Q: I plan to buy a solid state drive (SSD), which is basically flash memory packed into a hard drive case and accessed like a hard drive (through SATA, IDE, whatever). I have heard that it is bad to defrag a SSD. Is that true, and why is that so?

A: The "random access time" of an SSD is so fast that "defrag" it is pointless.

On a regular HD, the head have to move from track to track. On an SSD it's instantaneous (well, just about). Thus, defrag is pointless. If there is no appreciable "gap" between blocks of data, then defrag is not necessary. After all the idea of defrag is to move the data so it can be read as a contiguous segment as possible, so the read head doesn't have to jump around to get the data.

Also, on flash memory, each cell can only be read and written X number of times. (that X is actually in the millions) In order to improve reliability, flash drives (and SSDs) have what's called "load levellers" that purposefully SPREAD OUT the data so each cell is used equally as much as possible. This actually fragments the data. However, such is NOT a concern on SSD, as stated before.

Thus, if you defrag an SSD, not only you will NOT see any gains, you will actually defeat the load levelling feature, increasing those frequently used cell's chance of failure, and thus, decrease reliability of the entire drive. Of course, you may be shaving off only fraction of a percent in reliability, but why borrow trouble?