How Do SSDs Work?

To understand how and why SSDsSEEAMAZON_ET_135 See Amazon ET commerce 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.

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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 drives can spin. Western Digital’s 10,000 RPM VelociRaptor family is the fastest set of drives ever built 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.

NAND is nowhere near as fast as main memory, but it’s multiple orders of magnitude faster than a hard drive. While write latencies are significantly slower for NAND flash than read latencies, they still outstrip traditional spinning media.

There are two things to notice in the above chart. First, note how adding more bits per cell of NAND has a significant impact on the memory’s performance. It’s worse for writes as opposed to reads — typical triple-level-cell (TLC) latency is 4x worse compared with single-level cell (SLC) NAND for reads, but 6x worse for writes. Erase latencies are also significantly impacted. The impact isn’t proportional, either — TLC NAND is nearly twice as slow as MLC NAND, despite holding just 50% more data (three bits per cell, instead of two). This is also true for QLC drives, which store even more bits at varying voltage levels within the same cell.

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