ADR 0256: say exactly how a SAS drive, a USB drive and a stuck read are treated

The SAS sentence now holds for modes with spaces, a hung read is unhealthy in two to three minutes, not
one, every USB drive is told sat, and the limits of a stuck read are recorded.

02-DECISIONS/0256-a-machines-hardware-condition-is-read-by-one-module-on-every-machine-and-a-fault-is-said-through-its-health.md

@@ -168,12 +168,16 @@ as not read, never as cool. Nothing here conflicts with GENESIS.
 - The module installs a package (`smartmontools`) for `smartctl`, and runs two processes of its own as
   root, each writing one file under the runtime directory and nothing else. It does not run `smartd`.
 - **A drive is not woken by the module, as far as the drive can say so.** Each read is asked with
-  `-n standby`, and smartctl stops without reading when the drive says it is in standby or asleep (an ATA
-  drive by its power mode, a SAS drive or one the operating system powered down by smartctl's own words).
-  smartctl is told the drive's kind (`-d nvme`, `sat` or `scsi`) where it is known, so it does not probe a
-  drive to find out. A drive that cannot say whether it is asleep (a USB bridge that does not pass the
-  question on) is not read at all, and is said as not read. What remains: a drive of a kind the module
-  cannot tell is read with smartctl's own detection.
+  `-n standby`, and smartctl stops without reading when the drive says it is in standby or asleep. An ATA
+  drive says it by its power mode (STANDBY, STANDBY_Y, SLEEP). A SAS drive, or one the operating system
+  powered down, says it in smartctl's own words, "Device is in <mode> mode, exit(2)", where a SAS mode
+  holds spaces (STANDBY BY TIMER, STANDBY_Y BY COMMAND, LOW POWER); the module takes every such mode, and
+  an idle mode never stops smartctl. smartctl is told the drive's kind (`-d nvme`, `sat` or `scsi`) where
+  it is known, so it does not probe a drive to find out. A drive behind USB is always told `sat`: one
+  behind a USB-to-NVMe enclosure or a vendor's own bridge does not answer to it, and is said as not read.
+  A drive that cannot say whether it is asleep (a USB bridge that does not pass the question on) is not
+  read at all, and is said as not read. What remains: a drive of a kind the module cannot tell (a virtual
+  disk, a memory card) is read with smartctl's own detection.
 - **Reading a drive may keep it from spinning down.** Reading SMART from an awake drive can restart its
   standby timer, so a drive read more often than its timer is set for never reaches standby. Whether the
   servers' drives are set to spin down is not known: no mesh tool reads a drive's standby timer or power
@@ -184,7 +188,12 @@ as not read, never as cool. Nothing here conflicts with GENESIS.
   longer than an hour can still be kept awake. Reading the power mode and the timer is a follow-up for
   this module's tools.
 - **A drive that stops answering may look asleep**, and is then HIGH after a day, never unhealthy (rule
-  4). A drive whose read hangs instead is unhealthy within a minute of its bound.
+  4). A drive whose read hangs instead is unhealthy in about two to three minutes: its read is said once
+  it is past its 52 s bound, and the health check needs two looks 60 s apart to be unhealthy.
+- **Known limits of a stuck read.** A smartctl stuck in the kernel cannot be ended, and outlives the
+  drive reader: a restarted reader starts a new read of that drive, which may stick beside the first. The
+  reads of all drives run at once, with no cap per controller or bridge; a server with many drives on one
+  controller asks it all at once, once an hour.
 
 ## Follow-ups