[libvirt] Re: libvirk KVM save (migrate) terribly slow

Daniel P. Berrange berrange at redhat.com
Thu Dec 10 11:44:34 UTC 2009

On Thu, Dec 10, 2009 at 08:05:12AM +0100, Nikola Ciprich wrote:
> Hi,
> I noticed that using libvirt to save running domain is terribly slow,
> I mean it can take even 10 minutes to save VM with 2GB RAM, although
> the host is almost idle (8CPU, 16GB RAM). If I understand correctly, 
> libvirt first pauses the vm and then migrates it to file (optionally through gzip).
> Restoring it back to running state is almost instant.
> I guess this is not what should be expected, is there something particular 
> I should check?

This matches behaviour that I see when saving VMs. QEMU takes a really
very long time to save itself using migrate exec:.  It is not CPU limited
even in gzip mode - there is near zero CPU usage while this is going on.
QEMU just seems to be very slow at sending the data. I've not had time to
investigate whether this is a flaw in QEMU's exec: migration handling,
or whether it is being hurt by too small pipe buffers, or something else
entirely.  I must say I'm not entirely convinced that it is a good idea
for QEMU to be mixing use of FILE * / popen, with non-blocking I/O, but
that could be a red-herring.

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