NAND Flash Technology and Solid State Drives (SSDs)
Non-Volatile NAND Flash Memory
One of the benefits of NAND Flash is its non-volatile storage of data. Unlike DRAM memory which must be powered continuously to retain data, NAND memory retains data even when the power is off — making it ideal as storage for portable devices.
Types of NAND Flash
There are primarily two types of NAND Flash widely used today, Single-Level Cell (SLC) and Multi-Level Cell (MLC). NAND Flash stores data in a large array of cells. Each cell can store data — one bit for cell for SLC NAND, and two bits per cell for MLC. So, SLC NAND would store a “0” or “1” in each cell, and MLC NAND would store “00”, “01”, “10”, or “11” in each cell. SLC and MLC NAND offer different levels of performance and endurance characteristics at different price points, with SLC being the higher performing and more costly of the two.
NAND Cell Wear Leveling
NAND cells are not designed to last forever — unlike DRAM, their cells will wear out over time when written to (Reads do not wear out NAND Flash like Writes do). So, NAND storage devices have a limited number of write cycles, although the impact on many storage devices is not significant because of Wear Leveling being carried out by the Flash controller that always resides on the device. All USB Flash drives, SD cards and SSDs have a NAND controller that manages the NAND Flash and performs such functions as Wear Leveling and Error Correction.
So, to prolong the life of NAND storage devices, the NAND Flash controller ensures that all data written is spread evenly across all physical blocks of the device so as not to wear-out one area of the NAND faster than another.
Solid-State Drives (SSDs)
In the past two years, NAND Flash costs dropped enough to make new primary storage devices, solid-state drives, possible for client systems and servers. SSDs are direct replacements for the hard disks (or standard disk drives) in computers with compatible interfaces (such as SATA or SAS).
SSDs offer significant performance and durability advantages over standard hard drives. SSDs have no moving parts; they are all semiconductor devices. Because of this, SSDs do not suffer from mechanical latencies like hard drives do, and without moving parts, SSDs can be subjected to much more shock and vibration than a hard drive, making them ideal for a broad range of portable and mobile applications.
In years past, solid-state drives were designed with DRAM memory chips and were as expensive as luxury cars, making them suitable only for demanding server applications.
Today, with lower cost NAND Flash SSDs, SSDs are being used in a variety of applications ranging from consumer to enterprise and military computing.
SSD Endurance
Kingston's Client system MLC SSDs are rated at 20GB in typical writes per day over three years (Kingston remains conservative in its specifications). This is commonly known as the SSD's endurance.
Today's MLC SSDs are not intended for use in servers, which require much higher levels of writes per day; for those write-intensive applications, 64GB server SLC SSDs can be rated at upward of 1TeraBytes in 4K, 100% Random worst case writes per day over 3 years. Due to the higher cost and lower capacities of SLC SSDs for servers, the industry is working on a new category of server SSDs, called Enterprise MLC (or eMLC).
