NAND Flash technology has been widely used for many years in consumer applications such as mobile phones, cameras and MP3 players.  It is only more recently its power has been harnessed for hard disk replacement for laptops and ultimately the enterprise.

NAND flash can be read very quickly (50uS) and written more slowly (200uS) but once the device is full it cannot be rewritten without erasing flash blocks (2,000 pages) which can take up to 5mS.    It is this asymmetry that creates problems when a device fills up and empty space needs to be recovered to allow more writes.

This recovery process is know as “garbage collection” or “grooming” and requires that active pages be gathered and placed into a free block so the targeted block can be erased.  All of this cleanup happens in-band with user traffic so can severely impact performance in demanding enterprise environments.  It is commonly referred to as the “write cliff” and is seen by the user as:

• Dramatic fall in throughput
• Extreme latency spikes (as reads are queued behind 5ms erases)

Source: AnandTech.com