Re: [Qemu-devel] external snapshots freezes block device since qemu 2.8

[Kashyap Chamarthy <kchamart@redhat.com>]

On Thu, Mar 09, 2017 at 11:11:09AM +0100, Piotr Rybicki wrote:
> Hello there.
> 
> I discovered, that since qemu 2.8 , external snapshots (very similar to:
> http://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit),

When I wrote this Wiki page, I tested it only with regular QCOW2 files
on EXT4 file system.

> freezes block device after:
> 
> # virsh blockcommit (...)
> 
> There is no error message after completion of the command above.
> I'm using gluster + ZFS fuse mount as a storage for a VM on gentoo.
> libvirt debug showed nothing helpful.
> With qemu <2.8 -: snapshots are working as expected.
> 
> Is this a known issue?
> Is it qemu or libvirt ?
> 
> If it helps, how can i diagnose this further?a

To see what libvirt is sending to QEMU, you can enable the debug log
filters, which can give some clue:

- In /etc/libvirt/libvirtd.conf, set these config attributes

    log_filters="1:libvirt 1:qemu 1:conf 1:security 3:event 3:json 3:file 3:object 1:util"
    log_outputs="1:file:/var/log/libvirt/libvirtd.log"

- Restart libvirtd:

    $ systemctl restart libvirtd

- Perform your `blockcommit` test.

PS: A gentle reminder -- when reporting upstream, you'll likely better
help if you at least test with the latest releases (libvirt-3.1.0, and
QEMU 2.8), if not from Git.

[...]

-- 
/kashyap