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    SubjectRe: [PATCH] 64 bit scsi read/write
    Daniel Phillips wrote:
    >
    > On Saturday 14 July 2001 16:50, Chris Wedgwood wrote:
    > > On Sat, Jul 14, 2001 at 09:45:44AM +0100, Alan Cox wrote:
    > >
    > > As far as I can tell none of them at least in the IDE world
    > >
    > > SCSI disk must, or at least some... if not, how to peopel like NetApp
    > > get these cool HA certifications?
    >
    > Atomic commit. The superblock, which references the updated version
    > of the filesystem, carries a sequence number and a checksum. It is
    > written to one of two alternating locations. On restart, both
    > locations are read and the highest numbered superblock with a correct
    > checksum is chosen as the new filesystem root.

    But this assumes that it is the most-recently-written sector/block
    which gets lost in a power failure.

    The disk will be reordering writes - so when it fails it may have
    written the commit block but *not* the data which that block is
    committing.

    You need a barrier or a full synchronous flush prior to writing
    the commit block. A `don't-reorder-past-me' barrier is very much
    preferable, of course.

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