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Re: [libvirt] RFCv2: virDomainSnapshotCreateXML enhancements

On 08/23/2011 09:12 AM, Stefan Hajnoczi wrote:
Hmm...so there is no abstraction.  I still need to understand what the
underlying snapshot implementation does and what its limitations are.

We're still trying to get to that point. A while ago, I posted an RFC for adding virStorageVolSnapshot* APIs, which would be the ideal place to expose storage-volume independent wrappers around snapshot management. The idea is still that we can use APIs like that (instead of low-level access to the snapshot image file) to stream snapshot data from a remote host back to the client. We also need a new API that lets you quickly access all of the storage volumes associated with a domain (my patch to add 'virsh domblklist' is a start at getting at all the storage volume names, but not quite as good as getting the actual virStorageVolPtr objects).

This is what I meant when I asked about aiming for something more
high-level where the user doesn't need to be an expert in snapshots to
use this API.  In order to access the snapshot I need to use an
out-of-band (ssh?) mechanism to get to the libvirt host and know how
to access the external snapshot image file.  If that image file is
using an image format then I need to use libguestfs, qemu-io/qemu-img,
or custom code to access the format.

For now, yes. But hopefully the work I'm doing is providing enough framework to later add the additions that can indeed expose libvirt APIs rather than low-level tool knowledge to get at the snapshots.

I think we simply cannot expose all this complexity to users.  Each
application would have to support the many different cases.  Libvirt
needs to tie this stuff together and present an interface that
applications can use without worrying how to actually get at the
snapshot data.

I don't see any problem with exposing the lower layers as a start, then adding higher layers as we go. There are different classes of users, and both layers are useful in the right context. But at the same time, I agree with you that what I have done so far is just a start, and by no means the end of snapshot-related libvirt work.

Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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