* Posts by Doug Dumitru

1 post • joined 19 Sep 2007

Intel slobbers over flash-based server storage

Doug Dumitru

Drive Wear

Mr. Stephens,

You misunderstand how Flash drives write and how they wear out. A flash drive rated at 100,000 write/erase cycles can have an "erase block" updated 100,000 times. This erase block is usually 1 megabyte or larger in size. So on "bare" flash chips, you could kill the erase block by writing to it 100K times.

This is where the flash controller steps in. First, the controller keeps spare erase blocks (usually around 2%) and will use these if an erase block fails. Second, the controller will watch for erase blocks that are heavily hit with writes and then move them around. This is called wear leveling. With good wear leveling in place, you have to write the entire drive 100,000 times, not just one location 100,000 times.

Thus to estimate drive wear out, you are better off to use the write speed of the drive and see how long it will take to kill it. With a 32G drive, 100,000 write cycles, and 40 MB/sec write speed, the result is:

32,000,000,000 x 100,000 / 40,000,000 = 80,000,000 seconds

This is 2.5 years of continuous writing. In anything other than a saturated data logging application, this drive will outlive the server.

Now drives did used to wear out. The difference now is that the drives are a lot larger and also last longer. A 64 MB drive with 10,000 write cycles and a 10 MB/sec write rate can be killed off in in 17 hours.

Some vendors quote 5,000,000 write cycles. With these the wear out times really start to get silly.


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