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

[Daniel P . Berrangé]

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.

Regards,
Daniel
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