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Re: [libvirt] block pull/commit for non-local storage



On Thu, Feb 20, 2014 at 02:46:03PM +0100, Peter Krempa wrote:
> As you can see in the prototypes of these functions (and from the docs
> for them which I'm not going to copy here) the user can provide the disk
> specification in two possible options:
> 
> 1) a full path if it's unambiguous - this is file centric and requires
> the file to be in the local filesystem
> 
> 2) a disk name "vda" - this is selected automatically by libvirt but
> allows to specify only the top image.
> 
> For systems that want to use remote storage without local representation
> such as gluster+libgfapi, this doesn't allow to use the APIs to start
> block jobs.
> 
> To solve this issue we need a way to specify paths on remote storage in
> some way. Below are two options we've discussed on IRC.
> 
> 1) Use URIs along with the file path to specify disk images.
> 
> This option would add a new, possibly well documented URIs to specify
> paths for disk images. These would be libvirt defined URIs (but
> surprisingly "similar" to qemu URIS) so that hypervisors with different
> storage specification would need a conversion.
> 
> This would allow to specify the targets as:
> vda - disk name
> /path/to/file - legacy way, path
> file:///path/to/file - new way of file paths
> block:///dev/blah - new way, block devs
> gluster://server/vol/img - new way, remote images
> ...
> 
> Possible caveats: RBD for example allows to use multiple hosts and we'd
> need to introduce a possibility to specify it if we'd add support for
> this on rbd.

Superficially this is appealing, however, what I dislike about it
is that it has no correspondance to the way you specify the targets
in the XML file. So apps would need to know two different formats
for dealing with network disks. The RBD multiple hosts issues is
another concern.

> 2) Export the image chain in the XML and allow to use indexed disk names
> This option would require to export the backing chain in the XML in some
> way, either the existing disk source specification in multiple elements
> (which I don't like as it is a bit convoluted), or possibly again via URIs.
> 
> Then the user would be allowed to specify vda[2] for the second backing
> image of the vda disk.
> 
> With this the internal representations of the backing chain would be
> used without the need for the user to specify path.

To me this is more appealing because of its simplicity. I think I would
rather like us to expose the backing store info explicitly in the XML
if we go this route, so that the index values are explicitly visible to
apps using the XML.

> A possible caveat here is that if backing chains for some reason will be
> converted to backing trees, this approach will be invalid.

One possibility if we ended up with forking trees would be to
have multiple indexes  vda[1][2]. This gets a little more tedious
to deal with though.

Even if we have a tree though, the backing files will have to be
exposed in some order in the XML file. eg via a depth first sort.
Once we expose the files in the XML in this way, we do still have
a single index value we can use - the value resulting from the
depth first sort.

> 3) ? anyone suggesting something better? :)
> 
> Thanks in advance for suggestions and/or new ideas.

The only other option I'd have would be to actually use the
XML snippet of the  <source> element. Again this would require
us to expose XML for all backing files. I think this would be
a bit more ugly and not so user friendly compared to 2).

Daniel
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