The Linux kernel will query the SCSI "Block device characteristics"
VPD to determine the rotations per minute of the disk. If this has
the value 1, it is taken to be an SSD and so Linux sets the
'rotational' flag to 0 for the I/O queue and will stop using that
disk as a source of random entropy. Other operating systems may
also take into account rotation rate when setting up default
behaviour.

Mgmt apps should be able to set the rotation rate for virtualized
block devices, based on characteristics of the host storage in use,
so that the guest OS gets sensible behaviour out of the box. This
patch thus adds a 'rotation-rate' parameter for 'scsi-hd' and
'scsi-block' device types. For the latter, this parameter will be
ignored unless the host device has TYPE_DISK.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20171004114008.14849-2-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>