Flash and Removable Storage - Upgrading and Repairing PCs - page 626

TRIM Command
AnothertechniquetoimproveSSDenduranceandperformanceisanextensiontotheATA
interface called the TRIM command. This allows an SSD-aware OS (such as Windows 7 or
later) to intelligently inform the SSD which data blocks are no longer in use, thus allow-
ing the drive's internal wear leveling and garbage collection routines much more space
to work with, which allows the drive to maintain a high level of performance especially
after all blocks have been written to at least once. For this to work, both the drive and the
OS must support the TRIM command. Windows 7 and Server 2008 R2 and later are SSD
aware and support the TRIM command, whereas earlier versions of Windows do not. SSDs
released in 2009 or later generally support the TRIM command, whereas those that do not
maybeabletoaddsupportviaafirmwareupgrade.Whenyouareupgradingthefirmware
on an SSD, it is highly recommended to have a full backup because in some cases a firm-
ware upgrade reinitializes the drive, wiping all data in an instant.
When an OS deletes a file or otherwise erases data from a drive, it doesn't actually erase
data. Instead, the OS simply marks the file allocation or master file tables to indicate that
those blocks are available, while leaving the data in them untouched. This works fine on
a normal HDD because overwriting is the same as writing, but it greatly hinders a flash
drive since a flash drive cannot overwrite data directly. On a flash drive, any overwrit-
ing causes the drive to first write any previously existing data to a new block, then erase
the block, and finally write the new data. Over time this results in the SSD filling up and
slowing down, even though from the OS point of view there is a lot of empty space.
When TRIM is used, whenever a file is deleted, copied, or moved or the drive is reformat-
ted, the drive is immediately informed of all the blocks that are no longer in use. This al-
lows the drive controller to erase the unused blocks in the background, ensuring that there
is always a sufficient supply of erased blocks available to keep write performance at near
like-new levels.
To further improve SSD performance, Windows 7 and later disable features such as Su-
perfetch and ReadyBoost as well as prefetching on SSDs with random read, write, and
flush performance above a certain threshold.
When running a non- TRIM aware OS (Vista, XP, and earlier), you may still be able to take
advantage of TRIM by installing a TRIM -aware application. For example, Intel provides a
programcalled theIntel SSDOptimizer (partoftheIntel SSDToolbox)that youcanperi-
odicallyruntoreporttothedrivewhichfileshavebeendeleted.OtherSSDmanufacturers
providesimilar tools(oftencalled wiper.exe)aswell.Ifyouarerunninganon- TRIM aware
OS with an SSD, check with the SSD manufacturer to see if it has an optimization tool
available.