Re: HA for Xen

[River Tarnell <river.tarnell%wikimedia.de@localhost>]

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Adam Hamsik:
> Why do you need SCSI reservations, I would say that if you are not going to 
> use that disk in more than one place it should be fine with out it.

Because the reservation is required to make sure we don't use it in more than 
one place.

Without a reservation, it's possible for the heartbeat connection between the 
two nodes to fail, while both are still up and connected to the storage.  How 
does each node know which one is allowed to access the storage?  (Both can't 
access it at once without corrupting the data.)

With SCSI-3 PGR, if the heartbeat connection fails, both nodes can attempt a 
reservation on the storage; one will succeed, and one will fail.  The node that 
succeeds can mount filesystems and start VMs, while the other node will take 
some "split cluster" action (usually, it will panic and reboot).

Regards,
River.
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