Hardware Reference
In-Depth Information
SLC Versus MLC
As previously mentioned, SSDs use NAND flash technology. Two subtypes of this tech-
nology are used in commercially available SSDs: SLC (single-level cell) and MLC (mul-
tilevel cell). SLC flash stores 1 bit in a single cell, whereas MLC stores 2 or more bits in
a single cell. MLC doubles (or more) the density, and consequently lowers the cost, but
this comes at a penalty in performance and usable life. SSDs are available using either
technology, with SLC versions offering higher performance, lower capacity, and higher
cost. Most mainstream SSDs use MLC technology, whereas more specialized high-end
products (mostly for server or workstation systems) use SLC.
One major problem with flash memory is that it wears out. SLC flash cells are normally
rated for 100,000 Write/Erase (W/E) cycles, whereas MLC flash cells are normally rated
for only 10,000 W/E cycles. When used to replace a standard hard drive, this becomes
a problem because certain areas of 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,
SSDsincorporatesophisticatedwear-levelingalgorithmsthatessentiallyvaryorrotatethe
usageofcellssothatnosinglecellorgroupofcellsisusedmorethananother.Inaddition,
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
designedtolastatleast10yearsunderthemostdemandinguse,andmostlastmuchlonger
than that. As SSD capacity increases, so does the ability of the wear-leveling algorithm to
spread out data among available cells.
Note that due to the way SSDs work internally, the concept of file fragmentation is im-
material, 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 is
SSD aware
, which means it can tell an SSD from a standard magnetic drive.
It does this by querying the drive's rotational speed via the
ATA IDENTIFY DEVICE
com-
mand.(SSDsaredesignedtoreport1rpm.)WhenWindows7detectsthatanSSDisattached,
it automatically turns off the background Disk Defragmenter function, thus preserving drive
endurance.WhenusingSSDswithWindowsVistaandearlierversions,youshouldmanually
disableorotherwisepreventanyformofdefragmentationprogramoroperationfromrunning
on SSDs.
