Re: [Xen-devel] XEN migration architecture question

[Mark Williamson <Mark.Williamson@xxxxxxxxxxxx>]

> 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?

First of all, bear in mind that Xend doesn't migrate storage for you, so the 
backend on the destination machine needs to somehow be able to access the 
original VBD.

Assuming you have that, the migration works in the same way as a backend 
restart: messages sent by Xend allow it to detect the broken device channel 
connection, reinitiate a connection with the new backend and resend all 
requests that were pending in the ring to make sure they complete.

> (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?

The old suspend / resume functionality used to require a domain to stop doing 
IO before it was suspended.  Since the backend restart support was developed 
for the driver domains project, it has been adopted instead as it's a more 
general mechanism.

HTH,
Mark


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