IBM 43W7726 Benutzerhandbuch
Solid State Drives for IBM BladeCenter and System x servers
3
Multi-level cell (MLC) SSDs
The basic difference between SLC flash memory and MLC flash memory technologies is storage density.
In comparison with SLC flash memory, which allows only two states to be stored in a cell, thereby storing
only one bit of data per cell, MLC flash memory is capable of storing up to four states per cell, yielding two
bits of data stored per cell.
only one bit of data per cell, MLC flash memory is capable of storing up to four states per cell, yielding two
bits of data stored per cell.
Tables 2 and 3 illustrate the differences.
Table 2. SLC single-bit flash memory states
Value
State
0
Full
1
Erased
Table 3. MLC dual-bit flash memory states
Value
State
00
Full
01
Partially programmed
10
Partially erased
11
Erased
MLC flash memory can be further delineated into two categories:
Consumer-grade MLC (cMLC): Used in consumer (single user) devices such as USB storage
devices, memory cards, mobile phones, and so on.
Enterprise-grade MLC (eMLC): Designed specifically for use in commercial (multiple-user) enterprise
environments.
Both cMLC and eMLC flash memory have the advantage of higher data density and the resultant lower
cost-per-bit ratio. For practical reasons, this is where the similarities end. The high-density storage model
employed by both technologies results in lower write endurance ratios and higher rates of cell degradation
than SLC flash memory, greatly reducing the lifetime of the device. For cMLC devices, this does not pose
any issues, as the lifetime expectancies are considered adequate for consumer-grade devices. This
makes cMLC flash memory ideal for lower-cost, consumer-targeted devices such as memory cards and
mobile devices, where cost and market factors outweigh performance and durability.
employed by both technologies results in lower write endurance ratios and higher rates of cell degradation
than SLC flash memory, greatly reducing the lifetime of the device. For cMLC devices, this does not pose
any issues, as the lifetime expectancies are considered adequate for consumer-grade devices. This
makes cMLC flash memory ideal for lower-cost, consumer-targeted devices such as memory cards and
mobile devices, where cost and market factors outweigh performance and durability.
eMLC provides longer endurance through trimming of components and optimizing certain parameters in
the firmware. In addition, eMLC SSDs employ over-provisioning data storage capacity and wear-leveling
algorithms that evenly distribute data when the drives are not being heavily utilized. This results in a
sixfold increase in write cycles and reduced concerns about cell degradation. While it does not yet match
the performance and durability SLC flash memory, it still exceeds lifetime expectancy requirements for
enterprise applications.
algorithms that evenly distribute data when the drives are not being heavily utilized. This results in a
sixfold increase in write cycles and reduced concerns about cell degradation. While it does not yet match
the performance and durability SLC flash memory, it still exceeds lifetime expectancy requirements for
enterprise applications.
For industries where enterprise performance and durability is essential, IBM SSDs employ eMLC NAND
flash memory to leverage the cost-effective characteristics of MLC flash memory with the performance
and reliability of SLC technology.
and reliability of SLC technology.