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Flash disks' reliability

Uwe asks about fast, reliable, not noisy storage mechanisms - And yes, of course, he talks about Flash memory’s well known limitation: The relatively low number of write cycles each area of the memory is known to be able to withstand. I recently told this same argument to a friend who was enthusiastically telling me about his dream project of setting up a computer based exclusively on solid-state storage. Of course, he didn’t like the limitation. But after reading a bit (sorry, I don’t have the URLs at hand - but Google is a good friend), it seems that many (most? all?) controllers work around this limitation by rearranging the most used blocks around, so that the Flash gets as evenly used as possible. Quoting from Wikipedia’s article on Flash memory:

Another limitation is that flash memory has a finite number of erase-write cycles (most commercially available flash products are guaranteed to withstand 1 million programming cycles). This effect is partially offset by some chip firmware or file system drivers by counting the writes and dynamically remapping the blocks in order to spread the write operations between the sectors. This technique is called wear levelling. Another mechanism is to perform write verification and remapping to spare sectors in case of write failure, which is named bad block management (BBM).

So, Uwe, just check the media you get supports wear levelling technology, and you should be safe.


Ben Hutchings 2007-03-20 03:42:17

Re: Flash disks’ reliability

It seems hard to find out what does support wear-leveling; it’s not a commonly advertised feature. There are patents on it and at least one manufacturer has been sued and enjoined from using them.

Mark Brown 2007-03-20 03:13:40

Re: Flash disks’ reliability

The trouble with relying on wear leveling is that you’ve got to trust the quality of implementation of the firmware on the device. Frequently that’s quite low (think of all the trouble people have with ACPI implementations…).

Uwe Hermann 2007-03-19 19:40:14

Re: Flash disks’ reliability

Jep, I guess almost every flash device you buy nowadays supports wear-leveling. I don’t have any numbers about how much effect this has on the expected life-time of the device, though. It sure helps, but is it enough to make it usable 24/7 for a few years? I tried to find some studies or reports about stress tests, but I couldn’t find something good so far…