Re: Libvirt driver iothread property for virtio-scsi disks

[Daniel P . Berrangé]

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.

> 
> 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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