Migration filesystem coherency?

[John Byrne <john.l.byrne@hp.com>]

Hi,

I thought I had a workaround for live migration crashing (I've been 
looking at the SLES 3.0.2 9742c code.), but I found that I was getting 
filesystem errors. I'm wondering if the problem is races in data being 
written to the backing storage.

When migrating a domain, before the domain is started on the new host, 
you have to guarantee that all the domU vbd data is out of the block 
cache and written to the backing device. (In the case of a loopback 
device, whether this is sufficient depends on the cross-host coherency 
guarantees of the backing filesystem.) I cannot see that this takes 
place synchronously with the migration process. To me it looks like that 
the teardown/flush of the backing device depends on the action of the 
xenbus and the hotplug scripts and looks asynchronous to the migration 
process.

So, am I right that there is a really a problem here or is there some 
other way the vbd data is getting flushed during migrate?

Thanks,

John Byrne