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

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