[Linux-cluster] GNBD, CLVM and snapshots

[Mark Petersen]

I'm wanting to use gnbd with clvm to export block devices for 3 
(possibly more) hosts running Xen.  Each host will have access to the 
single gnbd export with LVM.  Only a single host will ever actually 
have the device mounted.  GNBD can support live migrations with a 
block device, which is the main attraction.

So a little info on Xen and what I want to do.  There are dom0's 
(privileged VM) that have full access to any running domU (VM 
instances started by the dom0.)  The dom0 will be running 
clvm/CCS/gnbd-Client/etc.  The dom0 will start a domU that mounts the 
lv, only the dom0 needs direct access to this resource.  In this 
configuration, would it be possible to take snapshots of the LV from 
the dom0?  What about from another dom0 in the cluster?  What about 
the gnbd-server?

Is work still be done on csnap?  There isn't much documentation on 
this, and it seems like it might be GFS specific.

If this won't work with clvm and gnbd, is there an alternative that 
would work?  I really want to be able to do snapshots and live 
migration with block devices.  I'm not sure this is possible.  I may 
fallback to only live migrations with gnbd if I have to.

Finally, ideally this would be backed by DRBD, but can gnbd handle a 
primary/secondary role instead of doing multipath (which won't work 
with drbd.)  Failover mode was mentioned in posts from over a year 
ago, and it sounds promising.