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PowerMax uses the service level that is associated with each storage group (SG) to maintain system performance among them. It accomplishes this task through the service level hierarchy.
Silver and Bronze are the only service levels with floor values. I/O to SGs that have either the Silver or Bronze service level is delayed for the entire floor value. This behavior is true even if the storage array can provide a better response time. This feature provides capacity for I/O requests for SGs with higher-priority service levels (without floor values) to complete promptly.
Increased load on the array threatens service level ceilings. The PowerMaxOS accordingly delays I/O (at the FA) to other SGs with lower-priority service levels, prioritizing according to the hierarchy. This enables I/O for higher-priority SGs to bypass that for the lower-priority SGs, ensuring, for example, that Diamond has a lower response time than Gold. I/O for the lower-priority SGs still maintains a response time in between the floor and ceiling values. The process of slowing down I/O for SGs with lower-priority service levels is known as throttling.
Throttling cascades down the service levels in the following hierarchy:
SGs that have the Optimized service level are exempt from being throttled. However, the response time for I/O to Optimized SGs may degrade as the system load increases.
Note: Data reduction does not impact the ceiling or floor limits of the service levels.
The optimized service level provides a unique use case in that it does not have a target I/O response time, no floor and cannot be throttled. Similarly, if an optimized storage group experiences high response times, no other storage groups are throttled.