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In Depth Information
Selecting an SSD
A new type of NAND memory was introduced called multi-level cell or
MLC NAND where two bits are stored in each cell. This allows MLC NAND
chips to offer higher capacities and lower costs compared to SLC NAND chips.
Today, all consumer SSDs use multi-level cell chips and even enterprise SSDs
are beginning to use a variation of MLC.
Wear durability or endurance is one item that is very important to consider
when selecting solid state drives. Each memory cell in NAND memory can be
written to a specific number of times before it wears out. How often the SSD is
written to determines the life of the drive. Reading from the drive does not affect
its life. For most consumer applications, even entry-level SSDs can sustain writes
of up to 10–20 GB per day and will last for many years before they need to be
replaced. The higher-end drives, usually in workstations or servers, can sustain
writes of several hundred gigabytes every day for up to five years. Because
consumers rarely write that much per day, higher-end SSDs will last much longer.
Solid state disks depend on advanced disk controllers to manage the life of
the NAND cells with special wear-leveling algorithms. When the operating
system needs to write something to disk, the OS talks to the storage
controller on the motherboard, which then talks to the SSD drive controller. The SSD
controller keeps track of the wear level of each block of cells and writes data
to the best location to maximize the life of the drive. Fragmentation is not a
concern because it has no moving parts that cause the mechanical latency on
traditional hard drives.
Because wear tolerance is very important when selecting an SSD for your
system, keep an eye on the endurance specifications when shopping. For example,
Kingston specifies total bytes written (TBW) for its SSDs to show you how many
terabytes can be written to each of its drives. Other manufacturers simply refer
to wear tolerance as drive endurance, but also give a number of terabytes or
gigabytes that can be written over some period. Remember that as the capacity
of the drive goes up, the endurance number should also increase.
For greater performance, many SSD controllers spread your data across
multiple chips, similar to striping on traditional hard drives. Additionally, some
SSD controllers incorporate RAID-like recovery features, where the controller
can recover your data even if one of the chips in the SSD fails.
Another part of selecting an SSD is finding one that will work well with
your hardware. Specifically, choose one that matches the speed of your storage
interface on your motherboard where the SSD will be connected. Right now all
the consumer solid state disks support two storage interfaces:
■ SATA Rev 2.0, which has a theoretical maximum performance of 3
gigabits per second.
■ SATA Rev 3.0, which has a theoretical maximum performance of 6 gigabits
per second. It is also backward compatible with SATA Rev 2, but runs at
the slower speed.