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Re: [Qemu-devel] [PATCH V4] qemu-img create: add 'nocow' option
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH V4] qemu-img create: add 'nocow' option |
Date: |
Mon, 30 Jun 2014 15:53:05 +0200 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Mon, Jun 30, 2014 at 02:29:58PM +0800, Chunyan Liu wrote:
> 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.
>
> Signed-off-by: Chunyan Liu <address@hidden>
> ---
> Changes to v3:
> * remove NOCOW option from .create_opts of those drivers calling
> bdrv_create_file to create file. Adding NOCOW to raw-posix.c is
> enough. No difference to users when using 'qemu-img create' interface.
> Make the patch cleaner.
>
> block/qed.c | 6 +++---
> block/raw-posix.c | 25 +++++++++++++++++++++++++
> block/vdi.c | 29 +++++++++++++++++++++++++++++
> block/vmdk.c | 6 +++---
> block/vpc.c | 29 +++++++++++++++++++++++++++++
> include/block/block_int.h | 1 +
> qemu-doc.texi | 16 ++++++++++++++++
> qemu-img.texi | 16 ++++++++++++++++
> 8 files changed, 122 insertions(+), 6 deletions(-)
Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block
Stefan
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