Three coupled changes that together meet the "stable max throughput
on any medium" bar:
1. Remove `is_nfs` from `skip_wait`. WAIT_AFTER runs on every medium
now. The `bounded_syscall` 30s safety net (already in place) covers
the wedged-FS case generically — no need to predict NFS-hangs at
compile time. Re-applies the 0.21.12 fix that landed the flat
25 MB/s on NFS in the first place.
2. Drop the `posix_fadvise(DONTNEED)` calls after WAIT_AFTER. Bounded
*dirty* pages (the actual invariant) does not require evicting
*clean* pages. DONTNEED was forcing read-modify-write on any
in-window seek-then-write — exactly the pattern matroska
cluster-size backpatches produce. Empirical 2026-05-15: write side
of mux fed NFS at 43 MB/s while the file grew at 25 MB/s, an 18
MB/s overhead almost certainly composed of those RMW cycles. The
kernel reclaims clean pages under LRU when memory is actually
needed — we don't have to ask.
3. Replace `detect_nfs(fd) -> bool` with `detect_storage_class(fd)
-> (StorageClass, chunk_bytes_seed)`. Medium detection happens
*once* at construction and produces *one* output: the initial
`chunk_bytes` seed for the autotuner. NFS gets 64 MiB (commit ack
~10-30 ms; needs bigger chunks to amortize); other media use the
caller's hint. No hot-path branches on medium. The autotuner drives
all subsequent decisions from measured WAIT_AFTER p95 latency,
identically on every (OS, FS) combination.
Module-level doc rewritten to match: no more "NFS escape hatch", no
"is_nfs" framing. The whole writeback module is now medium-agnostic
except for one labelled detection point.
MattJackson
f4a881b25d