Cargo on this box was suspected of writing the disk to death. Measuring
it says otherwise: 4 CPUs, load peaks at 3.0 during a full build, while
writes peak at 30 MB/s and sit at zero for most samples. The long poles
are single-crate rustc compiles that cannot be parallelised, so the
binding constraint is CPU and `nice -n 19` is the first-line tool.
The disk itself reads at 210 MB/s and writes at 50 MB/s, identically
across block sizes and IO modes -- a volume-level write cap rather than
disk physics. A short burst reaches ~150 MB/s on what look like burst
credits; calibrating against that number is a mistake, and it is the one
that made an earlier 40 MB/s ceiling do nothing.
paced.sh is the fallback for the phase that does write hard -- linking
the 643 MB debug binary saturates writes for about 13 seconds. It
duty-cycles a process group against the observed rate in
/proc/diskstats, because every kernel-side lever is unavailable here:
/sys/fs/cgroup is read-only and cannot be remounted or re-mounted
elsewhere even under sudo (no CAP_SYS_ADMIN), /proc/sys is read-only,
and the scheduler cannot be switched to BFQ so ionice is a no-op. Each
of those is verified, not assumed; the header records them so the next
person does not re-derive it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
