Debunking Utilization As Justification

Posted by Howard Marks
October 29, 2009

If I sit through another presentation or sales pitch that tells me how a product will save me money by raising my storage utilization, I'm going to get violent. Ever since the development of the SAN, vendors have been extracting money from users based on promises that their new shared disk array, virtualization widget or storage management application will pay for itself by increasing the customer's disk utilization. For ten years now, we've spent a lot of money but the savings have been elusive.

Basically the myth of savings from improved utilization is based on two false assumptions.  The first is that a typical enterprise is using 3-30 times as much raw disk space as they have data, because of limitations on the tools the storage administrators have at hand.  

This is the basic "if we put all our data on a single storage array we'll raise utilization" situation:  if each server has its own RAID controller and DAS disks there must be at least a disk's worth of free space on each one.  By using a single large array, all of that free space can be in a single pool and we can allocate it more efficiently.

The fallacy of this argument can be demonstrated by several studies showing that organizations moving from DAS to shared disk arrays don't typically see huge changes in utilization. Using conventional arrays still builds multiple RAID sets with different service levels, so they don't have the single shared pool they dreamed of. And, of course, they are still over provision.

The other problem with this purchasing argument is that the cost per GB of the new system is sometimes significantly higher than on the old. Adding a 3TB SAS JBOD to a database server would cost around $5/GB from Dell or HP. The same capacity in a EVA, Clarion or similar midrange shared array would cost at least $15/GB once FC connectivity is factored in.  At that rate we'd have to use 1/3 as much disk space just to break even.  Even worse, I've heard sales pitches where vendors were telling the CIO that higher utilization would create savings over the storage they had already paid for.

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