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Date: Thu, 15 May 2014 01:13:12 +0200
From: Max Reitz <mreitz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2] block/raw-posix: Try both FIEMAP and
 SEEK_HOLE
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To: Christoph Hellwig <hch@lst.de>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

On 11.05.2014 19:26, Christoph Hellwig wrote:
> On Tue, May 06, 2014 at 09:00:54PM +0200, Max Reitz wrote:
>> The current version of raw-posix always uses ioctl(FS_IOC_FIEMAP) if
>> FIEMAP is available; lseek with SEEK_HOLE/SEEK_DATA are not even
>> compiled in in this case. However, there may be implementations which
>> support the latter but not the former (e.g., NFSv4.2) as well as vice
>> versa.
>>
>> To cover both cases, always try SEEK_HOLE/SEEK_DATA (as this will
>> probably be covered by POSIX soon) and if that does not work, fall back
>> to FIEMAP; and if that does not work either, treat everything as
>> allocated.
> Btw, while I think that SEEK_HOLE/SEEK_DATA generally is the better API
> for qemu, the NFS 4.2 SEEK operation will be sufficient for a proper FIEMAP
> implementation, and we'll implement it for the Linux NFS client.

Hm, great, in that case this patch will probably never be put to the 
test. *g*

Max