Flash and Removable Storage - Upgrading and Repairing PCs - page 509

a hard drive are written to frequently, whereas other areas may be written to only a few times over
the life of the drive. To mitigate this wear, SSDs incorporate sophisticated wear-leveling algorithms
that essentially vary or rotate the usage of cells so that no single cell or group of cells is used more
than another. In addition, spare cells are provided to replace those that do wear out, thus extending
the life of the drive. Considering the usage patterns of various types of users, SSD drives are
generally designed to last at least 10 years under the most demanding use, and most last much longer
than that. As SSD capacity increases, so does the ability of the wear-leveling algorithm to spread out
data among available cells.
Note that, because of the way SSDs work internally, the concept of file fragmentation is immaterial,
and running a defragmenting program on an SSD does nothing except cause it to wear out sooner.
Unlike magnetic drives, which must move the heads to access data written to different physical areas
of the disk, an SSD can read data from different areas of memory without delay. The concept of the
location of a file becomes moot with wear leveling, in that even files that are presented as contiguous
to the file system are actually scattered randomly among the memory chips and cells in the SSD.
Because of this, SSDs should not be defragmented like traditional magnetic drives.
Note
Windows 7 and 8 are SSD aware , which means they can tell an SSD from a standard magnetic
drive. These versions of Windows determine this information by querying the drive's
rotational speed via the ATA IDENTIFY DEVICE command. (SSDs are designed to report
1rpm.) When Windows detects that an SSD is attached, it automatically turns off the
background Disk Defragmenter function, thus preserving drive endurance. When using SSDs
with Windows Vista and earlier versions, you should manually disable or otherwise prevent
any form of defragmentation program or operation from running on SSDs. For maximum
performance with any Windows version, install Tweak-SSD, available from
www.totalidea.com .
TRIM Command
Another technique to improve SSD endurance and performance is an extension to the ATA 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 allowing 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 may be able to add support via a firmware upgrade. When you are upgrading the
firmware on an SSD, it is highly recommended to have a full backup because in some cases a
firmware 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 overwriting causes the drive to first write any