3 ms·

Since it's come up, let's talk about SSD failure modes. Motive: Even absent deduplication, how do I know my three metadata writes didn't end up in the same ECC

by jws 15y ago

Since it's come up, let's talk about SSD failure modes. Motive: Even absent deduplication, how do I know my three metadata writes didn't end up in the same ECC block and aren't going to fail simultaneously? Open questions: • In the real world, how common is a SSD unrecoverable read error? • Are they confined to a single sector? Or a larger grouping? The wikipedia article on TRIM claims that SSDs write in 4k pages. It's reasonable to assume that if I write 4 copies of a 1k block of metadata the SSD block remapping could drop them all into the same page. Is the ECC computed on sectors or pages? Will a failure take out all 4 copies? My Intel SSD 310 specification is mute on the matter. Intel specifies a mean time between failure of 137 years for the drive and a "useful life" of 5 years. From that, I think we can assume Intel's reliability numbers are fiction meant to serve some other purpose than to inform, but since they are all we have we'll give it a go… Intel says I can look forward to one unrecoverable read error at 1 sector/10^16 bits read. Which, if I read at maximum speed is 14 months. (But does reading fast really make it fail sooner? It they come from botched writes or degradation over time then specifying it in "bits read" is silly.) Conclusion: • There might be a danger. • The spec sheets do not illuminate. • Someone on HN may have had an SSD develop an unrecoverable read error and know what happens.

Symmetry 15y ago
"Intel specifies a mean time between failure of 137 years for the drive and a "useful life" of 5 years." Remember, almost all products have some sort of bathtub curve[1], and the MTBF is the inverse of the observed failure rate at the bottom of the curve, while the useful life (or whatever the manufacturer calls it) takes the shape of the curve into account. Many products, including magnetic hard drives, have similar discrepancies between their MTBF and their useful lifespans. [1]http://en.wikipedia.org/wiki/Bathtub_curve http://en.wikipedia.org/wiki/Bathtub_curve
- jws 15y ago
  A combined "infant mortality" and "wear out curve" to make a bathtub curve seems plausible for SSD. "useful life" of 5 years implies that the wear out curve has risen significantly at 5 years. I suppose if there are few units that will make it into the thousands of years they can balance it out. Perhaps they take 10% of the production, pack them in inert gas shielded inside a meter of lead and bury them deep in salt mines to keep the MTBF up.
wazoox 15y ago
So far, I can tell from my experience that many SSD fail catastrophically and randomly. Many SSD controllers and/or firmware lack reliability, and crash under heavy load (the drive disappears, then comes back after a power-cycle); some corrupt data randomly; and even sticking to well known and reputable brand and model (think Intel X25) isn't completely safe - on a large bunch some inexplicably fail after only a short while. I think that SSDs are great, but aren't yet as reliable as spinning rust, the standard deviation being really high. For now I heavily "burn" SSDs before putting them in production, and use them in RAID only. However of all the pains SSDs give me, I never had an unrecoverable read error so far, even on 3 years old 16GB M-TRON SSDs.

by jws 15y ago

Symmetry 15y ago

"Intel specifies a mean time between failure of 137 years for the drive and a "useful life" of 5 years." Remember, almost all products have some sort of bathtub curve[1], and the MTBF is the inverse of the observed failure rate at the bottom of the curve, while the useful life (or whatever the manufacturer calls it) takes the shape of the curve into account. Many products, including magnetic hard drives, have similar discrepancies between their MTBF and their useful lifespans. [1]http://en.wikipedia.org/wiki/Bathtub_curve http://en.wikipedia.org/wiki/Bathtub_curve

jws 15y ago
A combined "infant mortality" and "wear out curve" to make a bathtub curve seems plausible for SSD. "useful life" of 5 years implies that the wear out curve has risen significantly at 5 years. I suppose if there are few units that will make it into the thousands of years they can balance it out. Perhaps they take 10% of the production, pack them in inert gas shielded inside a meter of lead and bury them deep in salt mines to keep the MTBF up.

wazoox 15y ago

So far, I can tell from my experience that many SSD fail catastrophically and randomly. Many SSD controllers and/or firmware lack reliability, and crash under heavy load (the drive disappears, then comes back after a power-cycle); some corrupt data randomly; and even sticking to well known and reputable brand and model (think Intel X25) isn't completely safe - on a large bunch some inexplicably fail after only a short while. I think that SSDs are great, but aren't yet as reliable as spinning rust, the standard deviation being really high. For now I heavily "burn" SSDs before putting them in production, and use them in RAID only. However of all the pains SSDs give me, I never had an unrecoverable read error so far, even on 3 years old 16GB M-TRON SSDs.