[linux-lvm] Re: [PATCH] 64 bit scsi read/write

[Andrew Morton]

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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