Tuesday October 10, 2006

A Solaris engineer and I got to talking last week about whether an operating system needs a wear leveling file system if it is going to run on a flash memory device. There are several such file systems for Linux that are used when that operating system is run in a small, embedded environments that use such media. But is this necessary or might it be the case that modern flash memory cards perform wear leveling internally?

For those not familiar with the term, a wear leveling file system is used on media like flash memory that has a limited service lifetime--parts of the memory can wear out if read or written too many times. Wear leveling works to spread data across the memory device to reduce usage hotspots and extend the useful life of the device.

The answer to this question--whether flash memory devices perform wear leveling internally--should be of direct interest to consumers. Think about your digital camera. If neither the camera nor the flash memory card are doing wear leveling, are you prematurely aging your memory card if you don't fill it completely before transferring your photos?

I was interested enough to write to three memory card manufacturers and ask them whether their products support wear leveling internally. I wrote as a consumer and not as a Sun employee. One answer was surprising.

Lexar and Kingston both told me their products support wear leveling. Lexar even sent me a short writeup describing several additional methods they use in their controller to increase reliability and product lifetime.

The odd man out was SanDisk. Their technical support people told me "that information is proprietary and cannot be disclosed." I asked them to check whether this was really their response and confirmed that this was indeed their answer. Personally, I'll take that as a "No" when making future memory card buying decisions.

Update as of April '07. Thanks to GlitchC for posting a pointer to a 2003 SanDisk whitepaper describing their wear leveling technology. As a further update, I'm pretty sure I know why SanDisk was being evasive last October. According to this press release, SanDisk completed its acquisition of msystems in November, about a month after my posting. Among other capabilities, SanDisk seems to have acquired a more sophisticated wear leveling scheme than that described in their 2003 paper.