Document putting the disk ramp above normal working temperature

A ramp that starts inside the range drives normally occupy makes every
load spike move the fans, which cycles audibly. On iz-pve0 the rpool
mirror works at 53-61c against a ramp starting at 52c, holding the fans
at 7920 RPM (+27% over stock) to cool drives rated to 70c. Starting the
ramp at 62c holds stock RPM flat and the drives settle at 53-54c.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -111,6 +111,23 @@ holds 6240 RPM. Here 28% PWM == 6240 RPM, so `LOW_FAN_SPEED=28` and
 `CPU_RAMP_LOW_OFFSET=15` (ramp starts at 62c). An R730xd (`iz-pve1`) idling at 48c is
 fine on the 18/24 defaults. Put the values in the unit as `Environment=` lines.
 
+### Put the disk ramp above the drives' normal working temperature
+
+The single most common cause of a noisy host is a disk ramp that starts *inside* the
+range the drives normally live in. Then every load spike pushes the fan up, the fan drops
+back, and it cycles audibly forever.
+
+Worked example, `iz-pve0`: `sda`/`sdh` are the rpool mirror in rear bays — busy root SSDs
+that idle at 53-55c and touch 60-61c under load, rated to 70c. With the default ramp
+starting at 52c they were permanently part-way up it, dragging the fans to 7920 RPM
+(+27% over stock) to cool drives that were never in trouble. Starting the ramp at 60c
+still cycled, because load spikes crossed it. Starting it at **62c**, just above their
+working peak, holds 6240 RPM flat — and the drives settle at 53-54c anyway.
+
+The rule: find the drives' normal peak, put `DISK_RAMP_LOW_OFFSET` a couple of degrees
+above it, and let the alarm (`DISK_ALARM_OFFSET`, 5c below the limit) catch anything real.
+Cooling a healthy drive from 57c to 53c is not worth 27% more fan.
+
 ### Expect to run above stock on some hosts, legitimately
 
 Dell's profile does not look at drive temperature at all — only CPU, inlet and exhaust.