Density is a measure of the quantity of information bits that can be stored on a given length (linear density) of track, area of surface (areal density), or in a given volume (volumetric density) of a computer storage medium. Generally, higher density is more desirable, for it allows more data to be stored in the same physical space. Density therefore has a direct relationship to storage capacity of a given medium. Density also generally affects the performance within a particular medium, as well as price. Storage device classes Solid state media Solid state drives use flash memory to store non-volatile media. They are the latest form of mass produced storage and rival magnetic disk media. Solid state media data is saved to a pool of NAND flash. NAND itself is made up of what are called floating gate transistors. Unlike the transistor designs used in DRAM, which must be refreshed multiple times per second, NAND flash is designed to retain its charge state even when not powered up. The highest capacity drives commercially available are the Nimbus Data Exadrive© DC series drives, these drives come in capacities ranging 16TB to 100TB. Nimbus states that for its size the 100TB SSD has a 6:1 space saving ratio over a nearline HDD Magnetic disk media Hard disk drives store data in the magnetic polarization of small patches of the surface coating on a disk. The maximum areal density is defined by the size of the magnetic particles in the surface, as well as the size of the "head" used to read and write the data. In 1956 the first hard drive, the IBM 350, had an areal density of 2,000 bit/in2. Since then, the increase in density has matched Moore's Law, reaching 1 Tbit/in2 in 2014. In 2015, Seagate introduced a hard drive with a density of 1.34 Tbit/in2, more than 600 million times that of the IBM 350. It is expected that current recording technology can "feasibly" scale to at least 5 Tbit/in2 in the near future. New technologies like heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR) are under development and are expected to allow increases in magnetic areal density to continue. Optical disc media Optical discs store data in small pits in a plastic surface that is then covered with a thin layer of reflective metal. Compact discs (CDs) offer a density of about 0.90 Gbit/in2, using pits