[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] Migration filesystem coherency?

Ian Pratt wrote:
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


At the moment, I'm trying a shared physical disk. Should that work? If so, what code is guaranteeing the data is written to disk before the domain starts executing on the new host?

As to loopback, regardless of what kind of I/O it does, when the loopback device is torn down, all I/O should be committed to, at least, the VFS layer of the backing filesystem. If the backing filesystem makes the proper coherency guarantees, then this should be sufficient. My understanding is that both GFS and OCFS2 make these guarantees. So with these filesystems as the backing store, as long as Xen can guarantee the tear down before the domain starts executing on the new node, things should work, shouldn't they?


John Byrne

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.