Libvirt driver iothread property for virtio-scsi disks

[Nir Soffer]

On Wed, Nov 4, 2020 at 6:54 PM Daniel P. Berrangé <berrange at redhat.com> wrote:
>
> On Wed, Nov 04, 2020 at 05:48:40PM +0200, Nir Soffer wrote:
> > The docs[1] say:
> >
> > - The optional iothread attribute assigns the disk to an IOThread as defined by
> >   the range for the domain iothreads value. Multiple disks may be assigned to
> >   the same IOThread and are numbered from 1 to the domain iothreads value.
> >   Available for a disk device target configured to use "virtio" bus and "pci"
> >   or "ccw" address types. Since 1.2.8 (QEMU 2.1)
> >
> > Does it mean that virtio-scsi disks do not use iothreads?
> >
> > I'm experiencing a horrible performance using nested vms (up to 2 levels of
> > nesting) when accessing NFS storage running on one of the VMs. The NFS
> > server is using scsi disk.
>
> When you say  2 levels of nesting do you definitely have KVM enabled at
> all levels, or are you ending up using TCG emulation, because the latter
> would certainly explain terrible performance.

Good point, I'll check that out, thanks.

> > My theory is:
> > - Writing to NFS server is very slow (too much nesting, slow disk)
> > - Not using iothreads (because we don't use virtio?)
> > - Guest CPU is blocked by slow I/O
>
> Regards,
> Daniel
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