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Flash Memory

From OCAU Wiki



Flash memory is solid state memory that retains the information it stores, after the power has been turned off (Non-Volatile RAM, CompactFlash (CF), SD cards etc). It is often used in cameras and embedded devices because it is more rugged (no moving parts) and hence requires less power (no disks to keep spinning).

CF cards are reaching storage sizes of 8Gb or more, with increasing better data access and throughput speeds.

Pros / Cons

This type of memory has the benifit of having significantly reduced data access times (random or sequential), as the storage is not on a spinning disk. However currently hard disks provide greater data throughput/transfer for chunks of data once it is accessed (if the hard drive is accessing it sequentially from a disk).

Flash memory is not affected as much by filesystem fragmentation (perhaps making it a solution as a swap partition *unproven claim, feel free to find evidence/tests to back that up*)

Cons - Wear

All flash memory available is subject to wear. Each block on the device is only rated for so many writes/erases, before it wears out and is no longer able to write/erase data from that block. Most CF cards will have advance wear-levelling algorithms that spread the write load across the entire flash memory or across sections called zones (SanDisk). This is done automatically and transparently for you in the hardware.

If you plan to use CompactFlash or other flash memory as a storage device or in a product, you need to consider this before selecting a brand of flash media, as different brands have wildly different wear levelling schemes and flash memory MTBF.

Home enthusiasts need to understand this trade off between added speed, but limited writes (some CF can be rated at over 3 million writes per block, and with average usuage that might work out to 100 years of writing). Wear can depend on the following factors

  • the amount of free space
  • wear levelling scheme
  • average number of writes occuring
  • average number of Kb written per second
  • filesystem selection / overheads


Note: There are also many resellers packing generic flash memory into thumbsticks and selling them very cheaply (eg: Astone) The quality of these devices varies greatly, and thumbsticks should be treated with caution if you are using them to store critical data. Brand and build quality can mean the difference between "device IO errors" two days before your assignment is due, and no problems.

See also Fake Memory Cards.


It is also recommended that devices are removed / umounted correctly. Linux 2.6 does not currently support hotswap ejecting with USB thumbdrives or PCMCIA CF, they should be umounted properly. I am not sure about Windows but I seriously doubt it as I have seen file corruption on thumbdrives as a result of pulling them out without first stopping the device/filesystem.


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