Trim (computing)

A trim command (known as TRIM in the ATA command set, and UNMAP in the SCSI command set) allows an operating system to inform a storage medium which blocks of data are no longer considered to be "in use" and therefore can be erased internally. TRIM is primarily used on solid-state drives (SSDs), but is also used on some shingled magnetic recording (SMR) hard drives.

Trim commands were introduced soon after SSDs. Because SSDs use different technology from earlier magnetic hard drives, the preexisting techniques for storing data caused early SSDs to become slower as they were used more. With trimming, the SSD can efficiently clear unneeded data that would otherwise slow future write operations.

Although tools to "reset" some drives to a fresh state were already available before the introduction of trimming, they also delete all data on the drive, which makes them impractical to use for ongoing optimization. As of 2010, many SSDs and many flash removable storage devices had internal garbage collection mechanisms for certain filesystem(s) (such as FAT and NTFS) that worked independently of trimming. Although this successfully maintained their lifetime even under operating systems that did not support trim, it had the associated drawbacks of increased write amplification and wear of the flash cells, as well as decreased write performance of flash cells.