Re: [Qemu-devel] Live Migration with different block devices

[Eric Blake <eblake@redhat.com>]

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On 06/24/2014 03:29 PM, Brian Jackson wrote:
> 
> 
> On 6/24/2014 2:22 PM, Xiongzi Ge wrote:
>> Hi,
>>
>>
>> When I do live migration, in the source and destination host, there are
>> different block devices, but qemu can not detect this. I used virtio
>> as the
>> driver in kvm and in the vdi device in the guest is /dev/vda.  So, 
>> the vm
>> guest can read different data from the same /dev/vda device.  I am
>> studying
>> this code to let qemu understand that, this is a new device.
>>
>> Does qemu recognize different block devices after live migration?
> 
> You aren't supposed to have different command line options when live
> migrating (with the exception of -incoming if you migrate that way). So
> whatever you are trying to do is unsupported.

Caveat - it IS supported to change command line options for things that
do NOT impact guest ABI.  For example, if you have /path/to/file as the
host location containing the guest image on the source, but
/other/path/file as the location on the destination, you CAN rewrite
that parameter (and in fact, libvirt DOES supporting the rewrite such
parameters if you use the 'virsh migrate --xml' option - after first
proving that your changes do not affect guest ABI).  But the onus is on
you to ensure that src:/path/to/file and dst:/other/path/file have the
SAME guest-visible contents at the time of the migration (this generally
requires that the two names be mappings to the same underlying network
resource; you can get trickier, but it starts to be at your peril,
because the guest is hosed if the two files differ in guest contents).

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


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