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Re: [libvirt] [PATCH] storage: new backend: btrfs subvolumes



On Mon, Sep 02, 2013 at 02:48:12PM +0300, Oskari Saarenmaa wrote:
> This commit adds a new storage pool driver, btrfs, which can be used to
> manage btrfs subvolumes on an existing btrfs filesystem.  The driver can
> create new blank subvolumes and snapshots of existing subvolumes as well
> as delete existing subvolumes.
> 
> The subvolumes created are automatically made visible on the host side
> and can be attached to domains using the <filesystem> tags as defined in
> 'format domain' documentation.
> 
> Libvirt volumes do not implement quotas at the moment because the current
> (btrfs-progs-0.20.rc1.20130501git7854c8b-4.fc20.x86_64) support for quota
> management in btrfs-progs is lacking the necessary features, for example
> it's not possible to see the quota assigned to a certain subvolume and
> usage information is only updated on syncfs(2).  Quota support will be
> implemented once the tools gain the necessary features.
> 
> Signed-off-by: Oskari Saarenmaa <os ohmu fi>

You're going to hate me, but having played around with this, I don't
believe it should be a separate pool driver. I think it should be
part of the 'storage_fs.c' driver. The way I see it, a btrfs subvolume
is just a special kind of volume in a directory.

eg given a directory located on btrnfs  /foo/bar it is perfectly
valid to want to support

   /foo/bar/somedir         (plain directory)
   /foo/bar/voldir          (subvolume directory)
   /foo/var/somefile.qcow2  (qcow2 file)

etc.

I'd suggest that we should make it possible to create directories
in the FS storage pool, but supporting use of a new attribute

 type='file|network|dir'

on the top level <volume> element.

Then distinguish between plain directories, and those backed by
a subvolme using the <format> element,

So, eg to create a btrfs subvolume use


<volume type='dir'>
  <name>wizz</name>
  <key>/mnt/btr/wizz</key>
  <source>
  </source>
  <capacity unit='bytes'>0</capacity>
  <allocation unit='bytes'>0</allocation>
  <target>
    <path>/mnt/btr/wizz</path>
    <format type='volume'/>
  </target>
</volume>


This is a more generic approach that will work if any other FS grow
support for subvolumes like btrfs has.

Regards,
Daniel
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