Here are few common SSD glossaries.
Data retention is the timespan over which a ROM remains accurately readable. It is how long the cell would maintain its programmed state when the chip is not under power bias. Data retention is very sensitive to number of P/E cycle put on the flash cell and also dependent on external environment. High temperature tends to reduce retention duration. Number of read cycles performed can also degrade this retention.
P/E (Program/Erase) Cycle:
In NAND flash, storage is achieved using floating-gate transistors that form NAND gates. As such, the non-programmed state of a bit is 1, while the programming operation injects charge into the floating gate and its resultant bit becomes 0. The opposite operation, erase, extracts the stored charge and reverts the state to 1. The erase and program operations inherently cause degradation of the oxide layer isolating the floating gate; this is the reason for NAND flash's finite lifespan (30K-1M program/erase cycles for SLC typically, 2.5K-10K program/erase cycles for MLC, 10K-30K program/erase cycles for eMLC).
Flash Translation Layer (FTL):
Flash Translation Layer, is a software layer used in computing to support normal file systems with flash memory. FTL is a translation layer between the sector-based file system and NAND flash chips. It enables the operating system and file system access NAND flash memory devices as access disk drives. A FTL hides the complexity of flash by providing a logical block interface to the flash device. Since flash does not support overwriting flash pages in place, an FTL maps logical blocks to physical flash pages and erase blocks.
The metadata is used for the management of the stored information or data in the NAND flash memory. The metadata generally includes, a logical-to-physical address mapping table of the stored information, information of attributes of the stored information, and any other data that can assist in the management of the stored information.
Virtual pool is a set of NAND erased blocks ready to be programmed.
Over provisioning is a technique used in the design of flash SSDs and flash media cards. By providing extra memory capacity (which the user can't access) the SSD controller can more easily create pre-erased blocks ready to be used in the virtual pool. Overprovisioning improves:
Write performance & IOPS
Reliability & endurance
Error Correction Code (ECC)
The deterioration of the flash memory cell over time and the disruptions from neighboring flash memory pages can lead to random bit errors in the stored data. While the chances of any given data bit being corrupted is quite small, the vast number of data bits in a storage system makes the likelihood of data corruption a very real possibility.
Error detection and correction codes are used in flash memory storage systems to protect the data from corruption. Dell SSD drives are equipped with the industry’s most advanced ECC algorithm to achieve enterprise level of uncorrectable bit error rate of 10-17.
Write Amplification Factor (WAF)
The write amplification factor is the amount of data the SSD controller has to write in relation to the amount of data that the host controller wants to write. A write amplification factor of 1 is perfect, it means you wanted to write 1MB and the SSD’s controller wrote 1MB. A write amplification factor greater than 1 isn't desirable, but is an unfortunate fact of life. The higher your write amplification, the quicker your drive will wear out and the lower its performance will be.
data written to the Flash memory / data written by the host = write amplification
Certain Operating Systems support the TRIM function, which translates deleted files to the associated LBA (logical block address) on the storage device (SSD). For SATA, the command is also called TRIM, for SAS, the command is called UNMAP. The TRIM/UNMAP command notifies the drive it no longer needs data in certain LBAs (Logical Block Address) which then free up a number of NAND pages.
The TRIM/UNMAP command needs to be supported by the OS, the drive, and the controller in order to work. The TRIM/UNMAP command could result in higher SSD performance from both the reduced data needed to be rewritten during garbage collection and the higher free space resulting on the drive.
How is SSD Sanitized
SSDs can be sanitized by writing over the entire drive capacity several times. Not all Vendors have this secure erase and self-encrypting features on SED (Self Encrypting Drive) SSDs.
These techniques enable a faster and efficient way to sanitize a SSD.
The use of an endurance management algorithm ensures that sufficient Program/Erase (P/E) cycles are available for the warranty time period of the drive. The firmware will limit writes if a drive is written heavily. However, customers will rarely see performance throttling when an SSD is used under the intended application.