Re: bad virsh save /dev/null performance (600 MiB/s max)

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Wed, Mar 09, 2022 at 07:27:12PM +0100, Claudio Fontana wrote:
> > 
> > One difference I could see looking at the qmp commands issued by libvirt in the "virsh save" case,
> > is "detach:true" in the migration command (which seems to have no effect in qemu),
> 
> That is a bug in libvirt - it should not be setting that in QMP.
> 
> To quote the QAPI spec for 'migrate'
> 
>   # @detach: this argument exists only for compatibility reasons and
>   #          is ignored by QEMU
> 
> 
> > 
> > 
> > and maybe more interestingly this stuff about the "fd":
> > 
> > 
> > 2022-03-09 17:29:34.247+0000: 20390: info : qemuMonitorSend:995 : QEMU_MONITOR_SEND_MSG: mon=0x7faa9003ebf0 msg={"execute":"getfd","arguments":{"fdname":"migrate"},"id":"libvirt-390"}^M
> >  fd=34
> > 2022-03-09 17:29:34.247+0000: 20387: info : qemuMonitorIOWrite:452 : QEMU_MONITOR_IO_WRITE: mon=0x7faa9003ebf0 buf={"execute":"getfd","arguments":{"fdname":"migrate"},"id":"libvirt-390"}^M
> >  len=73 ret=73 errno=0
> > 2022-03-09 17:29:34.247+0000: 20387: info : qemuMonitorIOWrite:457 : QEMU_MONITOR_IO_SEND_FD: mon=0x7faa9003ebf0 fd=34 ret=73 errno=0
> > 2022-03-09 17:29:34.248+0000: 20387: info : qemuMonitorJSONIOProcessLine:240 : QEMU_MONITOR_RECV_REPLY: mon=0x7faa9003ebf0 reply={"return": {}, "id": "libvirt-390"}
> > 2022-03-09 17:29:34.249+0000: 20390: info : qemuMonitorSend:995 : QEMU_MONITOR_SEND_MSG: mon=0x7faa9003ebf0 msg={"execute":"migrate","arguments":{"detach":true,"blk":false,"inc":false,"uri":"fd:migrate"},"id":"libvirt-391"}^M
> >  fd=-1
> > 2022-03-09 17:29:34.249+0000: 20387: info : qemuMonitorIOWrite:452 : QEMU_MONITOR_IO_WRITE: mon=0x7faa9003ebf0 buf={"execute":"migrate","arguments":{"detach":true,"blk":false,"inc":false,"uri":"fd:migrate"},"id":"libvirt-391"}^M
> >  len=113 ret=113 errno=0
> > 
> > 
> > in qemu I am currently looking at the code in migration/socket.c
> > vs the code in migration/fd.c , wonder if the difference would
> > stem from there..
> 
> When saving to a file, libvirt passes in a pre-opened FD for
> QEU to use. IIRC this should always be a pipe FD connected to
> libvirt's iohelper program, sometimes indirectly via a compression
> program.

In which case, what is a simple 'top' showing on the system?

Dave

> Regards,
> Daniel
> -- 
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK