[libvirt] [PATCH v4 0/5] Expose FSFreeze/FSThaw within the guest as API

[Eric Blake]

On 03/27/2014 04:48 PM, Jon Bernard wrote:

> 
> One thing comes to mind: once volumes are quiesced, qemu-img could be
> used directly to create the snapshot.  In the case of iSCSI-attached
> volumes, a remote snapshot command could be issued to create a snapshot.
> Once all have completed the guest then thawed.
> 
> For local volumes, using qemu-img seems cumbersome to me.  I feel there
> should be a libvirt call to handle at least the local volumes.

virDomainSnapshotCreateXML already handles a snapshot of a subset of
local volumes via qemu.  The reason we are adding FSFreeze/FSThaw is to
cater to situations where creating the snapshot via qemu is not feasible.

But be aware that you generally must NOT use qemu-img on a disk that is
in-use by qemu, as qemu may have pending changes to the image metadata
that would cause qemu-img to misbehave.  If a guest is live (even if
paused or quiesced), the only safe things you can do to that guest are
to backing files (which are read-only to qemu) or actions supported
directly by qemu (and yes, we still need qemu to have a way to pivot to
a new disk file if you have a way of doing a fast snapshot while the
disk is quiesced but in a way different than how qemu does snapshots
with qcow2 files).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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