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The PowerMax uses the concept of service levels which control IO at the front-end adapters to meet the desired response times. The following sections will detail how service levels are implemented on the PowerMax.
The PowerMax comes fully pre-configured from Dell with all disk groups, data (thin) pools, and data devices (TDAT) created. An overview of how these objects are constructed on the array is included below:
Note: The physical makeup of the default SRP cannot be modified; however, if multiple SRPs are available (Dell involvement is required), the SRP designated as default can be changed.
A service level is a property of a storage group (SG). The service level defines the maximum and minimum response times for IO operations that involve the SG.
The minimum response time (known as the floor) defines the shortest time that each IO operation on the SG takes to complete. The maximum response time (known as the ceiling) defines the longest, acceptable time for any IO operation on the SG. PowerMax uses the floor and ceiling values to monitor whether each storage group complies with its quality-of-service criteria.
The storage administrator can use the service levels to help ensure that SGs associated with high-priority applications are not impeded by lower-priority applications.
There are six service levels on the PowerMax are the following, beginning with the highest priority:
The names can be customized. Each service level has its own ceiling and floor value as listed in Table 1.
Service Level | Ceiling | Floor |
Diamond |
| None |
Platinum | 0.8 ms | None |
Gold | 1 ms | None |
Silver | 3.6 ms | ~3.6 ms |
Bronze | 7.2 ms | ~7.2 ms |
Optimized (default) | Exempt | Exempt |
When multiple service level are employed on the array, it is possible that the PowerMax will throttle IO of lower service levels to meet the demands of the higher service levels, regardless of the target response time.
A graphical display shown in Figure 125 is available in Unisphere for PowerMax which also includes what the average response time is for each service level.
As service levels are applied at the storage group level, any VMFS datastore created on a device in that storage group inherits that service level. Through service levels, VMware administrators can offer a variety of response time options for VM users and their virtual machines. Note that both NFS and virtual volumes (vVols) are also supported with service levels.
PowerMax uses the service level associated with each storage group (SG) to maintain system performance among them. It accomplishes this through the service level hierarchy.
IO to SGs that have the Silver or Bronze service level, the only ones to have a floor value, are delayed for the duration of the floor value, even if the storage array can provide a better response time. This provides capacity for IO requests for SGs with higher-priority service levels (those without floor values) to complete promptly.
If the load on the array increases, threatening service level ceilings, the PowerMaxOS will delay IO (at the FA) to other SGs with lower-priority service levels, prioritizing according to the hierarchy. This enables IO for higher-priority SGs to bypass that for the lower-priority SGs, ensuring for example that Diamond has a lower response time than Gold. IO for the lower-priority SGs will still maintain a response time in between the floor and ceiling values. The process of slowing down IO 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 IO 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.