TOPIC:

Hello:
I'm brand new to XenServer and evaluating it for our environment. We are interested in HA-Lizard iSCSI-HA to create shared local storage from our XenServer hosts. Recommendation is to set up a minimum of 3 XenServer hosts for Xen HA, so in doing that we'd have a single HA-Lizard master and 2 slaves. Based on the HA-Lizard documentation more than one slave can be set up so it sounds like this is a valid setup.

But, most of the HA-Lizard documentation mentions a 2-node pool. With 3 nodes, I'm assuming that the local storage is just mirrored among all 3? Just confirming that for sizing requirements. The DRBD documentation mentions in order to do 3 nodes the devices are stacked... so does that also apply to HA-Lizard? I haven't seen a 3-node example configuration which would be helpful if one is out there.

Thank you for the information and your help.

Below is a long winded answer to your question intended to provide some clarification on what our SW does.

The most common use of our software is in a 2-node XenServer environment w/ local storage. This setup specifically addresses two issues:
1- HA in a 2 node pool is not reliably supported by XenServer- HA-Lizard was designed to provide predictable failover in a 2-node pool
2- local storage is converted to a form of internal SAN such that VMs can be moved live and restarted without any of the limitations of using DAS. This is done with our iSCSI-HA package and allows users to build 2-node pools without any external SAN while realizing the all the functionality (mainly VM agility) of a SAN environment.

With that said many users are implementing HA-Lizard+iSCSI-HA to realize the scenario described above. This design is well documented in a reference design and how-to that we publish on our site.

With all that said, it is possible to use just our HA-Lizard component which provides an alternative HA solution to XenServer's inbuilt HA. When running HA-Lizard there are no restrictions on how many hosts are part of the pool. So, in your example a 3-node pool is fully supported.

The limitation in your case will be that iSCSI-HA is designed to work in a 2-node pool only. Trying to cascade a third DRBD device may be too cumbersome to effectively deal with.

Thank you very much for the clarification.

I did do some more research on the DRBD side of things and as of version 9.0 it has direct support for more than 2 nodes without stacking:

www.drbd.org/users-guide-9.0/s-4node-example.html

In my case I would definitely be interested in this but I understand that it is not directly available as of now.