[libvirt] [PATCH V2] storagevol: add nocow to vol xml

[Ján Tomko]

On 07/08/2014 08:47 AM, Chunyan Liu wrote:
> Add 'nocow' to storage volume xml so that user can have an option
> to set NOCOW flag to the newly created volume. 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,

s/turn off NOCOW/turn off COW/

> 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 'nocow' option, it could set
> NOCOW flag per file:
> for raw file images, handle 'nocow' in libvirt code; for non-raw file images,
> pass 'nocow=on' option to qemu-img, and let qemu-img to handle that (requires
> qemu-img version >= 2.1).
> 
> Signed-off-by: Chunyan Liu <cyliu at suse.com>
> ---
> Changes:
>   - now qemu-img can handle 'nocow=on' option, just pass 'nocow=on' to
>     qemu-img for non-raw file images. No need to handle all file type
>     in libvirt code.
> 
> Pervious version is here:
>   http://www.redhat.com/archives/libvir-list/2013-December/msg01257.html
> 
> ---
>  docs/formatstorage.html.in    |  7 +++++++
>  docs/schemas/storagevol.rng   |  5 +++++

Adding a test case to storagevolxml2argvtest would be nice. Also, by adding
the volume XML to storagevolxml2xmlin for this test, it will get validated
against the rng schema in 'storagevolschematest'.

>  src/conf/storage_conf.c       |  3 +++
>  src/storage/storage_backend.c | 22 ++++++++++++++++++++++
>  src/util/virstoragefile.h     |  1 +
>  5 files changed, 38 insertions(+)
> 
> diff --git a/docs/formatstorage.html.in b/docs/formatstorage.html.in
> index 1cd82b4..e8862bf 100644
> --- a/docs/formatstorage.html.in
> +++ b/docs/formatstorage.html.in
> @@ -385,6 +385,7 @@
>              <label>virt_image_t</label>
>            </permissions>
>            <compat>1.1</compat>
> +          <nocow/>
>            <features>
>              <lazy_refcounts/>
>            </features>
> @@ -424,6 +425,12 @@
>          1.1 is used. If omitted, qemu-img default is used.
>          <span class="since">Since 1.1.0</span>
>        </dd>
> +      <dt><code>nocow</code></dt>
> +      <dd>Turn off COW of the newly created volume. So far, this is only valid
> +        to a file image in btrfs file system. It will improve performance when

s/to a file/for a file/

> +        the file image is used in VM. To create non-raw file images, it
> +        requires QEMU version since 2.1. <span class="since">Since 1.2.6</span>

1.2.7

Otherwise looks good to me.

Jan

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