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Solid-state revolution: in-depth on how SSDs really work

| Thursday, June 7, 2012
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Solid-state drives are odd creatures. Though they sound simple in theory, they store some surprisingly complex secrets. For instance, compare an SSD to a traditional magnetic hard drive. A modern multi-terabyte spinning hard disk plays tricks with magnetism and quantum mechanics, results of decades of research and billions of dollars and multiple Nobel Prizes in physics. The drives contain complex moving parts manufactured to extremely tight tolerances, with drive heads moving around just thousandths of a millimeter above platters rotating at thousands of revolutions per minute. A modern solid-state drive performs much more quickly, but it's also a more mundane on the inside, as it's really a hard drive-shaped bundle of NAND flash memory. Simple, right?

However, the controller software powering an SSD does some remarkable things, and that little hard drive-shaped bundle of memory is more correctly viewed as a computer in its own right.

Given that SSDs transform the way computers "feel," every geek should know at least a bit about how these magical devices operate. We'll give you that level of knowledge. But because this is Ars, we're also going to go a lot deeper—10,000 words deep. Here's the only primer on SSD technology you'll ever need to read.

Varying degrees of fast

It's easy to say "SSDs make my computer fast," but understanding why they make your computer fast requires a look at the places inside a computer where data gets stored. These locations can collectively be referred to as the "memory hierarchy," and they are described in great detail in the classic Ars article "Understanding CPU Caching and Performance."

It's an axiom of the memory hierarchy that as one walks down the tiers from top to bottom, the storage in each tier becomes larger, slower, and cheaper. The primary measure of speed we're concerned with here is access latency, which is the amount of time it takes for a request to traverse the wires from the CPU to that storage tier. Latency plays a tremendous role in the effective speed of a given piece of storage, because latency is dead time; time the CPU spends waiting for a piece of data is time that the CPU isn't actively working on that piece of data.

The table below lays out the memory hierarchy:

Registers"instantaneous" under 1KB
Level 1 Cache 1-3 ns64KB per core
Level 2 Cache3-10 ns 256KB per core
Level 3 Cache 10-20 ns2-20 MB per chip
Main Memory30-60 ns 4-32 GB per system
Hard Disk 3,000,000-10,000,000 nsover 1TB

Read more: ArsTechnica
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