Re: XEN migration architecture question

[Andrew Warfield <andrew.warfield@gmail.com>]

Hi Eric,

Migration will handle network interfaces and *network attached* block
devices, even under i/o load.  For network devices, an ARP
advertisement is sent indicating the new location of the VM.  We've
tested this a fair bit in cluster environments and are able to migrate
heavily i/o loaded servers very fast.  You may see a few dropped
packets, but TCP will take care of that.  Block devices will see the
backend disconnect when they are suspended, and should reconnect and
reissue any outstanding requests when they resume.

In our migration tests, we generally use network storage.  One
approach that seems to work pretty well is to import GNBD mounts in
domain 0 and export them to the migrating domains --
/dev/gnbd/my-dom-disk will have relevance on both ends of the
migration.

Migration does not currently have explicit support for local block
devices.  It is possible to do, for instance using the block tap to
tunnel the request stream over TCP between the two hosts, it just
isn't clear that it's all that useful -- most people seem to want to
use migration to offload a running VM completely.

hth,
andy.

On Thu, 20 Jan 2005 12:44:40 -0800 (PST), Eric Tessler
<maiden1134@yahoo.com> wrote:
> I have an important question about XEN's migation operation related to the
> XEN block and network back/front drivers.  If I migrate a domain from one
> machine to another in the middle of heavy disk/network activity, what
> happnens to the back/front end XEN drivers? (I see they have suspend/resume
> operations but I don't believe they are being used)  I see that the front
> end drivers are passing machine addresses to the back-end drivers - are
> these pointers somehow still valid when a domain is moved to another
> machine?  Shouldn't the drivers wait until there is no I/O activity before
> being migrated? 
>   
> Thanks, 
> Eric
> 
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