[libvirt] [PATCH 6/6] storage:dir: adapts .refreshVol .refreshPool for ploop volumes

Olga Krishtal okrishtal at virtuozzo.com
Thu Feb 18 16:39:37 UTC 2016

On 18/02/16 18:17, Ján Tomko wrote:
> On Thu, Feb 18, 2016 at 05:04:16PM +0300, Maxim Nestratov wrote:
>> 18.02.2016 16:46, Ján Tomko пишет:
>>> On Wed, Feb 17, 2016 at 02:40:05PM +0300, Olga Krishtal wrote:
>>>> To update information about ploop volumes inside the a single pool we need
>>>> to be sure that it is the ploop directory and not some other directory.
>>>> Ploop volume directory obligatory contains root.hds - image file and disk
>>>> descriptor - DiskDescriptor.xml. If path to a volume is a path to some
>>>> directory, we check the existance of this files.
>>> With each ploop volume being a directory with a ploop disk image and the
>>> XML, I think they deserve a separate pool type.
> On second thought, the pool is still just a directory, it's the volumes
> that are different, so it does belong in the directory-based pools.
The main idea of ploop is to have an image file, use it as a block device,
and create and use a file system on that device. It is looks like loop 
However, we do need DiskDescriptor.xml store at the same directory
that root.hds (file, that contains ploop).

> That leaves the mixing of the ploop volumes with the ploop disk images,
> don't we need a new STORAGE_VOL type?
At the beginning I also had thoughts about it, but:
1) New STORAGE_VOL type will be exclusively for us. No one else will use it.
So, we have volume time only for one format.
2) We can avoid such situation, because our ploop format looks a bit 
like qcow format.
(Of course they have have different internal structure, but still, the 
only difference in such case -
DiskDescriptor.xml, that can not be stored in the same file)
3) And we are able to work with STORAGE_VOL_FILE, because we are a file 
upon loop device.

>>> The ploop image (root.hds) could be detected as such by the fs pool,
>>> but creating and deleting the directories feels out of place in this
>>> backend.
>> Actually one of the main intention of implementing ploop disk support in
>> storage pool was
>> ability to make it possible to work with existing storage pools like
>> NFS, DIR, FS.
> The 'image in a directory' still won't work with older libvirtd.
>> Creating a
>> separate  storage pool makes it impossible. Also, our future plans to
>> expand storage pools
>> with new pool type like VZ storage and CEPH FS becomes worthless either.
> Jan

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