Re: HA for Xen

[Adam Hamsik <haaaad%gmail.com@localhost>]

On Oct,Monday 4 2010, at 1:47 AM, River Tarnell wrote:

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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).

Yeah but still you can use different ways to not allow split brain. e.g. you 
can use one 
small lun on disk array as another heartbeat device which is used to keep 
quorum ok.

Regards

Adam.