Thursday, August 27, 2009

Clustered NAS or Scale Out NAS?

Clustered or Scale-out NAS? Let us look at what each of these terms mean and how they would relate in a NAS environment and the impact. Historically clustering has always been done on servers - with Vertias (or any other product) Clustering setting up a NFS share and serving out clients. Then, with NetApp coming into NAS market, started its own file servers and then advanced them to clusters. In the recent few years, another technology - scale-out NAS is becoming popular.

As it goes with the name, clustering implies 2 controllers in an active-active or active-passive configuration. The main advantage of this is that there is no interruption in service when one of the controller fails; the second controller takes on the functions of the first. When designing clustered NAS, keep in mind of the failure scenario when a single controller need to do the functions of two. It might get overloaded and bring all services to a screeching halt if they are both running at >75% of utilization all the time. The other disadvantage in a clustered NAS solution is that each controller "owns" its storage and thereby the NAS shares. So if you have a single share that needs the performance of both controllers combined, it cannot be done. You will end up using 100% of one and the other doing nothing for that one share which needs more performance. This also implies there is no global namespace for the shares - each share can be accessed by an IP that is tied to one hardware controller.

Scale-out solutions started showing few years ago, and there are a lot of them now in the market. The idea here is to have a horizontal scalability by adding more controllers that can add more performance for that single file share you need. Keep an eye on what you need and what the solution offers as the scale-out solutions are unique to each other. Some scale out solutions are storage-agnostic i.e you can attach anything in the backend as the storage, and they just provide the controllers. This provides more flexibility for an Enterprise if they already have an existing relationship with a hardware vendor they procure storage from. You can also move/migrate from one backend storage to another seamlessly without any downtime. You can leverage the storage virtualization features for better performance, scalability and availability.

The other major advantage of a scale-out solution is global-namespace. The IP space is virtualized for the front-end (which NAS clients use) behind more IPs that are logically tied to the controllers. So any access request can be coming in from one-ip (virtualized backend ip) and leaving through another. This makes it a good candidate to leverage more network ports and utilize all available bandwidth rather than getting stuck with one-controller/1-ip/few-network-ports. And any controller failure, (multiple with some vendors) will not impact client communication accessibility.

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