Do you need to defrag SSD drives?

Some people seem to be unsure as to whether super fast SSDs - Solid State Disk drives - need to be defragmented. They know know disks become fragmented over time and that this slows them down, and that defragmenting them speeds them up again. So why won't Windows defragment the SSD in their PC?

In a nutshell, SSDs don't need to be defragmented. To understand why, you need to know how solid state disks and old style mechanical disks work.

wdfDesktop_Blue_6GbAbove is a very crude drawing of a disk drive, while on the left is the real thing - a Western Digital drive with the cover removed. My crude drawing shows how a disk is divided up into circular tracks and each track is divided up into sectors. The disk spins and the read/write head moves back and forth to read files that are stored in the sectors.

The three red areas show a single file that is fragmented. It is stored in three sectors that are widely spaced. When this file is read from the disk, the disk has to spin around until the sector is near the read/write head, the read/write head then moves in and out to position itself over the sector. It can then read the first part of the file. To read the next part, which is in a different sector, the read/write head again waits for the disk to spin around and then moves in or out as required to position itself over the second sector so that it can read the data. It then goes through the whole procedure again to get the third and final part of the file in the remaining sector.

All this spinning of the disk and moving of the read/write head in and out to position itself over a sector takes time. It is much simpler if the three sectors follow each other on the same track and it could position itself over the first sector and read it, then without needing to move, it could read the next two sectors as they pass under the read/write head. Three reads - the whole file - in one go. That's why defragmenting speeds up the disk drive and the computer feels so much snappier.

An SSD does not have any moving parts and data is simply read from memory chips. Whether the memory locations are next to each other or far apart makes no difference. No time is wasted reading from a memory location far away because there are no mechanical parts to move to go and get it. If a file is split over three locations, so what? Reading from one memory chip is no different from reading from another.

The speed difference between a fragmented SSD and a defragmented one is so small you can't measure it, if any difference exists at all. The memory chips used for storage in an SSD can only be read/written a certain number of times, so it is important not to waste read/write operations needlessly. This is why Windows will not defragment SSDs. 1) It makes no difference. 2) It wears out the drive.

(Disk drives are a bit more complicated than I've described here, but the principal is the same.)

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