disk access times.

    6. Support for SATA devices. No big advantage as far as I'm concerned,
    but for others or down the road that may be otherwise.

    7. I gather read/write time should be reduced because SAS eliminates
    drive skew.

    8. On the cost issue, while I did happen upon some unusually expensive
    drives, and while it may be that the premium for SAS is small, I
    did read: "SAS permits SCSI devices and cabling to simultaneously be
    significantly cheaper and faster (initial 3GB/sec bus speed, soon
    going to 20GB/sec and up)." If true, that in itself would seem a
    compelling reason to migrate from SCSI to SAS.

    --

    Haines Brown, KB1GRM




  5. Re: What is present situation with SAS bus drives?

    Haines Brown wrote:
    > "SAS permits SCSI devices and cabling to simultaneously be
    > significantly cheaper and faster (initial 3GB/sec bus speed..."


    That should read "3Gb/sec bus speed". IOW, the bus speed is 3 Giga
    *bits* per second, *not* 3 Giga *bytes*. Usable bandwidth is in the
    area of 300 MBytes/sec, or similar to that of U320 SCSI. Of course with
    multiple U320 disks that bandwidth is shared by all disks on the bus,
    but that's not the case with SAS.

    However, no disk can sustain those transfer rates. Even the best single
    disk speed I've seen (~135 MB/sec from a very pricey 300 GB Seagate
    Cheetah 15K.5 series drive, according to http://www.storagereview.com/)
    is well short of the available bus bandwidth.

    Personally, I don't understand why people get so excited by theoretical
    numbers (as in another recent thread about SATA vs. ATA) that can't be
    sustained. Those burst rates only apply when reading from the drive's
    16 MB (or smaller) buffer---so it's a 0.05 second (or shorter) burst at
    the full bus speed. Yet everybody seems to want a higher burst speed
    without regard to sustained transfer rate. Even sustained transfer rate
    isn't always a good measure of performance. For many uses a higher seek
    speed is what counts (and that's where 15K RPM drives shine, regardless
    of interface IMHO) since few applications require streaming large
    amounts of data continuously.

    Personally, I haven't seen any compelling reason to use SAS. (But I
    don't really have a compelling reason *not* to use SAS, either.) I've
    got SCSI and I'll stick with it as long as these systems keep running
    for the simple reason that they all meet my current needs. For any new
    systems I purchase, SAS vs. SCSI will be a non-issue. I'll choose
    systems based on other criteria and use whatever disks are appropriate
    for those systems' on-board controllers. That's a strategy I'd
    recommend to anyone who doesn't know what to do.

  6. Re: What is present situation with SAS bus drives?

    Haines Brown wrote:
    .....
    >
    > Anyone in hearing range have any experience using a SAS drive and
    > controller under Linux?
    >


    I have a DL380-G5 (SAS) running HP's hardware raid controller using
    RHEL 4 (the one it comes with by default). I'm sure that SLES10 will
    have no problem as well. Machine has dual 5160's.. it's pretty
    fast.

    No issues to report.

+ Reply to Thread