[Libguestfs] Concurrent scanning of same disk

[Richard W.M. Jones]

On Thu, May 28, 2015 at 10:57:51AM +0300, NoxDaFox wrote:
> 2015-05-28 10:40 GMT+03:00 Richard W.M. Jones <rjones at redhat.com>:
> 
> > On Thu, May 28, 2015 at 10:33:48AM +0300, NoxDaFox wrote:
> > > To create the snapshots I'm using the libvirt command snapshotCreateXML
> > > with no flag set. Does libvirt support consistent snapshotting or shall I
> > > rely on QEMU backup new feature only?
> >
> > According to: http://wiki.libvirt.org/page/Snapshots
> > virDomainSnapshotCreateXML is only consistent if the guest is paused
> > during the operation.
> >
> > The new qemu feature is called drive-backup
> > (http://wiki.qemu.org/Features/IncrementalBackup).  Unless things
> > changed recently, it is not exposed through libvirt, so the only way
> > to use it is by sending qemu monitor commands
> > (
> > http://kashyapc.com/2013/03/31/multiple-ways-to-access-qemu-monitor-protocol-qmp/
> > ).
> >
> > This is all pretty bleeding edge.  I still think you'd be better off
> > just ignoring snapshots that fail and moving on to the next one.
> >
> > Rich.
> 
> 
> I might be missing something then as the guest is actually paused during
> the acquisition of the snapshot.
> 
> I pause the guest, take a screenshot, a core dump and a snapshot, then I
> resume the guest. Proof is that I can clearly analyse the memory core dump
> without any problem.

Note a core dump doesn't normally include the guest's disk.  It just
contains the guest's memory, so it's not relevant for consistency.

> Maybe I am breaking the disk's consistency once I extract the dump through
> the qemu-img command?
> 
> The command is:
> qemu-img convert -f qcow2 -o backing_file=guest_disk.qcow2 -O qcow2 -s
> snapshot_n guest_disk.qcow2 new_disk_for_libguestfs.qcow2

Is the guest paused when you do this?  If not, then this will create
an inconsistent snapshot.

> Could it be that, as the backing file is pointing to the guest's disk which
> will evolve in time, when guestfs tries to read the data sees
> incosistencies?

qemu-img convert makes a full copy, so it's not guestfs that's the
problem, but qemu-img.  The copy is not done instantaneously.

> The guest_disk.qcow2 is a COW clone of a base_disk.qcow2, what if I rebase
> the new_disk_for_libguestfs.qcow2 to the base_disk.qcow2?

Many copies and snapshots.  I'm thoroughly confused ...

Anyhow, unless you either make a full copy while the guest is paused,
*or* you use a point-in-time snapshot feature of either qemu
(drive-backup) or your host filesystem, you're not making a consistent
snapshot.

Rich.

-- 
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