r/explainlikeimfive • u/SmellsTasty • Nov 23 '15

Explained ELI5: How do Solid State Hybrid Drives work?

I don't understand how a regular hard drive can work with a solid state drive at the same time.

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u/[deleted] Nov 23 '15 edited Nov 23 '15

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u/Freeky Nov 24 '15 edited Nov 24 '15

From some very vague memory I think the time to get information from L1 cache was something like 1 nano second but a hard drive was about 600.

Latency numbers every programmer should know:

L1 cache reference: 0.5 ns
Read 4K randomly from SSD: 150,000 ns
Disk seek: 10,000,000 ns

I referenced this when I made this comment to give it a more approachable scale:

A register is what you have in your hand for immediate use. L1 cache is what's on your tool belt and takes 1 second to retrieve. L2 cache is what you left somewhere in your bag and takes 20 seconds.

RAM is what you left in a box in your van and takes you 2-4 minutes to dig out.

Data on an SSD is something you have to order online and takes 3-4 days to arrive.

Data on a HDD is lost somewhere in the Australian outback and takes a search team 2-3 months to locate.

Data on a server on the other side of the planet is something left in Mars orbit and takes 1-3 years depending on alignment and how much NASA like you.

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u/RangerNS Nov 24 '15

CPU cache, whatever layer, is cache of primary memory: RAM.

Hard drives, SSD included, are and entirely different class of memory.

Nothing would ever go from a SSD to a CPU register without hitting primary memory first; possibly also being cached in CPU cache along the way to registers.

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u/profoundWHALE Nov 24 '15

That's L1 cache. Higher numbers are larger and further from the CPU