Document critical-load-hibernate-seconds

(cherry picked from commit 5299c7c)

Author: John Nicholls

doc/command-line-flags.md

@@ -63,6 +63,12 @@ Comma delimited status-name=threshold, same format as [`--max-load`](#max-load).
 
 This may sometimes lead to migrations bailing out on a very short spike, that, while in itself is impacting production and is worth investigating, isn't reason enough to kill a 10 hour migration.
 
+### critical-load-hibernate-seconds
+
+When `--critical-load-hibernate-seconds` is non-zero (e.g. `--critical-load-hibernate-seconds=300`), `critical-load` does not panic and bail out; instead, `gh-ost` goes into hibernation for the specified duration. It will not read/write anything from/to any server during this time.  Execution then continues upon waking from hibernation.
+
+If `critical-load` is met again, `gh-ost` will repeat this cycle, and never panic and bail out.
+
 ### critical-load-interval-millis
 
 When `--critical-load-interval-millis` is specified (e.g. `--critical-load-interval-millis=2500`), `gh-ost` gives a second chance: when it meets `critical-load` threshold, it doesn't bail out. Instead, it starts a timer (in this example: `2.5` seconds) and re-checks `critical-load` when the timer expires. If `critical-load` is met again, `gh-ost` panics and bails out. If not, execution continues.

go/cmd/gh-ost/main.go

@@ -130,7 +130,7 @@ func main() {
 	maxLoad := flag.String("max-load", "", "Comma delimited status-name=threshold. e.g: 'Threads_running=100,Threads_connected=500'. When status exceeds threshold, app throttles writes")
 	criticalLoad := flag.String("critical-load", "", "Comma delimited status-name=threshold, same format as --max-load. When status exceeds threshold, app panics and quits")
 	flag.Int64Var(&migrationContext.CriticalLoadIntervalMilliseconds, "critical-load-interval-millis", 0, "When 0, migration immediately bails out upon meeting critical-load. When non-zero, a second check is done after given interval, and migration only bails out if 2nd check still meets critical load")
-	flag.Int64Var(&migrationContext.CriticalLoadHibernateSeconds, "critical-load-hibernate-seconds", 0, "When nonzero, critical-load does not panic and bail out; instead, gh-ost goes into hibernate for the specified duration. It will not read/write anything to from/to any server")
+	flag.Int64Var(&migrationContext.CriticalLoadHibernateSeconds, "critical-load-hibernate-seconds", 0, "When non-zero, critical-load does not panic and bail out; instead, gh-ost goes into hibernation for the specified duration. It will not read/write anything from/to any server")
 	quiet := flag.Bool("quiet", false, "quiet")
 	verbose := flag.Bool("verbose", false, "verbose")
 	debug := flag.Bool("debug", false, "debug mode (very verbose)")