Re: [Xen-devel] reliable live migration of large and busy guests

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, Nov 7, 2012 at 3:10 PM, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote:

> From an idealistic point of view, it might be quite nice to have several

> live migrate mechanisms, so the user can choose whether they value
> minimum downtime, minimum network utilisation, or maximum safety.

Agreed.  IIRC, when post-copy was suggested for Xen years ago,
Ian Pratt was against it, though I don't recall why, so Michael
Hines work was never pursued (outside of academia).  Probably
worth asking IanP before investing too much time into it.

Was he against a hybrid approach, where you "push" things first, and then "pull" things later?  Or just against a pure "pull" approach?  I'm pretty sure a pure "pull" approach would result in lower performance during the migration.

Just tossing another idea out there: What about throttling the VM if it's dirtying too many pages?  You can use the "cap" feature of the credit1 scheduler to reduce the amount of cpu time a given VM gets, even if there is free cpu time available.  You could play around with doing N iterations, and then cranking down the cap on each iteration after that; then the application wouldn't have a several-second pause, it would just be "running slowly" for some period of time.

Overall doing a hybrid "send dirty pages for a while, then move and pull the rest in" seems like the best approach in the long-run, but it's fairly complicated.  A throttling approach is probably less optimal but simpler to get working as a temporary measure.

 -George

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