Re: [Xen-devel] vbd flushing during migration?

[John Byrne <john.l.byrne@xxxxxx>]

It would be a bit ugly, but mostly straightforward to watch for the 
destruction of the vbds (or all devices) after the destroyDomain() is 
done and then sending an all-clear. (The last time I looked there wasn't 
a waitForDomainDestroy() anywhere, so it would probably be best to write 
one.) This would guarantee correctness: which is the most important thing.
The problem I see with that strategy is the effect on downtime during a 
live-move. Ideally you'd like to start the vbd cleanup when the final 
suspend is done and hope to parallelize the any final device operations 
with the final pass of live-move. How to do that and play nice with 
domain destruction on the normal path and handle errors seems a lot less 
clear to me.
So, are you just ignoring the notion of minimizing downtime for the 
moment or is there something I'm missing?
John

Andrew Warfield wrote:
> It's slightly more than a flush that's required.  The migration
> protocol needs to be extended so that execution on the target host
> doesn't start until all of the outstanding (i.e. issued by the
> backend) block requests have been either cancelled or acknowledged.
> This should be pretty straight forward given that the backend driver
> ref counts a blkif's state based on pending requests, and won't tear
> down the backend directory in xenstore until all the outstanding
> requests have cleared.  All that is likely required is to have the
> migration code register watches on the backend vbd directories, and
> wait for them to disappear before giving the all-clear to the new
> host.
>
> We've talked about this enough to know how to fix it, but haven't had
> a chance to hack it up.  (I think Julian has looked into the problem a
> bit for blktap, but not yet done a general fix.) Patches would
> certainly be welcome though. ;)
>
> a.
>
> On 7/31/06, John Byrne <john.l.byrne@xxxxxx> wrote:
> > Hi,
> >
> > I don't see any obvious flush to disk taking place for vbd's on the
> > source host in XendCheckpoint.py before the domain is started on the new
> > host. Is there a guarantee that all written data is on disk somewhere
> > else or is something needed?
> >
> > Thanks,
> >
> > John Byrne
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
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