TestNPassOverwrite detaches after every pass, so viperblock's drain and GC
only ever run at reopen — it cannot show whether GC reclaims superseded
chunks while a volume stays open, and its serial write rate never nears the
predastore rate limiter's burst. TestContinuousChurn attaches once and drives
concurrent 4 MiB-aligned raw overwrites of per-writer regions for several
minutes without detaching, so a sustained superseded-chunk backlog builds
under one long open, spanning multiple GC ticks.
The workload is bounded and self-contained: each dd writes one chunk and the
clock is re-checked after every write, the loop runs as root under a single
sudo bash with a process-group kill trap, and it is wrapped in a guest-side
timeout. Root ownership is load-bearing — the writers open a raw device, so
only a root-owned process group can reap them; a best-effort guest pkill runs
before detach as a backstop so a stray writer cannot wedge the detach.
joshsiv-mulga