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Date: Fri, 26 Sep 2014 19:04:47 +0200
From: Max Reitz <mreitz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 2/2] block/raw-posix: use seek_hole
 ahead of fiemap
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To: Tony Breeds <tony@bakeyournoodle.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, =?UTF-8?B?UMOhZHJhaWcgQnJhZHk=?= <pbrady@redhat.com>, Markus Armbruster <armbru@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

On 26.09.2014 01:14, Tony Breeds wrote:
> try_fiemap() uses FIEMAP_FLAG_SYNC which has a significant performance
> impact.
>
> Prefer seek_hole() over fiemap() to avoid this impact where possible.
> seek_hole is more widely used and, arguably, has potential to be
> optimised in the kernel.
>
> Reported-By: Michael Steffens <michael_steffens@posteo.de>
> Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Markus Armbruster <armbru@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Cc: Max Reitz <mreitz@redhat.com>
> Cc: P=C3=A1draig Brady <pbrady@redhat.com>
> Cc: Eric Blake <eblake@redhat.com>
> ---
> Changes since v1:
>   - split in to 2 patches
>   - tried to make the commit messages better

I used this order in at least one of the non-final versions of 4f11aa8a,=20
so I'm fine with it. The reason it got rejected was (as far as I=20
remember) that the FIEMAP ioctl() returns ENOTSUP if not supported,=20
whereas lseek() with SEEK_HOLE/SEEK_DATA returns some failsafe value,=20
probably reporting no holes at all. By reversing the order of=20
try_fiemap() and try_seek_hole(), try_fiemap() therefore may very well=20
become dead code. I don't object, however.

Reviewed-by: Max Reitz <mreitz@redhat.com>