An SSD is commonly composed of
DRAM volatile memory or primarily
NAND flash non-volatile memory.
[8]
[edit] Flash drives
Most SSD manufacturers use non-volatile flash memory to create more rugged and compact devices for the consumer market. These flash memory-based SSDs, also known as flash drives, do not require batteries. They are often packaged in standard disk drive form factors (1.8-, 2.5-, and 3.5-inch). In addition, non-volatility allows flash SSDs to retain memory even during sudden power outages, ensuring data persistence. Flash memory SSDs are slower than DRAM SSDs and some designs are slower than even traditional HDDs on large files, but flash SSDs have no moving parts and thus
seek times and other delays inherent in conventional electro-mechanical disks are negligible.
SSD Components:
- Controller: Includes the electronics that bridge the NAND memory components to the SSD I/O interfaces. The controller is an embedded processor that executes firmware-level software and is one of the most important factors of SSD performance. [9]
- Cache: A flash-based SSD uses a small amount of DRAM as a cache, similar to the cache in Hard disk drives. A directory of block placement and wear leveling data is also kept in the cache while the drive is operating.
- Energy storage: Another component in higher performing SSDs is a capacitor or some form of batteries. These are necessary to maintain data integrity such that the data in the cache can be flushed to the drive when power is dropped; some may even hold power long enough to maintain data in the cache until power is resumed.
The performance of the SSD can scale with the number of parallel NAND flash chips used in the device. A single NAND chip is relatively slow, due to narrow (8/16 bit) asynchronous IO interface, and additional high latency of basic IO operations (typical for SLC NAND - ~25 μs to fetch a 4K page from the array to the IO buffer on a read, ~250 μs to commit a 4K page from the IO buffer to the array on a write, ~2 ms to erase a 256 KB block). When multiple NAND devices operate in parallel inside an SSD, the bandwidth scales, and the high latencies can be hidden, as long as enough outstanding operations are pending and the load is evenly distributed between devices.
