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Re: [libvirt] RFCv2: virDomainSnapshotCreateXML enhancements



On 08/23/2011 04:28 AM, Stefan Hajnoczi wrote:
I forgot to ask the obvious question:

I am writing a backup program that is using the new snapshot APIs.  A
snapshot has been created, how do I read out the data from the
snapshot?

Here's how to access the data in the snapshot, at least for the first round implementation of qcow2 snapshots:

If you created an internal snapshot (virDomainSnapshotCreateXML with no flags), then the only way right now to read data out is to shut down any qemu process (since qemu-img should not be used on a file in active use by qemu), then:
qemu-img convert [options] -s snapshot file backup
to extract the named internal snapshot from 'file' into a new file 'backup'.

If you created an external snapshot (virDomainSnapshotCreateXML with the new _DISK_ONLY flag), then the data from the snapshot is the old file name. That is, if you start with '/path/to/old', then create a snapshot with a target file of '/path/to/new', then /path/to/old _is_ the snapshot, and /path/to/new is a qcow2 file with /path/to/old as its backing file. The snapshot (old file) can safely be accessed even while qemu is still running.

As for how to access which blocks have changed in the delta since the snapshot, that is not yet exposed in libvirt, due to lack of support in qemu and qemu-img.

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


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