Do SSD drives slowdown when near full?

[ImaNihilist]

typically you don't want to fill your SSD to more than 75% capacity or it will slow down a bit, garbage collection needs a bit of room to work right...that way the nand cell usage can be spread over the whole drive so one part of the drive will not die before another

regular spinning HD's (especially the main OS drive.partition) shouldn't exceed 85% or you will run into fragmentation/defragging problems....

This is not true.

A spinning HDD sees performance degradation for every additional bit written, because of the distance you have to travel across the platters. Once you go over 50%, it will really start to feel a lot slower than it was when it was new. In some cases, 50% capacity means a 25% performance penalty. A good drive will see a 25-30% reduction in performance around 90% capacity. Either way, you see a marginal decline for both reads and writes with every bit written.

In theory, an SSD at 10% capacity and an SSD at 90% capacity should be the same speed, at least with respect to read performance. They will even have the same write performance the first time you write to the entire disk. The problem with an SSD comes with "random writes" and fragmentation. If all your writes are sequential, in nice little blocks, you won't really see any performance degradation until the entire drive has been written to once. The performance problems with SSDs start when you have to re-write, and when you do it in a non-sequential way.

As kpxgq pointed out, this really isn't a problem for most people. In fact, it isn't a problem AT ALL until you fill the drive at least once. TRIM is useful, but not necessary unless you are "deleting" a lot things. When you delete something in your OS, the file isn't actually deleted—the markers are just removed that indicate the file exists. On an SSD a file that has been deleted still takes up space on the SSD—the cell is never "emptied", just looked over. When you go to re-write, the SSD has to look around and see if there are any empty cells. That takes time. If there aren't any empty cells, it has to empty the cell first, then write, which takes more time. With TRIM, when you delete a file at the OS level, the computer will tell the SSD, "Hey, you can empty that cell now." That way when the computer goes to write, it sees empty cells instead of full cells that need to be emptied and then re-written. The controller also keeps track of these blocks so it knows where they are.

The problem that TRIM solves is a problem that most people don't have, TBH. Most people will never write the full capacity of their drive, let alone write and re-write. If you are doing video editing TRIM can be quite helpful. It's also helpful for applications like Steam which are constantly updating (deleting, re-writing) large amounts of files.

The biggest problem with SSDs is how we interact with them at an OS level. We still use this idea of "free space" and "used space", but with an SSD that isn't really how it works. Instead there are three states: full, not in use, and empty. The middle state, "not in use", is not something that the SSD inherently understands—that's where TRIM comes in. It would be helpful to see this at an OS level. Knowing how much "empty" space you have (and where it is) is really the most important thing in determining write performance. Most controllers are pretty good about managing this kind of fragmentation in the background, but you can still get some slow downs in write performance in you have large blocks of "not in use" space that needs to be rewritten to. You really only get read performance slow downs in extreme fragmentation situations. I've never encountered them in the real world. I suppose you would have these kinds of problems in a database which is writing lots of small files all over the place, but it's unlikely to encounter a scenario on the desktop where a modern SSD is going to experience massive read performance degradation.

For a more thorough explanation of exactly what goes on with an SSD, see here: http://www.anandtech.com/show/2738/8 The whole article is worth reading, but that page in particular highlights the main issue with SSDs.