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    SubjectRe: FYI: RAID5 unusably unstable through 2.6.14
    Cynbe ru Taren wrote:

    >Just in case the RAID5 maintainers aren't aware of it:
    >
    >The current Linux kernel RAID5 implementation is just
    >too fragile to be used for most of the applications
    >where it would be most useful.
    >
    >In principle, RAID5 should allow construction of a
    >disk-based store which is considerably MORE reliable
    >than any individual drive.
    >
    >In my experience, at least, using Linux RAID5 results
    >in a disk storage system which is considerably LESS
    >reliable than the underlying drives.
    >
    >What happens repeatedly, at least in my experience over
    >a variety of boxes running a variety of 2.4 and 2.6
    >Linux kernel releases, is that any transient I/O problem
    >results in a critical mass of RAID5 drives being marked
    >'failed',
    >
    What kind of "transient io error" would that be?
    That is not supposed to happen regularly. . .

    You do replace failed drives immediately? Allowing
    systems to run "for a while" in degraded mode is
    surely a recipe for disaster. Degraded mode
    has no safety at all, it is just raid-0 with a performance
    overhead added in. :-/

    Having hot spares is a nice way of replacing the failed
    drive quickly.

    >at which point there is no longer any supported
    >way of retrieving the data on the RAID5 device, even
    >though the underlying drives are all fine, and the underlying
    >data on those drives almost certainly intact.
    >
    >
    As other have showed - "mdadm" can reassemble your
    broken raid - and it'll work well in those cases where
    the underlying drives indeed are ok. It will fail
    spectacularly if you have a real double fault though,
    but then nothing short of raid-6 can save you.


    Helge Hafting

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