Wednesday, 18 October 2006

Cisco, Microwaves and Virtualisation

So I've not posted in October so far and the month is half over. To my defence I spent a week in South Africa doing an equipment (Cisco) installation. No it wasn't a holiday but I did see the fantastic table mountain, which is spectacular and well worth a visit.

Part of the installation involved deployment of SRDF between two symmetrix arrays using Cisco 9216i switches. These have two IP ports and can be used for iSCSI or in this case FCIP as we had to provide the SRDF links across IP. Whilst this all sounds fine, the IP link was in fact over a microwave connection rather than a fixed line installation. Telecoms are prohibitively expensive in SA and sometimes microwave is the only option due to the high cost of fixed line installations.

Installation and configuration of the links was simple, including enabling SRDF. However my big concern is performance (I should note that this is no way a synchronous implementation, but an async one). The IP link is not dedicated to storage and being used for other purposes so fcping responses from the Cisco switches were very variable. At this point we haven't fully enabled SRDF but the next stage is to test performance of the link. As I see it, we need to monitor SRDF stats with ECC Performance Monitor, SRDF/A response times and the lag time of unwritten I/Os not committed to the remote site. The solution needs significant monitoring to ensure all links are active; with SRDF it is possible to monitor the status of a remote SRDF link using the "symrdf ping" command; this needs to be being issued reguarly if not every 5 minutes or less. More updates as I have them.

I spoke to an analyst today (it's Storage Expo in the UK) on the subject of virtualisation and in particular HDS' Universal Volume Manager. He was asking my view on where virtualisation is headed. I still think the switch is the right place, long term, for virtualisating the storage infrastructure. In the short term though, discrete hardware array virtualisation is a good thing. UVM can provide cost savings (on significant virutalised volumes of data) and additional functionality, however HDS will have to up their game to retain the virtualisation crown as time progresses. Specifically they need to address the issue of a failure in the USP when so much storage could be dependent on one subsystem. A clustered solution could be the answer here. Time will tell.