From be440e95a3d4e4544e53784e314a5e8928638d72 Mon Sep 17 00:00:00 2001 From: Zeb Hering Date: Fri, 28 Aug 2026 22:17:46 -0700 Subject: [PATCH] Document ramp width as the lever for fan-noise steadiness Span sets sensitivity: 22 points over 7c needs only 1.6c of drift to cross a 5% deadband, over 11c it needs 2.5c. Replaying 180 recorded samples, widening the ramp and raising the deadband took the same window from 19 fan changes to 1. Confirmed live under a zpool migration: 19 changes became 2, average fan speed fell from 40% to 38%, and CPU temperature was unchanged at 64-68c. Co-Authored-By: Claude Opus 5 (1M context) --- README.md | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/README.md b/README.md index 1031214..3c51db4 100644 --- a/README.md +++ b/README.md @@ -120,6 +120,30 @@ hour, at which point the alarm is the outage. `smartctl -H` across two dozen drives runs hourly, not per loop. Absolute and trend checks stay on the fast loop. +### Ramp width controls how *bouncy* the fans are + +Changing fan noise is more irritating than constant fan noise, and ramp width is the lever +for it. The span sets the sensitivity: 22 points of fan over a 7c ramp is 3.14%/degC, so +1.6c of CPU drift is enough to cross a 5% deadband. The same 22 points over 11c is +2%/degC, needing 2.5c. + +Replaying 180 recorded samples from `iz-pve0` under load (CPU 57-69c) through candidate +curves, counting how often the fan would actually move: + +| Ramp | Deadband | Fan changes | Distinct speeds | +|---|---|---|---| +| 62-69c | 5 | 19 | 28,31,34,37,40,43,46,50 | +| 62-69c | 8 | 6 | 28,37,46,50 | +| 62-73c | 5 | 5 | 28,34,40 | +| **62-73c** | **8** | **1** | 28,36 | + +Confirmed live against a multi-hour zpool migration: **19 fan changes became 2** over a +comparable window, average fan speed dropped slightly (40% to 38%), and **CPU temperature +did not move** (64-68c against 64-69c before). The extra airflow had been buying nothing. + +Widen the ramp and raise the deadband together. The cost is lag - a deadband of 8 at +2%/degC is about 4c of slack - which is affordable when you are 8-14c below any threshold. + ## Calibrating a host **The defaults are not portable and the fans will be wrong on an uncalibrated host.**