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Re: [PATCH] xfs: di_flushiter considered harmful

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: di_flushiter considered harmful
From: Ben Myers <bpm@xxxxxxx>
Date: Wed, 24 Jul 2013 12:16:47 -0500
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1374644850-5512-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1374644850-5512-1-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Wed, Jul 24, 2013 at 03:47:30PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> When we made all inode updates transactional, we no longer needed
> the log recovery detection for inodes being newer on disk than the
> transaction being replayed - it was redundant as replay of the log
> would always result in the latest version of the inode woul dbe on
> disk. It was redundant, but left in place because it wasn't
> considered to be a problem.
> However, with the new "don't read inodes on create" optimisation,
> flushiter has come back to bite us. Essentially, the optimisation
> made always initialises flushiter to zero in the create transaction,
> and so if we then crash and run recovery and the inode already on
> disk has a non-zero flushiter it will skip recovery of that inode.
> As a result, log recovery does the wrong thing and we end up with a
> corrupt filesystem.
> Because we have to support old kernel to new kernl upgrades, we
> can't just get rid of the flushiter support in log recovery as we
> might be upgrading from a kernel that doesn't have fully transaction
> inode updates.  Unfortunately, for v4 superblocks there is no way to
> guarantee that log recovery knows about this fact.
> We cannot add a new inode format flag to say it's a "special inode
> create" because it won't be understood by older kernels and so
> recovery could do the wrong thing on downgrade. We cannot specially
> detect the combination of zero mode/non-zero flushiter on disk to
> non-zero mode, zero flushiter in the log item during recovery
> because wrapping of the flushiter can result in false detection.
> Hence that makes this "don't use flushiter" optimisation limited to
> a disk format that guarantees that we don't need it. And that means
> the only fix here is to limit the "no read IO on create"
> optimisation to version 5 superblocks....
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>


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