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by addaon
397 days ago
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An okay overview of some high level context for on-disk storage, but it's perhaps more useful to say that disk hardware (and memory hardware) present an abstraction of a bunch of bits. Even for DRAM, there isn't a one-to-one mapping between capacitors the fab etches into the silicon and bits that your software can access at a given physical address. At the lowest level, defective rows are bypassed and remapped. At the next level up, ECC means that a single bit can never be (reliably) pointed at on its own -- instead, the data of, say, 64 bits is smeared across 72 capacitors. For disks, this gets even worse, both because the hardware itself is less reliable and because the slow speed allows more and more tricks to be played. A bunch of bits get mapped to a bunch of blocks, but blocks get remapped, bits within blocks get error corrected, multiple bits are stored in a single physical element, etc. |
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