I had to check the Microsoft clustering terminology a few days ago, so I used Google to find the most relevant pages for “Windows cluster” and landed on the Failover clustering home page where the Multisite Clustering link immediately caught my attention. Dreading the humongous amount of layer-2 DCI stupidities that could lurk hidden behind such a concept, I barely dared to click on the link ... which unveiled one of the most pleasant surprises I’ve got from an IT vendor in a very long time. Microsoft actually understands that some people prefer to keep their IT infrastructure stable and supported multi-subnet clusters for quite some time. What a revolutionary concept for the L2-crazed flat-earth world some other vendors are busy promoting.
The details of Microsoft’s multisite cluster implementation made me smile (and some people know that’s a tough call) – cluster resources register their A and AAAA records with DNS (yes, contrary to some networking vendors, Microsoft does support IPv6 in the data center) and there are two property settings that you can fine-tune: the DNS TTL and the registration of all (or just one) IP address.
The failover between subnets is completely controlled by DNS – not suitable for web servers due to browsers’ DNS pinning, but let’s hope your application architects know a scale-out approach is a better fit for a web server farm than a failover cluster. However, the DNS-based failover is a pretty good fit for other back-end services, for example the SQL server (don’t forget that a service needs to be restarted in a failover cluster and the restart time is significantly longer than the DNS failover time). What Microsoft does is almost exactly the same concept I was recommending in my Data Center Interconnects webinar; the only difference is the implementation method – they use their own DNS, I recommended using local and global load balancing.
Assuming more OS vendors see the same light Microsoft did, the shiny new L2 DCI technologies just might land where they belong – the same technology graveyard where LANE and ATM-to-the-desktop are lovingly preserved. Oh, and whenever the application people or server administrators tell you they need L2 DCI for Windows failover clusters, tell them Microsoft solved their problem over a year ago in Windows Server 2008.