[Qemu-devel] [PATCH v1 0/2] Reporting of rotation rate for disks

[Daniel P. Berrange]

While looking at libvirt tagged questions on serverfault I saw
someone ask how to indicate that a virtual disk is an SSD rather
than rotating rust.

https://serverfault.com/questions/876467/how-to-add-virtual-storage-as-ssd-in-kvm

IIUC, IDE / SCSI don't really have an explicit concept of a
disk being SSD vs HDD, but they do have a means of reporting
the disk rotation rate and both specify that SSDs should
report RPM==1.

Linux uses this to determine whether to set the 'rotational'
property to 0 for the I/O queue, and also whether to allow
the disk to contribute random entropy (only HDDs contribute).

This felt like something QEMU ought to allow the mgmt application
to set based on the storage it is using to back the virtual block
devices. So this series adds a 'rotation_rate' property to the
SCSI and IDE disks, taking an RPM value per their respective
specifications.

There is no mechanism to report this information to virtio-blk.
We could perhaps argue that people should use virtio-scsi instead,
because fixing virtio-blk would require enhancement to both QEMU
and Linux virtio-blk drivers (and other guest OS drivers too)

I'm unclear if there is anything I should have done wrt the
device migration vmstate when adding this extra field, or if it
is safe to just expect the mgmt app to set the property correctly
on src+dst ?

Daniel P. Berrange (2):
  scsi-disk: support reporting of rotation rate
  ide: support reporting of rotation rate

 hw/ide/core.c             |  1 +
 hw/ide/qdev.c             |  1 +
 hw/scsi/scsi-disk.c       | 20 ++++++++++++++++++++
 include/hw/ide/internal.h |  8 ++++++++
 4 files changed, 30 insertions(+)

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2.13.5