Re: [Qemu-devel] [libvirt-users] Adjust disk image migration (NBD)

[Stefan Hajnoczi]

On Fri, Feb 14, 2014 at 11:58:56AM +0100, Michal Privoznik wrote:
> On 14.02.2014 10:40, Joaquim Barrera wrote:
> > Hi all,
> > 
> > As I am doing some tests with qemu, I realized that the way it does 
> > 'migrate -i tcp:DEST:444' is not the same as 'libvirt migrate 
> > --copy-storage-inc'. Basically qemu uses the same stream as RAM 
> > migration and libvirt takes advantage of NBD transfer.
> > 
> > With virsh migrate-setspeed I observed that one can only control the 
> > transfer throughput of RAM, but not disk synchronization. At least this 
> > is what I can see in bmon when doing a migration with incremental copy.
> > 
> > The question is: Am I missing something or it is not implemented?
> > 
> > Thank you guys!
> > 
> 
> I think this is actually a qemu bug. Libvirt passes the correct values:
> 
> 
> 2014-02-14 10:52:08.010+0000: 27701: debug : qemuMonitorIOWrite:504 : 
> QEMU_MONITOR_IO_WRITE: mon=0x7f06cc00ea20 
> buf={"execute":"drive-mirror","arguments":{"device":"drive-virtio-disk0","target":"nbd:masina:49153:exportname=drive-virtio-disk0","speed":1048576,"sync":"full","mode":"existing"},"id":"libvirt-15"}
> 
> 
> ...
> 
> 2014-02-14 10:53:51.169+0000: 27701: debug : qemuMonitorIOWrite:504 : 
> QEMU_MONITOR_IO_WRITE: mon=0x7f06cc00ea20 
> buf={"execute":"migrate_set_speed","arguments":{"value":1048576},"id":"libvirt-221"}
> 
> 
> 2014-02-14 10:53:51.204+0000: 27701: debug : qemuMonitorIOWrite:504 : 
> QEMU_MONITOR_IO_WRITE: mon=0x7f06cc00ea20 
> buf={"execute":"migrate","arguments":{"detach":true,"blk":false,"inc":false,"uri":"fd:migrate"},"id":"libvirt-223"}
> 
> 
> However I observe what you do - disk migration is not shaped, while internal 
> state is.

Thanks for raising this.

I noticed that mirror_run() does not throttle the first loop where it
populates the dirty bitmap using bdrv_is_allocated_above().  The main
copy loop does take the speed limit into account but perhaps that's
broken too.

Paolo, Jeff: Any ideas?

Stefan