SSDs: Some Cold, Hard Numbers to Flavor Your Opinions

There’s nothing like cold, hard numbers to shore up an opinion. One of the slides that Jim Handy used in his Bell Micro seminar on solid-state disks (SSDs) compared what he labeled a “capacity HDD” (that’s a consumer-grade hard disk drive), an enterprise HDD (that’s a short-stroked speed demon), and an enterprise SSD. The one graph tells you a lot about how SSDs stack up against “rotating-rust” storage.


Handy HDD vs SSD Graph


As you can see from the graph, an enterprise-class SSD has more than twice the read bandwidth of an enterprise-class HDD and more than three times the read bandwidth of a “capacity” HDD. The enterprise-class SSD bests the enterprise-class HDD’s write bandwith by only 10%, reflecting the slow write/erase cycles of NAND Flash compared to the NAND Flash read latency.

The IOPS line in the graph is slightly misleading. While SSDs may perform 35,000 IOPS for reads, they’re about 90% slower for write IOPS. In reality, there’s a mix of reads and writes in any system, so real-life IOPS performance will fall somewhere between the extremes.

However, focus on the last line of the graph for a moment. This line describes latency and it’s here that SSDs truly shine due to their lack of mechanical components. A “capacity” HDD has a 15-msec latency, mostly due to the average amount of time required to move the read/write head from one disk track to the next. The short-stroked enterprise-class HDD has been restricted so that head seeks are radically reduced. The benefit is a nearly 10:1 reduction in latency, from 15 msec to 2 msec. The cost is a huge loss in disk capacity. Yet many enterprise system designers are willing to pay this cost to get the huge latency cut.

But look at that last number—the latency number for an enterprise SSD. There’s no mechanical head to move. There’s no disk platter to rotate into the right position. There’s just a large array of addressable NAND Flash blocks, all accessible in essentially the same amount of time. Latency is one of the biggest SSD benefits and it is one reason why server architects are beginning to intermix SSDs and HDDs to balance capacity with speed.

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