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Re: [libvirt] [PATCH 2/2] qemu: Clear source path of removable block if it is ejected.



On Fri, Aug 12, 2011 at 09:34:42PM +0800, Osier Yang wrote:
> This patch is mainly to solve the migration problem:
>   After the media is ejected inside guest, and one removes the
> source of the media outside, migration will fail early, as we
> start the qemu daemon on dest host with the source of the removable
> block still existing, this causes failure when trying to do cgroup
> setting on the media source then.
> 
>   All the migration functions try to get the domain XML before
> doing preparation on dest host. Except one specify the domain
> XML externally (e.g. option "--xml" of "virsh migrate"). So this
> patch changes the function "qemuDomainFormatXML" to check the
> media status of removable block using the new introduced monitor
> functions, and clear the source path if the media ejected, so
> that it won't start the qemu daemon on dest host along with the
> media path.
> 
>   Public migration APIs invokes "driver->domainGetXMLDesc";
> "driver->domainMigratePeer2Peer" uses "qemuDomainFormatXML"
> for v2 migration protocol and "qemuDomainBegin" for v3 migration
> protocol, to get the domain XML before preparation on dest host.
> All of these 3 functions are based on "qemuDomainFormatXML", that's
> why making changes on it.
> 
>   The changes affect the running domain config, (e.g. for a running
> domain, "virsh dumpxml" won't display the source path of the media
> anymore once it's ejected inside guest. But think it's reasonable
> to do like so.

No, IMHO, we really do not want to be going to the QEMU monitor every
time we dump XML. We want the dump XML operation to be fast & reliable
even in the face of a stuck/deadlocked QEMU [1].

If we want Dump XML to always show the corect state, then we need
QEMU to be giving us an event notification.


Daniel

[1] Yes we have a problem with the balloon query update in this respect
    too, but we need to fix that, not make the problem worse.
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