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Re: [PATCH 14/14] xfs: only update the last_sync_lsn when a transaction

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 14/14] xfs: only update the last_sync_lsn when a transaction completes
From: Ben Myers <bpm@xxxxxxx>
Date: Wed, 10 Oct 2012 09:28:41 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <1349693772-8064-15-git-send-email-david@xxxxxxxxxxxxx>
References: <1349693772-8064-1-git-send-email-david@xxxxxxxxxxxxx> <1349693772-8064-15-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
Hey Dave,

On Mon, Oct 08, 2012 at 09:56:12PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> The log write code stamps each iclog with the current tail LSN in
> the iclog header so that recovery knows where to find the tail of
> thelog once it has found the head. Normally this is taken from the
> first item on the AIL - the log item that corresponds to the oldest
> active item in the log.
> 
> The problem is that when the AIL is empty, the tail lsn is dervied
> from the the l_last_sync_lsn, which is the LSN of the last iclog to
> be written to the log. In most cases this doesn't happen, because
> the AIL is rarely empty on an active filesystem. However, when it
> does, it opens up an interesting case when the transaction being
> committed to the iclog spans multiple iclogs.
> 
> That is, the first iclog is stamped with the l_last_sync_lsn, and IO
> is issued. Then the next iclog is setup, the changes copied into the
> iclog (takes some time), and then the l_last_sync_lsn is stamped
> into the header and IO is issued. This is still the same
> transaction, so the tail lsn of both iclogs must be the same for log
> recovery to find the entire transaction to be able to replay it.
> 
> The problem arises in that the iclog buffer IO completion updates
> the l_last_sync_lsn with it's own LSN. Therefore, If the first iclog
> completes it's IO before the second iclog is filled and has the tail
> lsn stamped in it, it will stamp the LSN of the first iclog into
> it's tail lsn field. If the system fails at this point, log recovery
> will not see a complete transaction, so the transaction will no be
> replayed.
> 
> The fix is simple - the l_last_sync_lsn is updated when a iclog
> buffer IO completes, and this is incorrect. The l_last_sync_lsn
> shoul dbe updated when a transaction is completed by a iclog buffer
> IO. That is, only iclog buffers that have transaction commit
> callbacks attached to them should update the l_last_sync_lsn. This
> means that the last_sync_lsn will only move forward when a commit
> record it written, not in the middle of a large transaction that is
> rolling through multiple iclog buffers.
> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Reviewed-by: Mark Tinguely <tinguely@xxxxxxx>

I think this one is appropriate for the 3.7 release.  What say you?

-Ben

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