Example of restore from multiplexed database backup (Oracle)
Suppose that MPX_RESTORE_DELAY is not set in the bp.conf file, so its value is the default of 30 seconds. Suppose also that you initiate a restore from an Oracle RMAN backup that was backed up using 4 channels or 4 streams. You also use the same number of channels to restore.
RMAN passes NetBackup a specific data request, telling NetBackup what information it needs to start and complete the restore. The first request is received by NetBackup in 29 seconds, which causes the MPX_RESTORE_DELAY timer to be reset. The next request is received by NetBackup in 22 seconds; again the timer is reset. The third request is received 25 seconds later, resetting the timer a third time. But the fourth request is received 31 seconds after the third. Since the fourth request was not received within the restore delay interval, NetBackup starts three of the four restores. Instead of reading from the tape once, NetBackup queues the fourth restore request until the previous three requests are completed. Note that all of the multiplexed images are on the same tape. NetBackup mounts, rewinds, and reads the entire tape again to collect the multiplexed images for the fourth restore request.
In addition to NetBackup's reading the tape twice, RMAN waits to receive all the necessary header information before it begins the restore.
If MPX_RESTORE_DELAY is longer than 30 seconds, NetBackup can receive all four restore requests within the restore delay windows. It collects all the necessary header information with one pass of the tape. Oracle can start the restore after this one tape pass, for better restore performance.
Set the MPX_RESTORE_DELAY with caution, because it can decrease performance if set too high. Suppose that the MPX_RESTORE_DELAY is set to 1800 seconds. When the final associated restore request arrives, NetBackup resets the request delay timer as it did with the previous requests. NetBackup must wait for the entire 1800-second interval before it can start the restore.
Therefore, try to set the value of MPX_RESTORE_DELAY so it is neither too high or too low.