Stefano Garzarella
2020-Jul-16 09:33 UTC
Interesting qemu/virt-manager bug about the "rotational" attribute on virtio-blk disks
+Cc Michael, Stefan, virtualization at 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? Thanks, Stefano
Stefan Hajnoczi
2020-Jul-23 10:32 UTC
Interesting qemu/virt-manager bug about the "rotational" attribute on virtio-blk disks
On Thu, Jul 16, 2020 at 11:33:44AM +0200, Stefano Garzarella wrote:> +Cc Michael, Stefan, virtualization at 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 -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 488 bytes Desc: not available URL: <http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20200723/e2e0f7f8/attachment.sig>
Richard W.M. Jones
2020-Jul-23 10:40 UTC
Interesting qemu/virt-manager bug about the "rotational" attribute on virtio-blk disks
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, virtualization at 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.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. Rich.> 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-- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW