Factors Affecting SSD failure.
SSD store data in blocks of silicon transistors that have exceptionally fast read and write performance. Block sizes range from hundreds to thousands of bits. SSDs use NAND flash storage cells that are the basis onto which data is written and read. Each memory cell is able to register a number of bits as a value of one or zero.
SSD Cell Types.
Single-Level Cell (SLC) accept one bit per memory cell. SSD ‘s with SLCs are popular in enterprise environments where durability is key.
Multi-Level Cell (MLC) store two bits per cell and have slower access times than SLCs and have a lower durability factor.
Triple-Layer Cell (TLC) write three bits to each cell and are the most common type of SSD. TLC,s have more capacity but sacrifice relative speed, reliability, and durability.
- SLC, devices are better more expensive and suited to enterprise applications.
- MLC, performance is good but more expensive than TLC,
- TLC, is the most cost effective but uses relatively more power.
NAND Cell Types
NAND Compared
SSD Durability Ratings,
Typically SSD durability is expressed as terabytes written( TBW ) i.e. the number of terabytes that can be written to a drive before it fails. Typically a 500 GB model has a TBW rating of 600; TBWs ratings are conservative estimates;
Flash memory loses reliability when the cells are too close together. Stacking memory cells on top of each others increases capacity and reduces interference. This stacking is termed 3D NAND.
Wear Leveling:
SSD memory cells start degrade immediately from first use. To help keep drives in good shape for longer, manufacturers include wear technology, which uses an algorithm that writes data to memory cells as equally as possible. Instead of writing a certain block in one section of the drive all the time, it distributes data evenly, so all cells are filled at relatively the same rate.
Dynamic Wear Leveling
There are different types of wear leveling. Dynamic wear leveling creates a map that links logical block addresses (LBAs) from the OS to the physical flash memory cells. Each time the OS writes new data, the address map is updated and the original blocks are marked as invalid data. Every time a block of data is written to the flash memory it is written to a fresh location.
Static Wear Leveling
Static wear leveling works in a similar way as Dynamic Wear Leveling but with DWL the blocks that do not change are periodically moved and made available for use by other data. This moving around of cell blocks means the SSD will continue to operate and most cell blocks will have optimum usage.
Global Wear Leveling
DWL and SWL are implementd on a chip by chip basis. Ineffective blocks in physically different chips varies. A specific flash memory chip can have all its data blocks worn out while another one can have all the blocks still active. Global Wear Leveling GWL manages the blocks of data cells on the chips as a single pool and ensures all the cells in all the chips wear consistently
Wear Levels Compared
The following table is a brief comparison concerning wear leveling:
Item | Static | Dynamic |
Endurance | Longer life expectancy | Shorter life expectancy |
Performance | Slower | Faster |
Design complexity | More complex | Less complex |
Typical use | Industrial-grade flash drives | Consumer-grade flash drives |
Program Erase Cycle.
Program/Erase Cycle, the P/E Cycle translates to a drive’s endurance. When cell blocks write, erase, and rewrite data, an electrical charge damages the oxide layer of the cell’s floating gate transistor. Over time P/E cycle affected blocks no longer hold a charge are are of no use.
SSD Choices.
For those wishing to simply increase boot and load speeds for an improved browsing experience TLC drives are the best option. If you are constantly uploading, saving and downloading new files for work purposes then MLC series are the best option.
SSD Usage.
Media files are large and take up a lot of space. If you’re constantly adding photos, videos etc you’ll need the right capacity and high R/W speeds. If you’re not running resource heavy applications, then you don’t need the fastest drive available.
SSD Cache.
SSD,s use cache in which data is briefly stored before it’s written to the drive. Cache is critical for boosting SSD performance. They’re typically comprised of SLC or MLC NAND. When the cache is full, performance tends to drop significantly—this is especially true for some TLC and most QLC drives.
SSD SATA III
SATA 111 is a common interface used on SSD,s and is how the drive connects to the motherboard. SATA III has a maximum throughput of 600 megabytes per second.
NVMe SSD,s
This interface connects an SSD to the motherboard. NVMe travels over PCIe for blazing-fast speeds. NVMe drives are circa three times faster than SATA III.
Learn more about SSD Failures and Data Recovery.