[Bug 712392] Re: qemu-img convert blocks other tasks

[Alvin]

I still haven't had the chance to test this properly. (server is in
production), but the problem manifested itself by accident. So, here's a
little bit of information.

I started downloading (rsync) a qcow image from the file server and
noticed it was bit slow. 14MB/sec max - 790kB/s after a while. This is
writing to a simple single sata disk. Then the virtual machines started
to become unresponsive and  I had to interrupt the download, or the
server would have gone down. It's not memory pressure. Memory was at +/-
900MB (of 8GB). Only 2 small virtual machines were running, BUT there
was a snapshot present.

It's not qemu-img that causes the panics. Any I/O will do it. Then the famous messages start appearing in kern.log:
[149164.740056] INFO: task kvm:21354 blocked for more than 120 seconds.
[149164.740294] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[...]
[149164.740595]  [<ffffffff8155d557>] __mutex_lock_slowpath+0x107/0x190

Restarting the download without lvm snapshot present yields:
- 11MB/s, but more or less constant.
- Running virtual machines are responsive as ever
- No errors in kern.log

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https://bugs.launchpad.net/bugs/712392

Title:
  qemu-img convert blocks other tasks