[libvirt-users] drive-backup command permission denied.. and need some clarification

[Kashyap Chamarthy]

On Wed, Oct 15, 2014 at 01:56:20PM -0700, Jd wrote:
> On 10/15/14, 2:37 AM, Kashyap Chamarthy wrote:
> >On Tue, Oct 14, 2014 at 02:08:41PM -0600, Eric Blake wrote:
> >>On 10/14/2014 11:52 AM, Jd wrote:
> >>>Hi
> >>>
> >>>     * Trying to get drive-backup command, getting permission denied. :(
> >>>
> >>>        sudo virsh qemu-monitor-command --hmp my-instance --cmd
> >>>drive_backup drive-virtio-disk0 /tmp/foo.vda.img
> >>Ouch. qemu-monitor-command is explicitly unsupported, precisely because
> >>it goes behind libvirt's back and is likely to get libvirt confused.
> >>Most likely, the reason permission is denied is that you are failing to
> >>set the sVirt permissions of the file you are trying to use (this
> >>includes setting all of SELinux/apparmor labels, the disk lease manager,
> >>and cgroup ACLs).  You really are better off experimenting on raw qemu
> >>without libvirt, or else waiting for (or helping to patch) libvirt to
> >>drive the command directly, as trying to issue the raw monitor command
> >>while libvirt is still managing the domain is a recipe for disaster, as
> >>you just found.
> >>
> >>>        Looks like apparmor issue. What can I modify to make this work ?
> >>>
> >>>      * Couple of other questions
> >>>        drive-backup :
> >>>           * The doc seems to claim that it gives a point in time copy of
> >>>the drive. So I assume that no need to take any snapshot etc.. and merge
> >>>back in.
> >>My understanding of the qemu command is that given:
> >>
> >>[base] <- [active]
> >>
> >>calling drive-backup will create:
> >>
> >>[base] <- [snapshot](frozen at point of command)
> >>         \- [active](still modified by guest)
> >>
> >>
> >>>           * does it internally use snapshot ? Does this hook in to doing
> >>>fsfreeze and unfreeze using guest agent and does it automagically ?  I
> >>>do not see any options here.
> >>I have not yet had time to look into wiring up libvirt to drive the
> >>command; the libvirt solution will probably have the optional ability to
> >>quiesce the file system around the snapshot event, similar to the
> >>existing --quiesce flag of virDomainSnapshotCreateXML (in fact,
> >>virDomainSnaphostCreateXML might even BE the interface we use to wire up
> >>the qemu drive-backup command)
> >>
> >>>           * Suppose I have base <-- sn1 --<-- sn2 (QEMU active) .  does
> >>>it take data from sn2 only ? or base+sn1+sn2 .. full drive  and creates
> >>>a new qcow2 sparse file.
> >>If I understand the qemu command correctly, you have three choices via
> >>the 'sync' option: the entire disk (the snapshot is a flat image
> >>containing contents of base+sn1+sn2 with no backing file), a shallow
> >>copy (the snapshot is a qcow2 file containing contents of sn2 with
> >>backing file of sn1), or all new I/O (the snapshot file is populated
> >>only when additional writes occur to sn2; the more writes into the
> >>snapshot, the more sn2 has diverged from the point in time you created
> >>the snapshot; which might be more useful once persistent dirty bitmap
> >>tracking is added to qemu).  You may get better answers to questions
> >>like this on the qemu list, since libvirt can't drive it yet.
> >>
> >As an addendum, here's a small QMP example to test QMP 'drive-backup'
> >command:
> >
> >-----------------------------------------------------------------------
> >#!/bin/bash
> >
> >exec 3<>/dev/tcp/localhost/4444
> >echo -e "{ 'execute': 'qmp_capabilities' }" >&3
> >read response <&3
> >echo $response
> >echo -e "{ 'execute': 'drive-backup', 'arguments':
> >       { 'device': 'drive-ide0-0-0', 'sync': 'full', 'target':
> >       '/var/lib/libvirt/images/backup-copy.qcow2', 'mode': 'absolute-paths', 'format': 'qcow2' } }" >&3
> >read response <&3
> >echo $response
> >echo -e "{execute: 'query-block-jobs'}" >&3
> >-----------------------------------------------------------------------
> >
> >Of course, the above assumes a QEMU instance is running with QMP server:
> >
> >     $ qemu-system-x86_64. . . -qmp tcp:localhost:4444,server
> >
> 
> Thank you. This might come handy.  But I guess this can still throw libvirt
> off ? right ?

By "throw libvirt off" if you mean this: as the VM is running QEMU CLI
(with QMP server) behind the back of libvirt, assuming this same VM
was being managed by libvirt until that point -- in that sense, yes,
what you say is probably correct.

-- 
/kashyap