Re: [Qemu-devel] [PATCH v2 2/2] block/raw-posix: use seek_hole ahead of fiemap

[Eric Blake <eblake@redhat.com>]

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On 09/25/2014 05:14 PM, 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.

It also has the potential to be less precise than fiemap on older
kernels (any filesystem that supports fiemap but has not yet wired up
seek_hole will get the kernel's default fallback of claiming no holes at
all), but that problem will fade as more people use modern kernels.
This matches what coreutils does, so I like it on that grounds.

> 
> 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: Pádraig 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

Thanks; it is indeed better.

> 
>  block/raw-posix.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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