uses Transistor to store data rather than Hard disk drive (magnetic discs)
pros:
- no moving parts or physical limitations (disc can only move so fast)
- higher throughput, good random access
- more energy efficient
- better space density
cons:
- more expensive
- lower endurance (number of writes)
- more complicated to write Device driver for
SSD contains pages:
- on a silicon die, there are large “planes”
- on each plane, there are several blocks
- on each block, there are several pages - which may or may not correspond with OS page size
- typically 4 kB
reading page: 10 μs writing a page: 100 μs deleting a block (not a single page): 1 ms
you can only read complete pages and write to freshly erased pages erasing is done per block (128 or 256 pages), an entire block needs to be erased before writing writing is slow, since we may need to erase blocks
OS can help speed this process up
SSDs need to Garbage collector blocks
move any alive pages to a new block (with some overhead)
disc controller doesn’t know what blocks are still alive
SSD may think disc is full, when a file could be deleted
OS uses TRIM command to inform SSD a block is unused
SSD can freely erase the block without moving overhead
single devices - single large expensive disc (SLED) - just one large disc for data RAID