Re: [Qemu-devel] [PULL for-2.1 13/24] qemu-img create: add 'nocow' option

[Eric Blake <eblake@redhat.com>]

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On 07/01/2014 02:48 AM, Stefan Hajnoczi wrote:
> From: Chunyan Liu <cyliu@suse.com>
> 
> Add 'nocow' option so that users could have a chance to set NOCOW flag to
> newly created files. It's useful on btrfs file system to enhance performance.
> 
> Btrfs has low performance when hosting VM images, even more when the guest
> in those VM are also using btrfs as file system. One way to mitigate this bad
> performance is to turn off COW attributes on VM files. Generally, there are
> two ways to turn off NOCOW on btrfs: a) by mounting fs with nodatacow, then
> all newly created files will be NOCOW. b) per file. Add the NOCOW file
> attribute. It could only be done to empty or new files.
> 
> This patch tries the second way, according to the option, it could add NOCOW
> per file.
> 
> For most block drivers, since the create file step is in raw-posix.c, so we
> can do setting NOCOW flag ioctl in raw-posix.c only.
> 
> But there are some exceptions, like block/vpc.c and block/vdi.c, they are
> creating file by calling qemu_open directly. For them, do the same setting
> NOCOW flag ioctl work in them separately.
> 
> [Fixed up 082.out due to the new 'nocow' creation option
> --Stefan]
> 
> Signed-off-by: Chunyan Liu <cyliu@suse.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

I see this is already in qemu.git, so it will need a followup to fix the
user-visible typos (s/trun/turn/, s/Capitabl/Capital/)

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


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