How reliable will your flash or solid state storage device remain over time? This is the question I'll be exploring here, along with the basics of how these devices actually work. (I use the terms "flash" and "solid state" interchangeably.)
Your flash-based device will eventually die. And, it won't die a slow, horrible death like some hard drives. In most cases, the device will work one day and then, all of a sudden, it won't work anymore. The most common cause of flash death lies in the Achilles' heel of flash-based storage — limited write cycles. That is, each cell or block of a flash-based storage device can be written to only so many times before "wearing out." After enough erase and write operations, the insulating oxide layer around the cell breaks down to a point after which the cell is unusable.
Wear leveling is designed to spread erase and write operations out over the entire flash device rather than focusing on one specific area. For example, suppose you have a flash device on which you've stored a few dozen documents that you change on a regular basis. Without wear leveling, each time you modified your document, the data would, theoretically, be written to the exact same spot on your flash device. This write operation would, before the write actually happens, necessitate that the storage block be "flashed" or erased before the new data can be written to the same spot. This constant barrage on the same sectors will eventually lead to the failure of your flash device.
Wear leveling is a process designed to "spread the wealth." That is, instead of constantly writing information to the same locations over and over, erasures and writes are distributed across all blocks of the device, thus making sure that no single cell is constantly assaulted.
Most flash-based storage devices are rated for anywhere from 10,000 to 1 million write cycles although I've seen write cycles as high as 2 million, too. With wear leveling, your device will last a lot longer.
What happens when a cell does eventually fall into the bit bucket and become unusable? Some flash-based storage devices have spare sectors that can be brought into play to replace dead ones, thus extending the useful life of your storage device. When a cell fails, the data is written to one of the spare cells.
Flash-based storage uses ECC (Error Correcting Code) to prevent single-bit errors from laying waste to your data. There's not too much more to say about this data protection feature!
No moving parts
I'm not focusing only on what methods flash designers have implemented in order to protect flash storage. There is one feature inherent in flash-based storage that lends itself to reliability: the lack of moving parts. Moving parts create friction, increase heat output, and, in general, introduce an element of mechanical instability into a system.
- No worries about platter damage.
- Shock and vibration issues are non-existent.
- Able to withstand a greater temperature range since the moving parts aren't subject to expansion and c contraction. This, of course, doesn't mean that you should put your flash drive into a fire, but it can probably stand the heat a little more.
One of the people I work with recently put his flash drive through the dryer... literally. The plastic is somewhat melted, but the device still works! That's resilience!
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