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Re: [Xen-devel] disks under migration

> Lots of questions here...
> On the subject of network block devices (iSCSI, NBD, ENBD & GNBD), do you 
> have any recommendations?

iSCSI is definitely the most complex and heavy weight, but it
does have the advantage of being the one that's getting most
development, plus you can get h/w assistance from iSCSI initiator
NICs too. 

ResHat seem to care about GNBD for their cluster stuff, so maybe
that's worth looking at too.

> How should it be set up for migration purposes? Should dom0 be the initiator 
> and export it to the other domains, or should each domain be the client 
> directly? I'd expect the latter but am unfamiliar with how migration should 
> work.

Exporting directly to domains certainly makes migration easier as
you don't have to do anything special to provide remote access to
the block devices. 

However, there are advantages to doing it in dom0: 1) it's
totally transparent to the domains -- they just see hda etc as
usual. 2) If you have a h/w iscsi NIC, it probably makes sense to
use it...
> Assuming the latter, are any of the nbd technologies available bootable 
> without using initrd's? I don't have any particular problem with initrd, but 
> it's just an extra thing to set up...

To my knowledge, none work for root without an initrd, which is a

> How does swap work over the nbd's?

It should work just fine.

> Is there any way of providing high availability with any nbd
> devices, so that if one target goes down it can seamlessly
> failover to another? Maybe raid1 over iSCSI? 

Yep, you should be able to use MD to setup a RAID1 configuration
just fine. Even if you're using a local disk you can setup a
network block device to enable you to mirror writes remotely. 


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