Is it not the case that if you don't have ECC memory, ZFS could end up writing a checksum that does not match the data if you get a bitflip in just the right (wrong) spot?
The myth, popularized by a notorious thread on the TrueNAS forums [1], is specifically that ZFS requires ECC RAM, and will do worse than other filesystems without it, because scrubbing will multiply a single bitflip into a failed pool.
A ZFS core developer says that that isn't the case [2]. Here's some more reasoning [3], also about many other myths.
[1] https://www.truenas.com/community/threads/ecc-vs-non-ecc-ram...
[1]: https://arstechnica.com/civis/threads/ars-walkthrough-using-...
[2]: https://en.wikipedia.org/wiki/ZFS#2004%E2%80%932010:_Develop...
But everything is actually quite sensitive.
We’ve accepted lack of ECC because Intel decided it would be a product line differentiator, and serious customers who didn’t want random crashes or to lose data would buy chips with ECC.
It’s actually less of an issue these days because DDR5 has (by spec) some in-line ECC; won’t help with multi-bit errors but its an improvement on what came before.
AMD has been allowing ECC on lots of regular hardware for a long time.
People don't tend to buy ECC for desktop use because it costs significantly more (used server ram is/was often cheap... but it often doesn't work in desktop boards), and the performance specs are poor.
My home servers are mostly retired desktops, so they get my old desktop ram and I don't want to pay premium prices for jedec speed ecc ram on my desktops, thanks.
Since DDR5 doesn't include reporting on bit errors (afaik), it likely means much fewer single bit errors, but most experienced errors will be multi-bit. Although, I dunno what proportion of bit errors is on the ram chips and what's on the bus... there's no protection from bus errors.
If there were reporting, you could replace chips with high error rates, but without reporting you'll keep running them until they fail enough to notice.
Often unsupported or untested by the motherboard manufacturer, because the precedent set quite a while ago has stood so strongly.
> because it costs significantly more [...] and the performance specs are poor
Because it's niche. If a large share of the desktop market was using ECC memory, the extra cost would have been 10-12%, and a typical kit would push the clocks and timings at least as much as non-ECC memory.
> Since DDR5 doesn't include reporting on bit errors (afaik), it likely means much fewer single bit errors, but most experienced errors will be multi-bit. Although, I dunno what proportion of bit errors is on the ram chips and what's on the bus... there's no protection from bus errors.
Yeah that sucks. I'm really hoping that DDR6 standardizes a light form of end to end ECC. (Bursts are 24 bits wide and 24 bits deep, so on top of the 512 bits of payload there's a bunch of extra bits that can be used for ECC. If memory chips would just store 4 extra bits per 128, that's enough for reasonable ECC on a per-burst basis.)
I wasn't really around for it, but I appreciate the early penny pinchers that developed fake parity ram. The IBM PC required a parity bit per byte?, but you could add an xor chip to calculate it at read and save out on the expensive ram chip.
I wouldn't mind paying a little more for ECC, what with the extra chip and a little more circuitry, but desktop ECC often starts at 50% more, which I'm way too cheap to pay for.
Which means all copies of the data will be corrupted. This can be anything from an irrecoverable file to complete filesystem corruption.
But generally, yeah, not any more dangerous than any other filesystem, and ECC used to be cheap so it was a no-brainer, you should have backups anyway TBH if we're being honest about storage resiliency.
What has been debunked is the "scrub of death" issue, on a scrub a bad bit flip would cause an error, which would be copied over with good data -- well it was technically good before. It would be statistically difficult to have a fault on a read, then a clean read, then a second bit flip destroying the data.
ZFS without ECC is no more risky than any other file system / software RAID without ECC.
As no one owes you an explanation, it would take you five seconds to Google this and discover:
1. It's been disproven, with one of the original ZFS developers chiming in.
2. The original source of the rumor was a forum post that somehow became canon.
...(after much longer) that it's a rabbit hole with nuance. https://news.ycombinator.com/item?id=18481910