Messages in this thread | | | Date | Sun, 15 Jul 2001 11:21:04 +1000 | From | Andrew Morton <> | Subject | Re: [PATCH] 64 bit scsi read/write |
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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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