Turns out that some people are dubious that Exchange 2010 and 7.2k drives are a good fit. So I’m sometimes asked to justify spending less on storage for Exchange (a seemingly odd, but ultimately honest position to take).
Up until a short while ago, I just trotted out Microsoft’s graphs regarding the IO per “heavy use” mailbox they see in production. It shows a 10x decrease in IO per mailbox between Exchange 2003 and Exchange 2010.
This can be unconvincing to some of the folks who fought Exchange 5.5, 2000, and 2003 in the trenches, where it was common knowledge that not only did you put Exchange on the fastest storage available, you often had to allocate more disks than you needed for capacity to meet the performance demands of the application. A 10x reduction in IO workload doesn’t convincingly take you from “put it on the fastest storage you can afford” to “put it on the slowest storage you can buy”.
The story is actually two-fold – at the same time that IO per mailbox was being driven down, the size of the mailboxes were being driven up. Since storage sizing is and always has been a balance between performance (how quickly you need to move data on and off the drives) and capacity (how much data you need to store on the drive), IO density (or IO per GB) is far more relevant than IO per mailbox. In short, we don’t just look at how many IOs Exchange is driving per mailbox, we have to factor in how big those mailboxes are as well. Typical mailboxes were only 50MB-100MB during the Exchange 2003 days. Nowadays they’re between 10 and 100 times that. When we do factor in mailbox sizes, it turns out that IO density isn’t 10 times lower with Exchange 2010, it’s orders of magnitude lower in most modern Exchange configurations.
I’ve come up with a graph to illustrate this notion.
So while we saw “only” a 10x decrease in the IO per mailbox, we saw over a 400x decrease in IO per gigabyte.
Now let’s take a look at the IO density that spinning disks have offered throughout the years.
Clearly these are simple examples – the mailbox IO was far more variable in the 32-bit versions of Exchange, and the IO/GB for the disk drives doesn’t include RAID overheads, there’s no free space factored in and so forth. But when we interpolate the IO/GB driven by mailboxes and the IO/GB provided by disks, it matches up pretty well with the typical disk topologies I recommend:
In the Exchange 5.5 to 2003 days, every IO counted, and you had allocate more disks to the Exchange workload than you needed for capacity. Things got a lot better in Exchange 2007, where you could place your average mailboxes on either smaller 10k FC drives or larger 15k FC drives. With Exchange 2010 and large mailboxes, the sweet spot for the typical deployment is somewhere between 1TB and 2TB drives.
Now this isn’t to say that 2TB or 1TB drives are appropriate for all Exchange deployments. A large number of factors go into sizing storage for Exchange. But if you’ve done your homework and factored in all the variables, and it looks like the slower drives are the way to go, then you should trust your sizing and save your organization some money.
