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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?
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.
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...
How does swap work over the nbd's?
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? A really nice xen setup would be:
Machine A hosting iSCSI devices
Machine B hosting iSCSI devices
Machine C hosting xen domains
Machine D hosting xen domains
Raid1 over iSCSI would ensure that one of A or B could be down without affecting availability.
Xen domains can wander between C & D easily using migration.
A & C could be the same physical machine, as could B & D.
lots of possibilities... wish I had time to play with them all...

From: Ian Pratt
Sent: Fri 20/08/2004 4:16 AM
To: David Becker
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxxx; Ian.Pratt@xxxxxxxxxxxx
Subject: Re: [Xen-devel] disks under migration

> How does live (or normal) migration handle the disks involved?
> Are they transfered, or does the xfrd keep some sort of channel open?

xend currently doesn't give you any help and do this
automatically, but its something we might add in the future.

For most of our VMs we either use NFS root to a central server,
or iscsi direct to a SAN, so there's no block device as such to

If you do want to access back to a local drive, then you'll need
to set this up yourself. There are a bunch of network block
device options available, all of which should work: iscsi, NBD,

We could add the equivalent of the vif-bridge script to xend for
block devices. This would provide simple hooks to enable the
block device export/import to be set up.  However, chances are
we'd want this to be stateful and smarter, so its probably best
to do a xend plugin rather than just use scripts. 
> And I assume a saved domain that is restored on another host must have
> all its disk and swap space transfered to the new host as well, correct?

Yep. No auto magic, not yet ;-)


[*] Even using iscsi, there are some situations where you'd
prefer to have the iscsi driver in domain 0 and export the disk
as a block device to other domains. For example, if you have a
hardware iscsi initiator in the machine. In this instance, you'd
prefer to move the iscsi disk as part of the migrate. 

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