Inside Mac OS X 10.7 Lion: Apple adds SSD TRIM support

By Daniel Eran Dilger

Apple has added limited TRIM support to the latest developer release of Mac OS X 10.7 Lion, which should enhance the long term performance of Solid State Drives.

While Apple has already started incorporating SSDs into its Macs, it hasn't yet added TRIM, a specific type of operating system support intended to coordinate disk use between the system and the SSD controller. The feature was previously detailed as having "no" support in System Profiler (below, top).

The new Lion developer preview is reported to add TRIM support however, with users of Apple SSDs noting a "yes" (actually "oui") for support according to a report by the French blog MacGenerations (below, bottom).

Unlike conventional magnetic hard drives, SSDs must be erased before being rewritten with new data, somewhat similar to CD-RW disc. This housekeeping task can be managed by some SSD controllers, but the TRIM command is designed to keep SSDs efficiently optimized at all times, preventing a gradual decline in performance as garbage stacks up.

Cleaning up unused bits of deleted files on SSDs requires a sophisticated balancing act between making sure the drive is clean and ready for new write operations, while also limiting unnecessary wear, as the flash cells used by the devices wear out relatively quickly after a finite number of erase/write cycles, compared to the very long life of the recording surfaces of magnetic storage disks.

The TRIM command is part of the ATA interface standard. So far, it appears Apple's support in Lion is only activated for SSDs shipped by the company and not third party devices, but this is likely to be fleshed out more as Lion develops.