In an earlier posting, I noted that Solid-State Disks (SSDs) have certain behavioral differences from traditional spinning disks. One of these has to do with the speed of disk writes.
In a spinning disk, there’s very little difference in speed for a write to a block that is empty versus one that holds data. With SSDs, however, writes are often significantly faster to so-called “pre-erased” blocks. If a block contains data, many SSDs must first erase it and then perform the write. This is one reason that SSDs often start out with blazing speed, but then slow down over time, as they fill with data. (Some newer SSD designs obviate the need to erase before each write through “garbage collection” routines that perform pre-erasing “en masse.”)
In order to maximize longevity, SSD controllers may also modify write locations (you may have heard the term wear-leveling) so that the same locations aren’t reused over and over. This can also reduce SSD performance over time, and it’s another characteristic of SSDs that has no parallel in the world of spinning disks. One would expect that as the technology improves and SSD duty cycles increase, the need for wear-leveling will decrease, so this aspect of today’s technology may not persist forever; but for now, erase-before-write and wear-leveling are two more ways in which SSDs differ from spinning disks.
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Glenn Weadock is a longtime instructor for Global Knowledge and teaches Windows 7, Server 2008, and Active Directory. He has recently co-developed with Mark Wilkins two advanced Server 2008 classes in the Microsoft Official Curriculum. Glenn also consults through his Colorado-based company Independent Software, Inc. and is technical director of MarketCoach Investment Education Software LLC.
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