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Re: Interesting qemu/virt-manager bug about the "rotational" attribute o
Richard W.M. Jones
Re: Interesting qemu/virt-manager bug about the "rotational" attribute on virtio-blk disks
Thu, 23 Jul 2020 11:40:56 +0100
On Thu, Jul 23, 2020 at 11:32:39AM +0100, Stefan Hajnoczi wrote:
> On Thu, Jul 16, 2020 at 11:33:44AM +0200, Stefano Garzarella wrote:
> > +Cc Michael, Stefan, email@example.com
> > 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
I expect there's a pretty strong interaction between this feature and
the shared guest io_uring stuff isn't there? I'm not sure if it'll be
a positive or negative effect though.
> 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.
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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