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Re: [PATCH] [RFC] xfs_fsr: Improve handling of attribute forks

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] [RFC] xfs_fsr: Improve handling of attribute forks
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Sat, 6 Mar 2010 05:43:57 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <1267764049-30650-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1267764049-30650-1-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.19 (2009-01-05)
On Fri, Mar 05, 2010 at 03:40:49PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> If the file being defragmented has attributes, then fsr puts a dummy
> attribute on the temporary file to try to ensure that the inode
> attribute fork offset is set correctly. This works perfectly well
> for the old style of attributes that use a fixed fork offset - the
> presence of any attribute of any size or shape will result in fsr
> doing the correct thing.
> 
> However, for attr2 filesystems, the attribute fork offset is
> dependent on the size and shape of both the data and attribute
> forks. Hence setting a small attribute on the file does not
> guarantee that the two inodes have the same fork offset and
> therefore compatible for a data fork swap.
> 
> This patch improves the attribute fork handling of fsr. It checks
> the filesystem version to see if the old style attributes are in
> use, and if so uses the current method.
> 
> If attr2 is in use, fsr uses bulkstat output to determine what the
> fork offset is. If the attribute fork offsets differ then fsr will
> try to create attributes that will result in the correct offset. If
> that fails, or the attribute fork is too large, it will give up and just
> attempt the swap.
> 
> This fork offset value in bulkstat new functionality in the kernel,
> so if there are attributes and a zero fork offset, then the kernel
> does not support this feature and we simply fall back to the existing,
> less effective code.

Looks reasonable.  It would be good to have a testcase for this in
xfsqa to verify this works.

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