Using SSDs with Exchange Server

In my previous UPDATE column, I made some predictions for 2011. It's of course too early to tell whether any of them have come true or not (though it's really interesting to look at this Bloomberg article about analysts' predictions for iPad sales and see how many of them had absolutely no clue), so I figured I'd talk about something else instead: solid state disks (SSDs) and Microsoft Exchange Server.

First off, you should know that the SSD market is in the grip of Moore's law: prices and capacity are changing rapidly. As with the old days of electromechanical disk drives, the highest-capacity SSDs carry a stiff price premium compared to functionally identical, but smaller, units. Why? Because the manufacturers can get away with it. When you consider all the other areas where storage—and, indeed, other kinds of components—have become commoditized, it's hard to blame them. If you invest in SSDs for your Exchange deployment, you should know up front that your equipment will almost immediately be outpaced by new models that are less expensive and have higher capacity.

Worse still, here's what Microsoft has to say about SSDs in the "Understanding Storage Configuration" section of the Exchange 2010 SP1 Help files: "In general, Exchange 2010 Mailbox servers don't require the performance characteristics of SSD storage." With that kind of endorsement, you can certainly be forgiven for wondering why anyone would buy SSDs for use with Exchange. It's worth digging into the underlying reasoning, though, to better understand where SSDs might (and I emphasize might) be useful to you.

When most people think of SSDs, they think of speed. But the speed of SSDs isn't evenly distributed. Wikipedia's article on SSDs drolly notes that "performance of flash SSDs are difficult to benchmark." In general, SSDs have very high read speeds, and many of them offer faster sequential writes than equivalent electromechanical disks. However, their random I/O performance is often not that much better than conventional SATA or SAS disks. Guess what? Exchange 2010 still depends heavily on random I/O; around 40 percent of typical I/O is random. That means that you'll get little or no benefit for up to 40 percent or so of your I/O operations if you put your mailbox databases on SSDs—and you'll pay through the nose for enough storage to actually hold your databases. However, if you need fast sequential storage—say, on a server that generates a very high volume of transaction logs—the price penalty could be worth it.

SSDs also have the benefits that come with not having any moving parts: They are essentially shockproof, and they can tolerate a much wider range of environmental conditions than traditional disks. These factors might argue in their favor if you're putting an Exchange server on a ship, space vehicle, or oil rig, but then the problem you're likely to face is the reliability of the rest of your servers.

Don't get me wrong; I'm a big proponent of SSDs for desktop systems. I use one on my primary desktop machine and it's terrific, cutting boot times by more than 50 percent and generally making the system a great deal more responsive. For laptops—and, I suspect, for Exchange servers—the sweet spot right now is in so-called hybrid drives, which combine a large (4GB to 8GB) cache of flash RAM with a conventional electromechanical disk. The large cache provides excellent I/O performance without the high cost or limited storage space of a pure SSD. I think we'll continue to see hybrid drives nosing their way into Exchange deployments. In the meantime, you're probably better off saving your money and continuing to buy conventional disks for Exchange use.