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    SubjectFYI: RAID5 unusably unstable through 2.6.14

    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', 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.

    This has just happened to me for at least the sixth time,
    this time in a brand new RAID5 consisting of 8 200G hotswap
    SATA drives backing up the contents of about a dozen onsite
    and offsite boxes via dirvish, which took me the better part
    of December to get initialized and running, and now two weeks
    later I'm back to square one.

    I'm currently digging through the md kernel source code
    trying to work out some ad-hoc recovery method, but this
    level of flakiness just isn't acceptable on systems where
    reliable mass storage is a must -- and when else would
    one bother with RAID5?

    I run a RAID1 mirrored boot and/or root partition on all
    the boxes I run RAID5 on -- and lots more as well -- and
    RAID1 -does- work as one would hope, providing a disk
    store -more- reliable than the underlying drives. A
    Linux RAID1 system will ride out any sort of sequence
    of hardware problems, and if the hardware is physically
    capable of running at all, the RAID1 system will pop
    right back like a cork coming out of white water.

    I've NEVER had a RAID1 throw a temper trantrum and go
    into apoptosis mode the way RAID5s do given the slightest

    We need RAID5 to be equally resilient in the face of
    real-world problems, people -- it isn't enough to
    just be able to function under ideal lab conditions!

    A design bug is -still- a bug, and -still- needs to
    get fixed.

    Something HAS to be done to make the RAID5 logic
    MUCH more conservative about destroying RAID5
    systems in response to a transient burst of I/O
    errors, before it can in good conscience be declared
    ready for production use -- or at MINIMUM to provide
    a SUPPORTED way of restoring a butchered RAID5 to
    last-known-good configuration or such once transient
    hardware issues have been resolved.

    There was a time when Unix filesystems disintegrated
    on the slightest excuse, requiring guru-level inode
    hand-editing to fix. fsck basically ended that,
    allowing any idiot to successfully maintain a unix
    filesystem in the face of real-life problems like
    power failures and kernel crashes. Maybe we need
    a mdfsck which can fix sick RAID5 subsystems?

    In the meantime, IMHO Linux RAID5 should be prominently flagged
    EXPERIMENTAL -- NONCRITICAL USE ONLY or some such, to avoid
    building up ill-will and undeserved distrust of Linux
    software quality generally.

    Pending some quantum leap in Linux RAID5 resistance to
    collapse, I'm switching to RAID1 everywhere: Doubling
    my diskspace hardware costs is a SMALL price to pay to
    avoid weeks of system downtime and rebuild effort annually.
    I like to spend my time writing open source, not
    rebuilding servers. :) (Yes, I could become an md
    maintainer myself. But only at the cost of defaulting
    on pre-existing open source commitments. We all have
    full plates.)

    Anyhow -- kudos to everyone involved: I've been using
    Unix since v7 on PDP-11, Irix since its 68020 days,
    and Linux since booting off floppy was mandatory, and
    in general I'm happy as a bug in a rug with the fleet
    of Debian Linux boxes I manage, with uptimes often exceeding
    a year, typically limited only by hardware or software
    upgrades -- great work all around, everyone!

    Life is Good!

    -- Cynbe

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