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    SubjectRe: Accessing the same disk via multiple channels
    On 2003-02-14T09:20:12,
    steve cameron <> said:

    > Yay! We noticed that if a controller fails in such a way that
    > no interrupts are generated then md driver doesn't notice anything is
    > wrong. Commands don't fail, but don't complete either.

    Uhhhhh. They should timeout though and then be counted as errors; hard to
    catch this differently.

    You are saying it doesn't work?

    > (Better than putting a timeout on every command.) Also, md multipath
    > doesn't notice if the backup path has failed, to warn the user that
    > redundancy is no longer in effect. (Though if you set up things so i/o
    > is going down both paths, not such a big deal, as md will notice.
    > Probably you know all this already.

    Yes. We intentionally don't do active _monitoring_ on non-active paths, same
    as we don't do reprobing of failed paths to see whether they are alive again.

    (The LVM m-p patch does periodically send down live requests to failed paths
    to check this; I consider this intentional data corruption, but I'm

    This is something which can easily be implemented safely in user-space
    though; the md approach still exposes the lower level devices and you can
    check them periodically if you care, and be it with a

    while sleep 1 ; do
    if ! dd if=/dev/sdaX of=/dev/null ; then
    mdadm /dev/md0 --fail /dev/sdaX
    logger -p kern.alert "Path /dev/sdaX failed!"

    ;-) Obviously, not something we need to run in kernel space.

    > Well, the cciss driver is not a SCSI driver (except for purpsoes of
    > tape drives & tape changers) and HP/Compaq has sold more than one
    > million of those controllers (does popularity mean they aren't
    > "weird"? :-), and we have mulitpath capable storage boxes they
    > can connect to.

    Indeed. Yes, we'll need to figure out how to do this for 2.5/2.6; maybe
    porting forward the md m-p patch to 2.5 is indeed the best choice. It should
    be way easier, as md has been greatly cleaned up...

    However, past discussions on LKML regarding "How to do m-p cleanly in 2.5"
    have never reached a conclusion ;-) We'll see. The good thing about the SCSI
    m-p is that it can also handle multipathed tape drives...

    Lars Marowsky-Brée <>

    Principal Squirrel
    SuSE Labs - Research & Development, SuSE Linux AG

    "If anything can go wrong, it will." "Chance favors the prepared (mind)."
    -- Capt. Edward A. Murphy -- Louis Pasteur
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