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

[Peter Krempa]

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.

In this case I think the reorder happens as the PCI address of vdb is
larger than the address of vdc.

A partial workaround can be to use the data provided by the qemu guest
agent, for exampe via 'virsh domfsinfo':

$ virsh domfsinfo fedora32
 Mountpoint   Name   Type   Target
------------------------------------
 /            dm-0   xfs    vda
 /boot        vda1   xfs    vda

Here the guest-host matching is established via the PCI address so the
'Target' field accurately refers to the target in the VM XML.

Similarly the linux kernel recently changed enumeration of SCSI devices
so they are not guaranteed to match either. Libguestfs for example
needed a workaround.

https://github.com/libguestfs/libguestfs/commit/bca9b94fc593771b3801b09b95e477f160517909