Storage Informer
Storage Informer

A cluster of challenges

by on May.14, 2009, under Storage

A cluster of challenges

How scalable is a cluster of cache controllers when you need performance?
The initial description of V-Max, describes it as a cluster of two controller systems that can in turn cluster to very large configurations. The challenges of a cluster of storage controllers is maintaining cache consistencies.

In modular storage arrays with two controllers, LUNs are assigned to one controller or the other to avoid thrashing between the controller caches. The Hitachi AMS 2000 avoids this by switching the I/O request to the controller cache that contains the data and load balances the activity between the two caches.

So perhaps the V-Max is doing what the AMS 2000 has been doing for some time. That works for one V-Max node of two controllers. What gets harder to do is cluster multiple V-Max nodes and still present a global cache data image. Namely, have an I/O come in from one V-Max node and be routed to another V-Max node where the data resides in cache. There needs to be some sort of locking for cache consistency and some sort of arbitrator to tell which node has the correct status if the two of them become out of sync. That takes overhead if you have to go over an external wire instead of a backplane. Performance will be great if you happen to access the node where you data resides, but there will be a performance impact if it doesn’t.

If you happen to be going to EMC World next week, see if you can find out how this is done.


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