job history directory grows without bound, locks up job tracker on new job submission

An unpleasant surprise upgrading to 0.19: requests to jobtracker.jsp would take a long time or even time out whenever new jobs where submitted. Investigation showed the call to JobInProgress.initTasks() was calling JobHistory.JobInfo.logSubmitted() which in turn was calling JobHistory.getJobHistoryFileName() which was pegging the CPU for a couple minutes. Further investigation showed the were 200,000+ files in the job history folder – and every submission was creating a FileStatus for them all, then applying a regular expression to just the name. All this just on the off chance the job tracker had been restarted (see HADOOP-3245). To make matters worse, these files cannot be safely deleted while the job tracker is running, as the disappearance of a history file at the wrong time causes a FileNotFoundException.

So to summarize the issues:

• having Hadoop default to storing all the history files in a single directory is a Bad Idea
• doing expensive processing of every history file on every job submission is a Worse Idea
• doing expensive processing of every history file on every job submission while holding a lock on the JobInProgress object and thereby blocking the jobtracker.jsp from rendering is a Terrible Idea (note: haven't confirmed this, but a cursory glance suggests that's what's going on)
• not being able to clean up the mess without taking down the job tracker is just Unfortunate