Hardware Reference
In-Depth Information
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 reformatted, the
drive is immediately informed of all the blocks that are no longer in use. This allows 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 Superfetch 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 program
called the Intel SSD Optimizer (part of the Intel SSD Toolbox) that you can periodically run to report
to the drive which files have been deleted. Other SSD manufacturers provide similar tools (often
called wiper.exe) as well. If you are running a non
-TRIM
aware OS with an SSD, check with the
SSD manufacturer to see if it has an optimization tool available.
Partition Alignment
Another issue with SSDs is that they are normally designed to read and write 4K pages and to erase
data in 512K blocks. Windows XP and earlier OSs normally start partitions 63 sectors into a disk,
which means that the OS file system components and clusters overlap pages and blocks, resulting in
more pages being read or written, and more blocks being erased than necessary, which can cause a
noticeable performance hit.
SSDs perform at their best when partitions are created with the SSD's alignment needs in mind. All
the partition-creating tools in Windows 8/7/Vista place newly created partitions with the appropriate
alignment, with the first partition starting an even 2048 sectors into the disk. Because this is evenly
divisible by both 4K (8 sectors) and 512K (1024 sectors), there is no overlap between OS file
system cluster and SSD page/block operations.
Even if you are using Windows 8/7/Vista or another OS that normally creates aligned partitions, you
may still have misaligned partitions if the OS was installed into an existing partition or as an upgrade.
Many of the drive manufacturers have free partition alignment tools available that can check and even
correct the alignment of partitions on the fly. When creating new partitions on an SSD, you can
optionally use the
DISKPART
command to manually set the offset to the start of the first partition
such that all partitions on the drive will be properly aligned. With manual intervention, you can
ensure that even Windows XP and earlier will create partitions that are properly aligned for
maximum performance.
SSD Applications
SSDs are ideal for laptops because they are more rugged (no moving parts), weigh less, and consume
less power. The weight savings is fairly minor because the difference between an SSD and a
conventional drive of the same (or even greater) capacity is generally only a few grams. The power
savings is more real—SSDs only draw about a tenth of a watt compared to about 1 watt for an HDD
(average). But even that may be overstated. Although drawing one-tenth the power sounds like a
considerable savings, compared to other components such as the CPU, GPU, and display, each of
which draw 30 watts or more, the overall power savings in going from a standard HDD to an SSD is
