Architectural Impact of Hitachi SRA

I spent some time recently setting playing with VMware SRM and HDS modular storage. There are some interesting architectural impacts of the HDS Site Recovery Adapter that are worth discussing. Note that these impacts are the same whether you’re dealing with HDS modular or enterprise storage.

At the heart of the issue is this: The HDS Site Recovery Adapter talks to HDS Command and Control Interface (CCI), and CCI needs access to a command device on the array.

Do you run SRM on the same server (physical or virtual) as vCenter or not? If vCenter is virtualized, it’s not really a hard question. Provided you have the means to provision the CCI command device via iSCSI, then SRM can co-exist with vCenter. If you can’t do that....then SRM will be living elsewhere.

Personally, I don’t believe that SRM generally warrants its own physical hardware, particularly with the expense of FibreChannel ports and HBAs. But you might be able to co-locate SRM on another existing physical box (may I suggest the physical server housing your vCenter database?). My general preference is to see SRM running on a dedicated VM. To work with the HDS SRA, the CCI command device must be mapped to the physical ESX host and configured as a physical-mode RDM. This means you will be unable to vMotion your SRM machine. Shouldn’t be a show-stopper, just something to be aware of. You can still boot the SRM guest on any ESX node in the cluster to which the command device is mapped (be sure to keep LUN numbering consistent across all ESX nodes that potentially boot the SRM machine).

Recommendation Matrix

	SRM on vCenter server	SRM on dedicated server
SRM virtualized	Provision the command device via iSCSI to an initiator in the vCenter guest.	Provision command device via FibreChannel to the ESX host running the SRM guest. The command device must be a physical-mode RDM to the guest. This blocks vMotion of the SRM guest
SRM physical	Provision command device via FibreChannel to the SRM/vCenter server	Provision command device via FibreChannel to the SRM server. Generally overkill.

There are some other supported ways of solving this problem, including a separate Linux CCI server. But there are a lot of caveats to the other approaches, and I’m not comfortable recommending them for production use.

Architectural Impact of Hitachi SRA