RE: [Xen-devel] Migration filesystem coherency?

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 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?

The loop device doesn't do direct IO, so using it for migration is
fundamentally unsafe. See Andrew/Julians's blktap patches for a way to
do safe file-backed VMs. 

Ian 

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