Solid state drives, as the name suggests, don’t have any moving parts, unlike hard disk drives. Instead, data is stored on a series of NAND chips, which can retain their charge without a power source.
Hard disk drives (HDDs) store data on a series of spinning magnetic disks called platters. Hovering a few nanometres above the magnetic platters are the read/write heads, located on the tip of the actuator arm. The read/write heads, as the name suggests, are responsible for reading data from and writing data to the magnetic platters. The read/write heads must be aligned over a particular part of the platter in order to read and write data, which means there is a delay in the action. For example, the data the user requires might be located on different parts of the disk. Furthermore, if the hard drive is powered down or asleep, it can take a good few seconds for it to power up again. It was always clear that hard disk drives could never match the speeds at which CPUs operate, which is measured in nanoseconds rather than milliseconds. Although the hard drive industry has tried to continuously improve the read/write speeds of hard disk drives by introducing smaller platters and faster spindles, there’s only so far these measures can go. Even the most advanced hard drives are still nowhere near as fast as a CPU.
Solid state drives are a different matter. Called so because they don’t rely on spinning disks or moving parts, data on SSDs is written to a pool of NAND flash, which is made up of floating gate transistors. Unlike transistors in DRAM, NAND flash doesn’t require a power supply to retain its charge state, and is known as non-volatile memory. Although nowhere near as fast as main memory, NAND is many times faster than a hard drive. Even write operations, which are much slower for NAND flash than read operations, far out speed those of mechanical hard disk drives.
NAND chips in s solid state drives are organised in a grid, or a block, and rows, or pages. Reading and writing data is much faster on SSDs, but what about when it comes to overwriting data? Although writing data to an empty drive is incredibly fast, overwriting data is in fact much slower. Erasing data from an SSD can only been done at block level due to the amount of voltage involved, which would cause added stress to cells at a lower level. The drive controller in SSDs is also much more sophisticated than the type found in HDDs, due to the amount of NAND memory under control. Typically, solid state drives have 4 or 8 channels, which act like pipelines that continuously move data. Hard disk drives, on the other hand, only have a single channel to move traffic. This is what makes solid state drives so speedy. Nonetheless, the controllers found in hard disk drives - which program the read/write heads to hover nanometres above the platters - are nothing to be laughed at.