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Flash-based SSDs
After decades of hard-disk drive dominance, a new form of persistent
storage device has recently gained significance in the world. Generi-
cally referred to as solid-state storage, such devices have no mechani-
cal or moving parts like hard drives; rather, they are simply built out of
transistors, much like memory and processors. However, unlike typical
random-access memory (e.g., DRAM), such a solid-state storage device
(a.k.a., an SSD) retains information despite power loss, and thus is an
ideal candidate for use in persistent storage of data.
The technology we’ll focus on is known as flash (more specifically,
NAND-based flash), which was created by Fujio Masuoka in th e 1980s
[M+14]. Flash, as we’ll see, has some unique properties. For example, to
write to a given chunk of it (i.e., a flash page), you first have to erase a big-
ger chunk (i.e., a flash block), which can be quite expensive. In addition,
writing too often to a page will cause it to wear out. These two properties
make construction of a flash-based SSD an interesting challenge:
CRUX: HOW TO BUILD A FLASH-BASED SSD
How can we build a flash-based SSD? How can we handle the expen-
sive nature of erasing? How can we build a device that lasts a long time,
given that repeated overwrite will wear the device out? Will the march of
progress in technology ever cease? Or cease to amaze?
44.1 Storing a Single Bit
Flash chips are designed to store one or more bits in a single transis-
tor; the level of charge trapped within the transistor is mapped to a binary
value. In a single-level cell (SLC) flash, only a single bit is stored within
a transistor (i.e., 1 or 0); with a multi-level cell (MLC) flash, two bits are
encoded into different levels of charge, e.g., 00, 01, 10, and 11 are repre-
sented by low, somewhat low, somewhat high, and high levels. There is
even triple-level cell (TLC) flash, which encodes 3 bits per cell. Overall,
SLC chips achieve higher performance and are more expensive.
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