On 04/20/2011 05:28 PM, Christian Benvenuti (benve) wrote:
Daniel,
I looked at the patch-set you sent out on the 2/9/11
[PATCH 0/6] Introduce a new migration protocol
to QEMU driver
http://www.mail-archive.com/libvir-list@xxxxxxxxxx/msg33223.html
What is the status of this new migration protocol?
Is there any pending issue blocking its integration?
I would like to propose an RFC enhancement to the migration
algorithm.
Here is a quick summary of the proposal/idea.
- finer control on migration result
- possibility of specifying what features cannot fail
their initialization on the dst host during migration.
Migration should not succeed if any of them fails.
- optional: each one of those features should be able to
provide a deinit function to cleanup resources
on the dst host if migration fails.
This functionality would come useful for the (NIC) set port
profile feature VDP (802.1Qbg/1Qbh), but what I propose is
a generic config option / API that can be used by any feature.
And now the details.
----------------------------------------------
enhancement: finer control on migration result
----------------------------------------------
There are different reasons why a VM may need (or be forced) to
migrate.
You can classify the types of the migrations also based on
different semantics.
For simplicity I'll classify them into two categories, based on
how important it is for the VM to migrate as fast as possible:
(1) It IS important
In this case, whether the VM will not be able to (temporary)
make use of certain resources (for example the network) on the
dst host, is not that important, because the completion of the
migration is considered higher priority.
A possible scenario could be a server that must migrate ASAP
because of a disaster/emergency.
(2) It IS NOT important
I can think of a VM whose applications/servers need a network
connection in order to work properly. Loosing such network
connectivity as a consequence of a migration would not be
acceptable (or highly undesirable).
Given the case (2) above, I have a comment about the Finish
step, with regards to the port profile (VDP) codepath.
The call to
qemuMigrationVPAssociatePortProfile
in
qemuMigrationFinish
can fail, but its result (success or failure) does not influence
the result of the migration Finish step (it was already like this
in migration V2).
I *believe* the underlying problem is Qemu's switch-over. Once Qemu
decides that the migration was successful, Qemu on the source side dies
and continues running on the destination side. I don't think there are
more handshakes foreseen with higher layers that this could be reversed
or the switch-over delayed, but correct me if I am wrong... So now
whatever we do, we'd have to associate the port profile before the
actual switch-over, if we wanted to do something better than what is
there now and have the opportunity to terminate the migration before the
switch-over by Qemu happens in case of failure to associate profiles.
The problem is to know when the switch-over happens or when the
migration goes into the final phase where the source side doesn't run
anymore. The would allow us to not associate the ports right at the
beginning of the migration but maybe towards the time when for example
in live-migration the source is not running anymore *and* also we have
the result of the association before Qemu on the source dies for good. I
think some additional coordination between libvirt and Qemu would be
necessary so that if higher layer ops fail before the resume on the
destination side happens that Qemu can still fall back to the source
side. I believe what could happen now is that a VM could be transferred
too fast (by the Qemu process) while the association (in libvirt)
happens, Qemu on the source side dies, and then we only get the negative
result of the association. Maybe the simplest solution would be if Qemu
on the source side waited for a command before transferring the last
packet so we still have a chance to cancel and Qemu doesn't just 'run
away' underneath libvirt's feet ;-).
I assume that 2 associations with the same profile are possible with
802.1Qbg and Qbh. Both are also going through a Pre-associate state now.
Are there any side-effects if associating twice on the same switch like
no packets that can be sent on the source side or something like that --
obviously this would be bad if this happened early during live-migration
and we'd want to push the association close to the 'final migration
phase', which in turn may require more coordination with Qemu (don't
know whether the final phase can be determine now -- maybe via polling
Qemu's monitor).
Regards,
Stefan
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