[ovirt-devel] Issue: Device path changed after adding disks to guest VM

[Daniel P. Berrangé]

On Wed, Jan 06, 2021 at 04:36:46PM +0100, Peter Krempa wrote:
> On Wed, Jan 06, 2021 at 17:16:24 +0200, Nir Soffer wrote:
> > On Wed, Dec 2, 2020 at 4:57 PM Joy Li <joooy.li at gmail.com> wrote:
> 
> [...]
> 
> > Comparing to state before reboot:
> > 
> > # virsh -r domblklist disk-mapping
> >  Target   Source
> > ---------------------------------------------------------------------------------------------------------------------------------------------------------------
> >  sdc      -
> >  sda      /rhev/data-center/mnt/blockSD/84dc4e3c-00fd-4263-84e8-fc246eeee6e9/images/40018b33-2b11-4d10-82e4-604a5b135fb2/40f455c4-8c92-4f8f-91c2-991b0ddfc2f5
> >  vda      /dev/mapper/3600140594af345ed76d42058f2b1a454
> >  vdb      /dev/mapper/360014050058f2f8a0474dc7a8a7cc6a5
> >  vdc      /dev/mapper/36001405b4d0c0b7544d47438b21296ef
> > 
> > # ls -lh /dev/disk/by-id/virtio-*
> > lrwxrwxrwx. 1 root root 9 Jan  6 09:42
> > /dev/disk/by-id/virtio-b97e68b2-87ea-45ca-9 -> ../../vda
> > lrwxrwxrwx. 1 root root 9 Jan  6 09:42
> > /dev/disk/by-id/virtio-d9a29187-f492-4a0d-a -> ../../vdb
> > lrwxrwxrwx. 1 root root 9 Jan  6 09:51
> > /dev/disk/by-id/virtio-e801c2e4-dc2e-4c53-b -> ../../vdc
> > 
> > In the guest disks are mapped to the same device name.
> > 
> > It looks like libivrt domblklist is not correct - vdb and vdc are switched.
> > Peter, this expected?
> 
> The names in 'virsh domblklist' are unfortunately and confusingly chosen
> to match the expected /dev/ device node name, but it's at kernel's
> discretion to name /dev/ nodes.
> 
> This means that it's not guaranteed that what you see in 'virsh
> domblklist' will match the state in the guest.

Essentially the only thing the disk device name is used for is
sorting the <disk> elements within the XML document. This in
turn affects what order PCI addresses (virtio-blk) or SCSI
LUNS (virtio-scsi) are assigned in. This influences/hints as
to what order the guest OS *might* assign device names in.

The device name from the XML is not exposed to the guest
directly though.

Certainly when hotplugging/unplugging is involved all bets are
off wrt what disk names you'll see in the guest vs the XML. Dont
expect them to match except by luck.


Regards,
Daniel
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