4 ms·

You are quite right that 8GB memory and a cheap processor is good enough for home use. However I thought the consensus is that one cannot skip ECC with ZFS beca

by Laforet 11y ago

You are quite right that 8GB memory and a cheap processor is good enough for home use. However I thought the consensus is that one cannot skip ECC with ZFS because a silent bitflip can corrupt an entire zpool. In any case the extra cost is minimal so why not.

gh02t 11y ago
Running without ECC is no worse than doing so with any other filesystem [as far as I understand]. The thing about corrupting the entire zpool looks to be a bit of an urban legend (http://jrs-s.net/2015/02/03/will-zfs-and-non-ecc-ram-kill-your-data/ http://jrs-s.net/2015/02/03/will-zfs-and-non-ecc-ram-kill-yo...). Flipping bits can of course cause problems, but doing a scrub after a bit is flipped in memory doesn't hose your whole pool. There's been endless discussion and flame wars on the topic, but I've never once heard someone who actually had it happen (because it can't really happen). You can lose some data due to bit flips, you won't however lose everything in the whole volume like some people claim (unless you have something like an entire chip on the memory fail, then you're definitely screwed but neither ECC nor a different FS will help you there). ECC is a good idea and it's not hugely expensive, but there is one very big difference. That is, most people who might want to build a NAS have a decent bit of spare hardware lying around that they might be able to repurpose. That hardware is probably an old desktop machine and doesn't have ECC, so if you insist on using ECC then you can't use that old machine and have to buy a new mobo+RAM at least. I've been running ZFS for years without ECC doing weekly scrubs and never once destroyed anything. Edit: the counter-argument/source of the theory that you absolutely need ECC is primarily this post https://forums.freenas.org/index.php?threads/ecc-vs-non-ecc-ram-and-zfs.15449/ https://forums.freenas.org/index.php?threads/ecc-vs-non-ecc-... if you want to the other side for yourself.
- Laforet 11y ago
  Thanks for the response. I think JRS's account has some serious issues with the writer conflating hardware failure (stuck bits, unreadable blocks, etc) with random error that is more common and inevitable, not to mention undetectable in principle without ECC[0]. Bit flip has been shown to cause both data loss/corruption as well as filesystem failure in ZFS[1]. The scenario they presented in the paper is probably less relevant for a home lab where most data are accessed infrequently and the real chance of catastrophic failure is probably below 0.1% per bit flip event, but the risk is real and will only get higher as storage densities go up. People on FreeNAS forum tend to play on the extreme side of caution - after all if one is willing to cut corners on hardware other unsafe practices will probably follow, hence it is better to make it clear that certain acts are not worth the risk. [0]:DRAM Errors in the Wild: A Large-Scale Field Study http://www.cs.toronto.edu/~bianca/papers/sigmetrics09.pdf http://www.cs.toronto.edu/~bianca/papers/sigmetrics09.pdf [1]: End-to-end Data Integrity for File Systems: A ZFS Case Study http://research.cs.wisc.edu/adsl/Publications/zfs-corruption-fast10.pdf http://research.cs.wisc.edu/adsl/Publications/zfs-corruption...

by Laforet 11y ago

gh02t 11y ago

Running without ECC is no worse than doing so with any other filesystem [as far as I understand]. The thing about corrupting the entire zpool looks to be a bit of an urban legend (http://jrs-s.net/2015/02/03/will-zfs-and-non-ecc-ram-kill-your-data/ http://jrs-s.net/2015/02/03/will-zfs-and-non-ecc-ram-kill-yo...). Flipping bits can of course cause problems, but doing a scrub after a bit is flipped in memory doesn't hose your whole pool. There's been endless discussion and flame wars on the topic, but I've never once heard someone who actually had it happen (because it can't really happen). You can lose some data due to bit flips, you won't however lose everything in the whole volume like some people claim (unless you have something like an entire chip on the memory fail, then you're definitely screwed but neither ECC nor a different FS will help you there). ECC is a good idea and it's not hugely expensive, but there is one very big difference. That is, most people who might want to build a NAS have a decent bit of spare hardware lying around that they might be able to repurpose. That hardware is probably an old desktop machine and doesn't have ECC, so if you insist on using ECC then you can't use that old machine and have to buy a new mobo+RAM at least. I've been running ZFS for years without ECC doing weekly scrubs and never once destroyed anything. Edit: the counter-argument/source of the theory that you absolutely need ECC is primarily this post https://forums.freenas.org/index.php?threads/ecc-vs-non-ecc-ram-and-zfs.15449/ https://forums.freenas.org/index.php?threads/ecc-vs-non-ecc-... if you want to the other side for yourself.

Laforet 11y ago
Thanks for the response. I think JRS's account has some serious issues with the writer conflating hardware failure (stuck bits, unreadable blocks, etc) with random error that is more common and inevitable, not to mention undetectable in principle without ECC[0]. Bit flip has been shown to cause both data loss/corruption as well as filesystem failure in ZFS[1]. The scenario they presented in the paper is probably less relevant for a home lab where most data are accessed infrequently and the real chance of catastrophic failure is probably below 0.1% per bit flip event, but the risk is real and will only get higher as storage densities go up. People on FreeNAS forum tend to play on the extreme side of caution - after all if one is willing to cut corners on hardware other unsafe practices will probably follow, hence it is better to make it clear that certain acts are not worth the risk. [0]:DRAM Errors in the Wild: A Large-Scale Field Study http://www.cs.toronto.edu/~bianca/papers/sigmetrics09.pdf http://www.cs.toronto.edu/~bianca/papers/sigmetrics09.pdf [1]: End-to-end Data Integrity for File Systems: A ZFS Case Study http://research.cs.wisc.edu/adsl/Publications/zfs-corruption-fast10.pdf http://research.cs.wisc.edu/adsl/Publications/zfs-corruption...