Why no defrag on SSDs

Sampsy · 6 May 2011 at 22:23

I have heard a lot about how you should not defrag an SSD as it can damage it. Why is this? I would appreciate it if somebody could enlighten me.

rwm24 · 6 May 2011 at 22:25

Solid state drives are based on flash memory, which age very differently than standard spinning platter-based hard drives. Each sector has a limited number of write cycles, so the drives include wear-leveling features that allows the drive to last longer by essentially fragmenting the drive on purpose.

The read times are not greatly improved by defragmenting as they are in standard drives, so there is no real gain in defragmentation, but by doing so, you are increasing the number of writes to the drive, thereby decreasing the drive life.

Defragmenting puts your sectors near each other which is helpful for a disk that spins around. However, this is useless on an SSD which has constant access time for any sector. Defragmenting just causes extra writes on a disk (SSDs have a limited number of writes due to their design)

http://superuser.com/questions/1594/why-cant-you-defragment-solid-state-drives

Jokester · 6 May 2011 at 22:25

It wears the drive out and since there is no seek time it doesn't matter if data gets framented on the drive.

Phil99 · 6 May 2011 at 22:26

The time required to access data is the same across the entire SSD so files don't have to be stored sequentially for good performance.

Harddisk access times are much slower (~9ms vs ~0.1ms) and vary across the surface of the disk, so it's beneficial to have files arranged sequentially to avoid having to move the read head all over the disk to read a file.

23JMZ07 · 6 May 2011 at 22:27

SSDs have such negligable random access times (it takes them no time at all to seek for data) that it is pointless to defrag. Since they can find all the files needed so fast there is no need for the data to be structured in order. Also defragging requires many read and writes which is harmful to SSDs and shortens their lifespan.

DragonQ · 6 May 2011 at 22:29

Also note that your data won't be where Windows thinks it is anyway - SSDs have "wear levelling" that puts data in the least used NAND cells first. So even if you, for example, delete a file and then restore a backup of it immediately, it'll probably be put in completely different cells compared to the original. If Windows thinks a file has no fragments, it could in reality have loads.

Sampsy · 6 May 2011 at 22:30

Thanks for the info everyone, that clears things up :-)