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

Jd jd_jedi at convirture.com
Wed Oct 15 20:56:20 UTC 2014

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 ?


More information about the libvirt-users mailing list