Two types of Solid-State Media
- Solid-State Drive (SSD)
- PCIe Card/Solid-State Cared (SSC)
Solid State typically uses Flash Memory and the type is NAND Flash.
Flash memory is made up of flash cells which are grouped into pages. Pages are usually 4K, 8K or 16K in size, the pages are grouped into blocks which are 128K, 256K or 512K in size.
New flash cells are all set to 1 and can only be programmed from a 1 to a 0, to change it back again a block erase operation is needed to change the value back to a 1. A block erase will erase an entire block.
First time a block is written to performance is fast as you can write to an individual cell or page, after the first write any changes to the block requires the whole block to be read into memory, the entire block is then deleted and then reprogrammed with the update data.
Life expectancy of NAND flash is describe as program/erase (P/E) cycles
Write Cliff is a concept where performance of a SSD drops once each cell has been written to at least once and there is not enough space for in-flight I/O redirection. The SSD requires to be filled to about 70 – 80% for this to become noticeable.
When vendors quote performance numbers look for steady-state IOPS & MBps.
Four major types of NAND Flash
- Single-level cell (SLC) – Highest Performance and life expectancy though most expensive and lowest capacity. rated 100,000 P/E Cycles. Each cell stores 1 bit of data 1 or 0.
- Multi-Level Cell (MLC) – Less performance than SLC and life expectancy is around 10,000 P/E Cycles. Capacity is increased as each cell stores 2 bits of data.
- Enterprise-Grade MLC (eMLC) – Stronger error correction than standard MLC, tend to be heavily over-provisioned and a higher spec controller with more intelligence in the firmware. Overprovisioning is used to increase performance enabling to redirect writes to erased cells rather than go through the read/erase/program cycle.
- Triple-Level Cell (TLC) – stores 3 bits per cell, provides the lowest throughput and endurance around 5000 P/E cycles. high on capacity, good for consumer market.