How do SSDs work?

Here at ExtremeTech, we’ve often discussed the difference between different types of NAND structures — vertical NAND versus planar, or multi-level cell (MLC) versus triple-level cells (TLC). What we haven’t done is sit down and talk about the more basic relevant question: How do SSDs work in the first place?

How does a Solid State Drive (SSD)

To understand how and why SSDs are different from spinning discs, we need to talk a little bit about hard drives. A hard drive stores data on a series of spinning magnetic disks, called platters. There’s an actuator arm with read/write heads attached to it. This arm positions the read-write heads over the correct area of the drive to read or write information.

Because the drive heads must align over an area of the disk in order to read or write data (and the disk is constantly spinning), there’s a non-zero wait time before data can be accessed. The drive may need to read from multiple locations in order to launch a program or load a file, which means it may have to wait for the platters to spin into the proper position multiple times before it can complete the command. If a drive is asleep or in a low-power state, it can take several seconds more for the disk to spin up to full power and begin operating.

From the very beginning, it was clear that hard drives couldn’t possibly match the speeds at which CPUs could operate. Latency in HDDs is measured in milliseconds, compared with nanoseconds for your typical CPU. One millisecond is 1,000,000 nanoseconds, and it typically takes a hard drive 10-15 milliseconds to find data on the drive and begin reading it. The hard drive industry introduced smaller platters, on-disk memory caches, and faster spindle speeds to counteract this trend, but there’s only so fast that drives can spin. Western Digital’s 10,000 RPM VelociRaptor family is the fastest set of drives ever b uilt for the consumer market, while some enterprise drives spun up to 15,000 RPM. The problem is, even the fastest spinning drive with the largest caches and smallest platters are still achingly slow as far as your CPU is concerned.

How SSDs are different

“If I had asked people what they wanted, they would have said faster horses.” — Henry Ford

Solid-state drives are called that specifically because they don’t rely on moving parts or spinning disks. Instead, 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. This makes NAND a type of non-volatile memory.

The diagram above shows a simple flash cell design. Electrons are stored in the floating gate, which then reads as charged “0” or not-charged “1.” Yes, in NAND flash, a 0 means that data is stored in a cell — it’s the opposite of how we typically think of a zero or one. NAND flash is organized in a grid. The entire grid layout is referred to as a block, while the individual rows that make up the grid are called a page. Common page sizes are 2K, 4K, 8K, or 16K, with 128 to 256 pages per block. Block size therefore typically varies between 256KB and 4MB.

One advantage of this system should be immediately obvious. Because SSDs have no moving parts, they can operate at speeds far above those of a typical HDD. The following chart shows the access latency for typical storage mediums given in microseconds.
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