The Job Engine runs system maintenance jobs in the background and prevents jobs within the same classification (exclusion set) from running simultaneously. Two exclusion sets are enforced: restripe and mark.
Restripe job types are:
Mark job types are:
Note that MultiScan is a member of both the restripe and mark exclusion sets. You cannot change the exclusion set parameter for a job type.
The Job Engine is also sensitive to job priority, and can run up to three jobs, of any priority, simultaneously. Job priority is denoted as 1–10, with 1 being the highest and 10 being the lowest. The system uses job priority when a conflict among running or queued jobs arises. For example, if you manually start a job that has a higher priority than three other jobs that are already running, Job Engine pauses the lowest-priority active job, runs the new job, then restarts the older job at the point at which it was paused. Similarly, if you start a job within the restripe exclusion set, and another restripe job is already running, the system uses priority to determine which job should run (or remain running) and which job should be paused (or remain paused).
Other job parameters determine whether jobs are enabled, their performance impact, and schedule. As system administrator, you can accept the job defaults or adjust these parameters (except for exclusion set) based on your requirements.
When a job starts, the Job Engine distributes job segments—phases and tasks—across the nodes of your cluster. One node acts as job coordinator and continually works with the other nodes to load-balance the work. In this way, no one node is overburdened, and system resources remain available for other administrator and system I/O activities not originated from the Job Engine.
After completing a task, each node reports task status to the job coordinator. The node acting as job coordinator saves this task status information to a checkpoint file. Consequently, in the case of a power outage, or when paused, a job can always be restarted from the point at which it was interrupted. This is important because some jobs can take hours to run and can use considerable system resources.