[Openstack-operators] Inverted drive letters on block devices that use virtio-scsi

Logan V. logan at protiumit.com
Fri Jan 26 04:06:48 UTC 2018

If you are running Ocata or later, please see

There seems to be a broken series of patches related to virtio-scsi
that were backported to Ocata resulting in some unresolved virtio-scsi
drive ordering bugs. I see it primarily on virtio-scsi based instances
where 1) boot from volume instances with a secondary volume attached,
or 2) boot from image volumes where config drive is enabled.


On Thu, Jan 25, 2018 at 6:58 PM, Jean-Philippe Méthot
<jp.methot at planethoster.info> wrote:
> Hi,
> Lately, we’ve been converting our VMs block devices (cinder block devices)
> to use the virtio-scsi driver instead of virtio-blk by modifying the
> database. This works great, however, we’ve run into an issue with an
> instance that has more than one drive. Essentially, the root device has
> address <address type='drive' controller='0' bus='0' target='0' unit='1’/>
> while the second device has address <address type='drive' controller='0'
> bus='0' target='0' unit='0’/> . I believe this results in the root drive
> getting called sdb in the vm while the second drive gets called sda.
> If my assumption is right, what exactly controls which drive gets address
> unit=0 and which drive gets address unit=1 in the vm configuration?
> Jean-Philippe Méthot
> Openstack system administrator
> Administrateur système Openstack
> PlanetHoster inc.
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