Saturday Oct 25, 2008

Digital Photographers Beware: Flash memory is not as reliable as you may think

Using flash memory cards for long-term photo storage is a really bad idea. I didn't realize just how bad until I heard a recent talk on flash technology, which included reliability statistics for the two main varieties of flash, MLC and SLC.

Before sharing the numbers, you need to know that flash memory reliability is generally measured by two quantities: endurance and retention. Endurance measures the number of write cycles each sector of the memory can handle before serious or fatal errors begin to occur. Retention measures how long a flash memory part can be expected to hold its data reliably before attempts to read the data will fail. Of course this is all statistical, but nonetheless the numbers will give you a rough idea of the lifetimes of these memory devices.

Multi Level Cell (MLC) is the consumer-grade version of flash memory. It has an endurance of about 1000 write cycles. Think about how often you fill and erase your memory cards to decide if this number bothers you. If you are like me, you tend to keep and use memory cards for a long time, which means 1000 cycles is not a very comforting number. As an aside, I've noticed that my Canon G9 lets me erase a memory card with either a standard formatting operation or what is called a "low-level formatting." Since the low-level operation takes considerably longer than the standard version, I suspect it erases every sector on the card, which imposes unnecessary wear, i.e. more cycles are consumed from the 1000-cycle budget for these memories. From now on I'll be using the standard formatting option, which I suspect only affects metadata blocks. If I recall correctly, my Canon 10D just offers one formatting option. It is very fast, so I am guessing it does not erase every data block on the card.

Let's now talk about data retention.

Some photographers I know have decided to use flash memory for long-term photo storage since cards are getting so cheap and the form-factor is small and convenient. Really, really bad idea. The retention statistic for MLC parts is a mere 3-4 years. And, worse, retention and endurance are not independent: flash memory that has been used for many cycles can have a significantly reduced retention time.

The characteristics of Single Level Cell (SLC) are considerably better, but you still probably should not use these devices for long-term storage. While endurance for SLC is about 100,000 cycles, data retention is about 10 years. A decade is good, but not for archival purposes. And don't count on 10 years if you have heavily used the card---in which case, retention time can be considerably shorter.

While clearly SLC is more appealing than MLC, it is about 4X more expensive than MLC and about half as dense. I am hoping that the so-called "professional" flash cards sold as digital film are built using SLC, but I have found no way to determine that from vendor websites. If anyone has any concrete information on the products made by the major flash memory card vendors, please share.

Photography aside, I think it is safe to assume those ubiquitous USB dongles that are given away at conferences and others events are made with MLC. Keep than in mind when deciding what you store on these devices and how long you need it to be recoverable.

For an interesting discussion of retention and endurance along with several examples of how to determine suitability for use in several non-photographic circumstances, see this document titled Practical Guide to Endurance and Data Retention [PDF].

And to find out who is working to create ultra-reliable SLC flash memory parts, read this short article.


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Josh Simons

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