(We run ZFS in production and have not been hit by these issues, at least not that we know about. But I know of some historical ZFS bugs in this area and mysterious issues that AFAIK have never been fully diagnosed.)
While the advice is sound, this number isn't the right number for this argument.
That 10^15 number is for UREs, which aren't going to cause silent data corruption -- simple naive RAID style mirroring/parity will easily recover from a known error of this sort without any filesystem layer checksumming. The rates for silent errors, where the disk returns the wrong data that benefit from checksumming, are a couple of orders of magnitude lower.
itchingsphynx•57m ago
Is there a more specific 'rule of thumb' for scrub frequency? What variables should one consider?
nubinetwork•48m ago
For what its worth, I scrub daily mostly because I can. It's completely overkill, but if it only takes half an hour, then it can run in the middle of the night while I'm sleeping.
ssl-3•41m ago
If this cost is affordable on a daily basis, then do a scrub daily. If it's only affordable less often, then do it less often.
(Whatever the case: It's not like a scrub causes any harm to the hardware or the data. It can run as frequently as you elect to tolerate.)
toast0•38m ago
But you're balancing the cost of the scrub vs the benefit of learning about a problem as soon as possible.
A scrub does a lot of I/O and a fair amount of computing. The scrub load competes with your application load and depending on the size of your disk(s) and their read bandwidth, it may take quite some time to do the scrub. There's even maybe some potential that the read load could push a weak drive over the edge to failure.
On my personal servers, application load is nearly meaningless, so I do an about monthly scrub from cron which I think will only scrub one zpool at a time per machine, which seems reasonable enough to me. I run relatively large spinning disks, so if I scrubbed on a daily basis, the drives would spend most of the day scrubbing and that doesn't seem reasonable. I haven't run ZFS in a work environment... I'd have to really consider how the read load impacted the production load and if scrubbing with limits to reduce production impact would complete in a reasonable amount of time... I've run some systems that are essentially alwayd busy and if a scrub would take several months, I'd probably only scrub when other systems indicate a problem and I can take the machine out of rotation to examine it.
If I had very high reliability needs or a long time to get replacement drives, I might scrub more often?
If I was worried about power consumption, I might scrub less often (and also let my servers and drives go into standby). The article's recommendation to scan at least once every 4 months seems pretty reasonable, although if you have seriously offline disks, maybe once a year is more approachable. I don't think I'd push beyond that, lots of things don't like to sit for a year and then turn on correctly.