Re: [Xen-devel] Another migration question

[Jacob Gorm Hansen <jacobg@xxxxxxx>]

Harry Butterworth wrote:
> On Thu, 2005-03-17 at 14:31 +0000, M.A. Williamson wrote:
>
> > USB is a little trickier because there isn't a straightforward way to 
> > ensure the guest can still access the device after the migration nor to 
> > make the process transparent to the guest USB stack. For this reason, USB 
> > doesn't support suspend, so you can't suspend a USB frontend domain. We can 
> > probably do a bit better e.g. fake out port disconnects on suspend then 
> > allow a device reconnect on resume - it won't be transparent but it'll be 
> > more useful.
>
> If the inter-domain communication interface was network transparent it
> would allow you to migrate a domain within a cluster for load-balancing
> whilst leaving the usb devices where they were.
There is an argument against making any of this transparent. If you 
migrate stuff somewhere, you would like to be independent of the 
originating machine once you arrive. Network transparent IPC/RPC, 
filesystems, and process migration, were all hot topics in systems 
research some 10-15 years ago, and, apart from NFS (which is now being 
replaced by iSCSI) none of these fancy transparent schemes survived in 
the real world.
I've done some test with self-migration (the right way to do all of this 
IMHO) using software RAID-1. In this setup it is trivial to mirror a 
block device onto the target node, and when the mirror is synced up you 
migrate the OS itself. I guess it is not currently possible to upgrade a 
remote iSCSI mount to a local Xen-block device mount.
There is no need to forward any migration events into userspace, because 
userspace is what is controlling the whole thing anyway. The TCP 
connection is opened and fed from userspace, the ARP reply is sent from 
userspace, even the protocol implementation running at the remote end is 
uploaded from userspace.
You could argue that self-migration, implemented inside XenoLinux, is 
less portable. However, I recently counted, and it seems that today 
there are more different VMMs than there are operating systems.
But that is just my view. I am likely to be wrong, or at least suffering 
from the 'second system effect' ;-).
Jacob


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