A Cron Job Took Our Server to Load 41 by Attacking Itself

A */1 rsync took our staging box to a load average of 41 one afternoon, and it took me longer than I want to admit to work out why. The sync normally finished in about twenty seconds. That day the backup target's NFS mount went sluggish, the sync started taking ninety seconds, and cron — which does not know or care whether the last run is still going — launched a fresh copy every single minute on top of it. Inside ten minutes there were a half-dozen rsyncs all reading the same tree off the same slow disk, each one making the disk slower, each new minute adding another. The box wasn't under attack. It was attacking itself, one polite copy at a time. The thing that stung was that nothing was broken — every individual rsync was correct, the disk eventually recovered on its own, and the only reason it became an outage is that cron has no concept of "the last one is still running." That's the trap with scheduled jobs: a command that's perfectly fine when you run it by hand can take down a server the first time it runs longer than its interval with nobody watching. The fix everyone reaches for first is the wrong one The instinct is a PID file: write $$ to /var/run/job.pid on start, check whether that file exists on the next run, bail if it does. It almost works. Then one run gets kill -9 'd, or the box reboots mid-job, and the PID file is left behind pointing at a process that died on Tuesday. Now every future run sees a "lock" owned by a PID that no longer exists, and the job never runs again — the opposite failure, just as silent. There's also a race between the check and the write, and the times you most need the lock to be clean are exactly the times cleanup didn't happen, because the process died before it could clean up. flock has none of that. The lock isn't a file you create and delete — it's a lock the kernel holds on an open file descriptor , and the kernel releases it automatically the instant that descriptor closes. The process exiting closes it. So does crash