>Most systems that include ZFS schedule scrubs once per month. This frequency is appropriate for many environments, but high churn systems may require more frequent scrubs.

Is there a more specific 'rule of thumb' for scrub frequency? What variables should one consider?

I scrub once a quarter because scrubs take 11 days to complete. I have 8x 18TB raidz2 pool, and I keep a couple of spare drives on hand so I can start a resilver as soon as an issue crops up.

In the past, I've gone for a few years between scrubs. One system had a marginal I/O setup and was unreliable for high streaming load. When copying the pool off of it, I had to throttle the I/O to keep it reliable. No data loss though.

Scrubs are intensive. They will IMO provoke failure in drives sooner than not doing them. But they're the kind of failures you want to bring forward if you can afford the replacements (and often the drives are under warranty anyway).

If you don't scrub, eventually you generally start seeing one of two things: delays in reads and writes because drive error recovery is reading and rereading to recover data; or, if you have that disk behaviour disabled via firmware flags (and you should, unless you're reslivering and on your last disk of redundancy), you see zfs kicking a drive out of the pool during normal operations.

If I start seeing unrecoverable errors, or a drive dropping out of the pool, I'll disable scrubs if I don't have a spare drive on hand to start mirroring straight away. But it's better to have the spares. At least two, because often a second drive shows weakness during resilver.

There is a specific failure mode that scrubs defend against: silent disk corruption that only shows up when you read a file, but for files you almost never read. This is a pretty rare occurrence - it's never happened to me in about 50 drives worth of pools over 15 years or so. The way I think about this is, how is it actionable? If it's not a failing disk, you need to check your backups. And thus your scrub interval should be tied to your backup retention.

Once a month might be too high because HDDs are rated at ~ 180 TB workload/year. Remember, the workload/year limit includes read & writes and doesn't vary much by capacity, so a 10 TB HDD scrubbed monthly consumes 67% of the workload, let alone any other usage.

Scrubbing every quarter is usually sufficient without putting high wear on the HDD.

Once a month seems like a reasonable rule of thumb.

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.

Once a month is fine ("/etc/cron.monthly/zfs-scrub"):

    #!/bin/bash
    #
    # ZFS scrub script for monthly maintenance
    # Place in /etc/cron.monthly/zfs-scrub
    
    POOL="storage"
    TAG="zfs-scrub"
    
    # Log start
    logger -t "$TAG" -p user.notice "Starting ZFS scrub on pool: $POOL"
    
    # Run the scrub
    if /sbin/zpool scrub "$POOL"; then
        logger -t "$TAG" -p user.notice "ZFS scrub initiated successfully on pool: $POOL"
    else
        logger -t "$TAG" -p user.err "Failed to start ZFS scrub on pool: $POOL"
        exit 1
    fi
    
    exit 0

The cost of a scrub is just a flurry of disk reads and a reduction in performance during a scrub.

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.)

Total pool size and speed. Less data scrubs faster, as do faster disks or disk topology (a 3 way stripe of nvme will scrub faster than a single sata ssd)

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.