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@xxxxxxxxxxxxxxxxxxxxxxxxxx > > > > 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
