Re: Interesting qemu/virt-manager bug about the "rotational" attribute on virtio-blk disks

[Stefan Hajnoczi]

On Thu, Jul 16, 2020 at 11:33:44AM +0200, Stefano Garzarella wrote:
> +Cc Michael, Stefan, virtualization@lists.linux-foundation.org
> 
> On Thu, Jul 16, 2020 at 09:06:14AM +0100, Richard W.M. Jones wrote:
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1857515
> > 
> > A virtio-blk disk which is backed by a raw file on an SSD,
> > inside the guest shows rotational = 1.
> > 
> > I assumed that qemu must have a "rotational" property for disks and
> > this would be communicated by virtio to the guest, but qemu and virtio
> > don't seem to have this.  Pretty surprising!  Is it called something
> > other than "rotational"?
> > 
> 
> I'm not sure if we need to add this property in QEMU, but in Linux
> I found these flags (include/linux/blkdev.h) for the block queues:
> 
>     #define QUEUE_FLAG_NONROT 6       /* non-rotational device (SSD) */
>     #define QUEUE_FLAG_VIRT           QUEUE_FLAG_NONROT /* paravirt device */
> 
> xen-blkfront driver is the only one that sets the QUEUE_FLAG_VIRT,
> should we do the same in the virtio-blk driver regardless of the backend?

The ability to control this flag would be interesting for performance
experiments.

The problem with changing the default is that regressions can be
expected. Certain workloads benefit while others regress.

I suggest:
1. Make it controllable so that QUEUE_FLAG_NONROT can be set or clear
   (not hardcoded to a single value).
2. The device can communicate the optimal setting from the host. The
   SCSI protocol already conveys this information. Virtio-blk needs a
   feature bit and possibly config space field.
3. Make it migration-safe. It needs to be configured explicitly so the
   value doesn't change suddenly across migration.

Stefan

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