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Aftermarket SSD On A MacBook Pro: TRIM Gets Tested

TRIM On A Mac: Only For Apple's OEM SSDs

You already know that solid-state drives write data to 4 KB pages. In the process of relocating data, deleting stale data, and creating space for new data, they operate on larger blocks. This is what we refer to as garbage collection, and it's something that every SSD uses.

The trouble is that SSDs and operating systems typically don't have a very communicative relationship. When you delete a file, the OS marks the space consumed by that data as free, but doesn't tell the drive anything. Over time, you end up accumulating a lot of available space (according to the operating system) the SSD believes is still needed. It's only when an OS tells the drive to write to an address containing data that it figures out, "Ah, that information must be invalid." Up until then, those pages are still shuffled around by garbage collection, unnecessarily slowing down the SSD controller and wearing the drive's NAND cells prematurely.

This is where the TRIM command comes into play. When a file is deleted, the operating system sends a TRIM command to the SSD, marking those sectors as invalid data. At that point, the drive knows not to perform garbage collection on them.

Of course, support for TRIM comes from a few different places. The drive, controller, and operating system all need to be compatible. Fortunately, the process of building a TRIM-friendly ecosystem started a long time ago. Windows 7, Windows 8, Server 2008, Linux, OpenSolaris, FreeBSD, and OS X are all capable of the command. 

But just because a combination of hardware and software support TRIM doesn't mean it's always active. In the Windows world, we're fortunate enough that openness and compatibility are usually embraced. But Apple isn't so benevolent. It only allows TRIM to work on its OEM SSDs, and third-party drives installed aftermarket have this feature disabled.

The ways around this artificial limitation aren't secret. In fact, we showed you how to install a third-party SSD into a MacBook Air, turn TRIM on, and benchmark the drive in Upgrade And Benchmark Your 2012 MacBook Air's SSD. Today, we're showing you how to do the same thing using a different SSD and a different method, and then running another handful of tests.