The existing ZFS prefetch code (dmu_zfetch.c) has some problems:
1. It's nearly impossible to understand. e.g. there are an abundance of kstats
but it's hard to know what they mean (see below).
2. For some workloads, it detects patterns that aren't really there (e.g.
strided patterns, backwards scans), and generates needless i/os prefetching
blocks that will never be referenced.
3. It has lock contention issues. These are caused primarily by
dmu_zfetch_colinear() calling dmu_zfetch_dofetch() (which can block waiting for
i/o) with the zf_rwlock held for writer, thus blocking all other threads
accessing this file.
I suggest that we rewrite this code to detect only forward, sequential streams.
[... truncated ...]