[libvirt-users] libvirt live migration, qcow2 image, nbd server

[Eric Blake]

On 02/13/2015 03:15 AM, Kashyap Chamarthy wrote:
> On Thu, Feb 12, 2015 at 08:41:31PM -0600, Edward Young wrote:
>> Hi all,
>>
>>
>> When I live migrate a vm using
>>
>> "migrate --live --copy-storage-all mig-vm qemu+ssh://192.168.1.3/system
>> tcp://192.168.1.3"
>>
>>
>> I got the following error
>>
>>
>>              WARNING: Image format was not specified for
>> 'nbd://node2:49155/drive-virtio-disk0' and probing guessed raw.
>>
>> 258          Automatically detecting the format is dangerous for raw
>> images, write operations on block 0 will be restricted.
>>
>> 259          Specify the 'raw' format explicitly to remove the restrictions.
>>
>>
>> >From the error itself, I can tell that nbd server treat this image as raw,
>> rather than qcow2. My platform is libvirt+qemu. the image type of this vm
>> is indeed qcow2. I do not know how to let nbd know this image format.
> 
> Hmm, I wonder if `virsh --migrate` should have a --format (or -F) option
> that one can specify the format. Let's see what Eric Blake and others say.

No. The format used by the NBD connection under the hood is not
user-visible (for that matter, there may be more than one disk being
migrated, and a single 'migrate --format' would NOT be sufficient to
cover the case where multiple disks for the guest have different formats
on the host).  I think that it is merely a case where libvirt itself
needs to explicitly tell qemu that the NBD server it is setting up under
the hood will serve raw data, as there is no mention of the NBD server
on the command line.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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