Re: [Qemu-devel] "File too large" error from "qemu-img snapshot" (was Re: AW: Bug Repoting Directions Request)

[Eric Blake]

On 11/19/2014 10:32 AM, Prof. Dr. Michael Schefczyk wrote:
> Dear Eric, dear all,
> 
> Again, thank you very much. I now gather that I took the wrong path towards 
> nightly backups of running VM. I remain surprised that I did work for a 
> relatively long time.

[can you convince your mailer to wrap long lines?  also, we tend to
frown on top-posting on technical lists]

Yeah, you were just getting lucky that two different processes weren't
both trying to allocate a cluster for different purposes at the same
time.  When the collision finally did happen, it had catastrophic
results on your image.

> 
> A major book on KVM in German language by Kofler & Spenneberg recommends the 
> following approach for online backups (with and without "--disk-only"):
> 
> virsh snapshot-create-as vm XXX --disk-only
> qemu-img convert -f qcow2 -s XXX -O qcow2 XXX.img /backup/YYY.img
> virsh snapshot-delete vm XXX

Yes, virsh is using QMP commands under the hood, so this method is safe.
 One slight issue is that this sequence is incomplete (it does not
shrink the backing file chain after the copy is complete), so if you
keep repeating it, you will eventually cause reduced performance when
you have a really long chain of multiple qcow2 overlays, or even cause
qemu to run into fd exhaustion.  Also, this command does not show that
unless you clean things up, then the second time you run this you do not
want to copy XXX.img into backup, but instead the qcow2 wrapper file
that was created by the first snapshot (and which itself wrapped by the
second snapshot).

With new enough libvirt and qemu, you can shrink the chain back down
with an active commit, as in:

virsh blockcommit vm XXX --verbose --active --pivot

Also, the use of --disk-only means that your disks have a snapshot taken
as if at a point in time when you yank the power cord; reverting to such
a backup may require fsck, and may suffer from other problems from
anything that was depending on I/O that had not yet been flushed to
disk.  If you add the --quiesce option (which implies that you set up
your guest to use qemu-guest-agent, and told libvirt to manage the
agent), then you can guarantee that your guest has flushed and frozen
all filesystems prior to the point in time where the snapshot is
created; and you can even install hooks in the guest to extend that
stability to things like databases.  Then your backups are much easier
to use.  If you omit --disk-only, and take a full snapshot, then you
have the guest memory state that is necessary to restore all pending
I/O, and don't need to worry about freezing the guest file systems; but
then you have to remember to back up the memory state in addition to
your disk state.

> 
> Would this be any better than my script, because it uses virsh 
> snapshot-create-as instead of qemu-img snapshot? The second command is still 
> qemu-img convert which may be problematical.

No, remember what I said.  qemu-img may not be used on any image that is
opened read-write by qemu, but is perfectly safe to do read-only
operations on any image that is opened read-only by qemu.  That sequence
of commands goes from the initial:

disk.img [read-write]

then the snapshot-create command causes libvirt to issue a QMP command
to switch qemu to:

disk.img [read-only] <- overlay.qcow2 [read-write]

at which point you can do anything read-only to disk.img (whether
'qemu-img convert' or 'cp' or any other command that doesn't alter the
contents of the file)

finally, the 'virsh blockcommit' command would take you back to:

disk.img [read-write]

> 
> The problem I am facing is that the documentation is not easy to understand 
> for the average user/administrator who is not among the kvm developers and 
> experts. I have of course tried to read section 14.2.3 of RHEL 7 
> Virtualization Deployment and Administration Guide on backups, but I found 
> that too cryptic for someone like myself to draw practical consequences from 
> it.

If you are using libvirt to manage your guests, then yes, the qemu
documentation is cryptic, but that shouldn't matter - you should be
asking the libvirt community how to accomplish a job.  And yes, libvirt
could probably do a better job of advertising and documenting its level
of support for live backups, but that is more a question for the
address@hidden mailing list (the archive of that list show
that questions like this come up very regularly; you are not the first
to ask about live backups, so perusing those archives may give you some
ideas on what works).

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

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