9.3 Testing New Versions and Configurations
Moab supports a scheduling mode called MONITOR. In this mode, the scheduler initializes, contacts the resource manager and other peer services, and conducts scheduling cycles exactly as it would if running in NORMAL or production mode. Job are prioritized, reservations created, policies and limits enforced, and administrator and end-user commands enabled. The key difference is that although live resource management information is loaded, MONITOR mode disables Moab's ability to start, preempt, cancel, or otherwise modify jobs or resources. Moab continues to attempt to schedule exactly as it would in NORMAL mode but its ability to actually impact the system is disabled. Using this mode, a site can quickly verify correct resource manager configuration and scheduler operation. This mode can also be used to validate new policies and constraints. In fact, Moab can be run in MONITOR mode on a production system while another scheduler or even another version of Moab is running on the same system. This unique ability can allow new versions and configurations to be fully tested without any exposure to potential failures and with no cluster downtime.
To run Moab in MONITOR mode, simply set the MODE attribute of the SCHEDCFG parameter to MONITOR and start Moab. Normal scheduler commands can be used to evaluate configuration and performance. Diagnostic commands can be used to look for any potential issues. Further, the Moab log file can be used to determine which jobs Moab attempted to start, and which resources Moab attempted to allocate.
If another instance of Moab is running in production and a site administrator wants to evaluate an alternate configuration or new version, this is easily done but care should be taken to avoid conflicts with the primary scheduler. Potential conflicts include statistics files, logs, checkpoint files, and user interface ports. One of the easiest ways to avoid these conflicts is to create a new test directory with its own log and stats subdirectories. The new moab.cfg file can be created from scratch or based on the existing moab.cfg file already in use. In either case, make certain that the PORT attribute of the SCHEDCFG parameter differs from that used by the production scheduler by at least two ports. If testing with the production binary executable, the MOABHOMEDIR environment variable should be set to point to the new test directory to prevent Moab from loading the production moab.cfg file.
INTERACTIVE mode allows for evaluation of new versions and configurations in a manner different from MONITOR mode. Instead of disabling all resource and job control functions, Moab sends the desired change request to the screen and asks for permission to complete it. For example, before starting a job, Moab may print something like the following to the screen:
Command: start job 1139.ncsa.edu on node list test013,test017,test018,test021 Accept: (y/n) [default: n]?
The administrator must specifically accept each command request after verifying it correctly meets desired site policies. Moab then executes the specified command. This mode is highly useful in validating scheduler behavior and can be used until configuration is appropriately tuned and all parties are comfortable with the scheduler's performance. In most cases, sites will want to set the scheduling mode to NORMAL after verifying correct behavior.
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