Non-zero prefetch_* counters when zfs_prefetch_disable=1

[sean-]

Non-zero prefetch_* counters when zfs_prefetch_disable=1

[amotin]

@sean- Yes, that is one case. But if you search sources for dmu_prefetch, you'll find about a dozen other cases.

[amotin]

zfs_prefetch_disable=1 disables only speculative prefetches, based on tracked activity. Some prefetches ZFS executes internally, based on hard-coded logic. Many of those are accounted as prescient prefetch, that means ZFS reliably knows that data will be needed soon, but it some cases prefetches can be predictive. IIRC marking prefetches as prescient could take some love. If you have a workload when predictive or especially prescient prefetch is unreasonable, it would be good to analyze it.

[sean-]

Heh, so if a workload fadvise(2)es a FD with POSIX_FADV_SEQUENTIAL, that would show up? I think this all makes sense to me now. Thank you, @amotin .