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Re: [PATCH 3/3 V3] xfs: allow logical-sector sized O_DIRECT

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: [PATCH 3/3 V3] xfs: allow logical-sector sized O_DIRECT
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 23 Jan 2014 10:14:39 +1100
Cc: Eric Sandeen <sandeen@xxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <52DEF8BF.5070106@xxxxxxxxxxx>
References: <52DEF81D.4090100@xxxxxxxxxx> <52DEF8BF.5070106@xxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Jan 21, 2014 at 04:46:23PM -0600, Eric Sandeen wrote:
> Some time ago, mkfs.xfs started picking the storage physical
> sector size as the default filesystem "sector size" in order
> to avoid RMW costs incurred by doing IOs at logical sector
> size alignments.
> However, this means that for a filesystem made with i.e.
> a 4k sector size on an "advanced format" 4k/512 disk,
> 512-byte direct IOs are no longer allowed.  This means
> that XFS has essentially turned this AF drive into a hard
> 4K device, from the filesystem on up.
> XFS's mkfs-specified "sector size" is really just controlling
> the minimum size & alignment of filesystem metadata.
> There is no real need to tightly couple XFS's minimal
> metadata size to the minimum allowed direct IO size;
> XFS can continue doing metadata in optimal sizes, but
> still allow smaller DIOs for apps which issue them,
> for whatever reason.
> This patch adds a new field to the xfs_buftarg, so that
> we now track 2 sizes:
>  1) The metadata sector size, which is the minimum unit and
>     alignment of IO which will be performed by metadata operations.
>  2) The device logical sector size
> The first is used internally by the file system for metadata
> alignment and IOs.
> The second is used for the minimum allowed direct IO alignment.
> This has passed xfstests on filesystems made with 4k sectors,
> including when run under the patch I sent to ignore
> XFS_IOC_DIOINFO, and issue 512 DIOs anyway.  I also directly
> tested end of block behavior on preallocated, sparse, and
> existing files when we do a 512 IO into a 4k file on a 
> 4k-sector filesystem, to be sure there were no unexpected
> behaviors.
> Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>

Looks good. Nice work with the comments.

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>

Dave Chinner

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