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

[Chun Yan Liu]

>>> On 7/9/2014 at 07:25 PM, in message <53BD26A7.9090404 at redhat.com>, Ján
Tomko<jtomko at redhat.com> wrote: 
> 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'.

Thanks. Will add it.

- Chunyan

>  
> >  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 
>  
>