[libvirt] [PATCH v1] qemu-migration: Disallow migration of read only disk

[Corey S McQuay]

On 08/10/2016 09:16 AM, Koniszewski, Pawel wrote:

>> -----Original Message-----
>> From: libvir-list-bounces at redhat.com [mailto:libvir-list-
>> bounces at redhat.com] On Behalf Of Corey S. McQuay
>> Sent: Friday, August 5, 2016 8:34 PM
>> To: jjherne at linux.vnet.ibm.com; libvir-list at redhat.com
>> Cc: Corey S. McQuay <csmcquay at linux.vnet.ibm.com>
>> Subject: [libvirt] [PATCH v1] qemu-migration: Disallow migration of read only
>> disk
>>
>> From: "Corey S. McQuay" <csmcquay at linux.vnet.ibm.com>
>>
>> Currently Libvirt allows attempts to migrate read only disks. Qemu cannot
>> handle this as read only disks cannot be written to on the destination system.
>> The end result is a cryptic error message and a failed migration.
>>
>> This patch causes migration to fail earlier and provides a meaningful error
>> message stating that migrating read only disks is not supported.
> What will happen if read-only disk is copied to destination prior to migration start? Currently such scenario works, will it still work with this code?
Based on our testing, pre-copying a read only disk image to the 
destination system has no effect on the outcome of attempting to migrate 
a non-shared read only disk. I'm not sure what scenario you are 
referring to but here is what we tried:

Relevant guest xml:
     <disk type='file' device='cdrom'>
       <driver name='qemu' type='raw' cache='writethrough'/>
       <source file='/disk-images/guest.iso'/>
       <backingStore/>
       <target dev='sdz' bus='scsi'/>
       <readonly/>
       <alias name='scsi0-0-0-0'/>
       <address type='drive' controller='0' bus='0' target='0' unit='0'/>
     </disk>

The disk image exists at /disk-images/guest.iso on the source. Before 
migration we copied the image to the same path on the destination 
system. Then we attempted migration:

     virsh migrate --live --copy-storage-all --migrate-disks sdz 
--verbose kvm1 qemu+ssh://dstHost/system tcp://dstHost

The error message we get is:

error: internal error: info migration reply was missing return status

Running journalctl shows additional information:

Aug 10 16:02:16 collin-kvm libvirtd[41616]: operation failed: migration 
of disk sdz failed.

I'm pretty sure this patch does not stop the user from doing anything 
that works today. But if your scenario is different from ours in some 
way please let us know and we'll do some more testing.