Compared to normal hard disks the data on an SSD can’t be overwritten easily. When deleting data on a disk, the file system will only delete the files from the index, but not the actual cells containing the data since this would take some time. With a hard disk it is no problem to overwrite a cell, but not with an SSD. To write new data, the cell needs to be erased beforehand. If this would be done just before writing new data, the erase process would take additional time, making the write process quite slow. This is where TRIM comes in. TRIM is a command implemented in the OS and erases cells which are no longer in use. This happens when the SSD is idle, so it doesn’t slow down anything. If this would not happen regularly the SSD would become fragmented over time and it would get remarkably slower.
But not only TRIM is taking care of this, there also is Garbage Collection. Garbage Collection is not provided by the OS but by the SSD’s firmware. What Garbage Collection does is a little more complicated from TRIM, but the outcome is the same. Not indexed cells are erased in the background so they can be rewritten. A good Garbage Collection makes TRIM nearly obsolete.
Apple only supports TRIM for their built in (branded) SSDs but not for third party hardware. This is intended, but we don’t think out of viciousness but for a reason. The TRIM command needs to run accurate and the communication between the OS and the SSD happens in the range of nanoseconds. This is quite critical and needs to be maintained squeamishly. If not, the risk is that the SSD overheats constantly from “running too fast”, which will shorten its life span and lead to failure.
The are various TRIM-enabler-tools out there, but we don’t recommend any of them. This has several reasons: first of all the communication between the OS and the SSD needs to be very accurate as mentioned above. Secondly the Garbage Collection of our recommended SSDs works like a charm and our experience goes back some years now. No Intel SSD we worked with ever became slower, without using TRIM. As well Samsung has a totally new and well written Firmware in their new 830 series. Third the TRIM enablers floating around on the web are discussed controversial and we doubt their use. One of them is a kernel patch that replaces files with old versions and needs to be patched again with every update of the OS. Since the functionality of the OS relies on the interaction of all files, this could be counterproductive in another place. To keep it simple: this is working on the open heart of your computer by a remote manual by someone you don’t know. We would not want to do that.
All our recommended SSDs have a highly developed firmware with solidly functional garbage collection. This is enough to keep the SSD running fast and with high performance for a long time and makes the TRIM command almost unnecessary.
Since the SSD is the main hard drive of your work horse, we recommend spending a few euros more to buy a high-quality SSD rather than buying a cheap one, that may reveal its weaknesses during time. With those you won’t need TRIM at all on OS X.