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Re: [libvirt] RFC: APIs for managing a subset of a domain's disks



On Mon, May 02, 2011 at 05:31:00PM -0600, Eric Blake wrote:
> Consider the case of a guest that has multiple virtual disks, some
> residing on shared storage (such as the OS proper) and some on local
> storage (scratch space, where the OS has faster response if the virtual
> disk does not have to go over the network, and possibly one where the
> guest can still work even if the disk is hot-unplugged).  During
> migration, you'd want different handling of the two disks (the
> destination can already see the shared disk, but must either copy the
> contents or recreate a blank scratch volume for the local disk).
> 
> Or, consider the case where a guest has one disk as qcow2 (it is not
> modified frequently, and benefits from sharing a common backing file
> with other guests), while another disk is raw (for better read-write
> performance).  Right now, 'virsh snapshot' fails, because it only works
> if all disks are qcow2; and in fact it may be the case that it is
> desirable to only take a snapshot of a subset of the domain's disks.

There's a problem here, but I don't much like the solution.  It's
going to be very clumsy to extend (say) "virsh migrate" or
virt-manager to support this.

How about just adding flags into the disk XML, eg:

  <disk>
    ...
    <flags>
      <migrate>false</migrate>
      <snapshot>false</snapshot>
    </flags>
  </disk>

(Don't sweat the details; the important point is that these are a
property of the disk which is permanently attached to that disk
through the XML).

Rich.

-- 
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