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Solid State Drive Buyer's Guide

Drive Tech Compared

So in one corner, we have hard disks with several critical moving parts, and in the other corner we have SSD with absolutely zero moving parts. All an SSD has to do is move electrons around. As you might expect, this has serious ramifications on several fronts. First off, there’s the time it takes to access any given bit of data within the drive. Even a late model, high-performance unit like Seagate’s Momentus 7200.4 notes an average “latency” of 4.17 milliseconds (ms). The latest Intel X25-M drive (using 34 nm NAND, see below) specifies 75 microseconds. You’ve got three decimal places between milli and micro, so 75 microseconds is 0.075 milliseconds, or a 55X difference between the two access times. When you’re dealing with lots of little data seeks, those gaping latency differences add up in a hurry.

Aside from that, SSD is often just flat out faster. Even the chart-topping Momentus 7200.4 tops out with an average sustained transfer speed around 80 MB/sec. Meanwhile, the latest SSDs can easily average more than 250 MB/sec on streaming read throughput. Now, when it comes to writing to the drive, SSD loses a lot of its advantage. SSDs still remain generally faster than HDDs on writes, but there are plenty of break-even cases as well as applications (video editing for one) in which hard drives keep an advantage.

As you might suspect, "reads" refer to reading data from the drive, copying it from the drive into system memory, while "writes" occur when writing data from the system out to the drive.

Because there are no moving parts, SSDs consume very little power. A power-efficient notebook drive such as Western Digital’s Scorpio Black draws 0.2W in standby mode and 2.1W when active. Intel’s latest X25-M SSDs consume only 0.15W when active and a nearly comatose 0.075W when idle. There are ways to minimize hard drive power consumption, such as running in at slower rotation speeds unless more performance is needed, but this is almost splitting hairs. SSD simply consumes less power, and that can translate into longer battery times in notebooks.

With less delicate innards, SSDs are also more rugged than their HDD counterparts. Take a unit such as Super Talent’s UltraDrive GX, which, like many modern SSDs, sports a shock resistance of 1500G, 0.8 ms. A 1500G shock is roughly equivalent to a 27-foot drop, and the 0.8 ms notes the time delay between shock pulses. In contrast, a laptop HDD such as Hitachi’s Travelstar 7K500 boasts a shock resistance of 400G during operation, meaning while the drive is in the process of reading or writing. This is the number that’s important for times when a heavy object might accidentally fall on the system while it’s active.