Hi,

I'm doing some test for to see if I can change my actual configuration (2 host + SAN) that have a single point of failure with a "2 host No SAN" but seen that I've also the NAS I would like to add it as a third disk replication for to have a full copy also in a "real" iSCSI device.

I know that DRBD allow to add a third replication.
I don't need to add this unit in the HA logic it's just a "disaster recovery" unit that I showld can connect to a single xen host for to recover all.

How I've to change after the noSAN installation to add it ? And it's possible to add it with working system or it's dangerous ?

Thank you
Have a nice day

DRBD does allow for stacking of resources which could replicate to a third node, however, it would require that the third node be a Linux O/S with DRBD with access to a raw partition. In your case, the third node is a NAS which could not act in this capacity unless you somehow tethered it to another Linux host.

Another consideration is that DRBD will blindly copy all data to all nodes which does not protect you from data corruption.

Since the third SR is just a NAS, I would consider storing entire VM backups there instead which gives you the added benefit of storing multiple backup images per VM. There are many options for doing this in an automated way. There is also one bundled with halizard (however it is not documented). You can get more details by typing "/etc/ha-lizard/scripts/vm_backup.sh --help" from the shell of any host that has halizard installed.