Re: [Xen-devel] disks under migration

[Mark Williamson <Mark.Williamson@xxxxxxxxxxxx>]

Setting up dom0 as the initiator and re-exporting to other domains is 
currently more complex to manage under migration: you'd have to work out a way 
of getting dom0 on the new host to initiate a connection and do the 
re-exporting.  If you have the migrated domain talk directly to the target 
then you don't need to explicitly support this...

OTOH, you can boot directly off disks exported by dom0 without requiring an 
initrd, which would be useful.

If you choose to use NFS then you can use it (as you probably realise) as a 
root filesystem without an initrd.  I believe there's a patch available 
somewhere that enables swapping over NFS (Adam Heath's Xen 1.2 Debian packages 
included this).

The ideal way to do this sort of stuff would modify the Xend migration tools 
to export a domain's block devices over the network when it is moved away from 
its "home" node.  Domains would use Xen disk interface but when on the "home" 
node, this would be backed directly by storage in dom0, whereas if migrated to 
"remote" nodes, the remote dom0 would additionally have to import this block 
device over the network, then re-export to the domain.  This would all be 
transparent to the domain in question, whilst avoiding network overheads when 
on the "home" node.

We've talked about this scenario and it would be interesting to try it out.  
We may get round to implementing it for some later release but nobody (AFAIK) 
is working on it right now.

HTH,

Mark



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