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The optimized service level provides a unique use case in that it does not have a target IO response time, no floor and cannot be throttled. Similarly, if an optimized storage group experiences high response times, no other storage groups will be throttled.
A report is available which will graphically display what percentage of a storage group is in compliance with the service level over a selected period. The compliance is shown in terms of what percent of the storage group meets the service level (stable), is below the service level (marginal), or is well below the service level (critical). A three-month sample report is included in Figure 126. The percentage is listed for each compliance.
Figure 126. Service level Compliance Report
Unisphere for PowerMax provides a view of the Storage Groups Dashboard which shows the default SRP and the service levels along with the service level compliance in Figure 127.
Figure 127. Unisphere for PowerMax 10.0 Storage Groups Compliance Dashboard
There is an important concept to understand about storage groups and the PowerMax. The PowerMax internal engine, known as FAST, manages all host related data on the array; however, that does not necessarily mean a storage group is “FAST managed”. In order for a storage group to be “FAST-managed”, the user must explicitly provide an SRP and/or a service level. This is easiest to demonstrate in the Solutions Enabler CLI. Figure 128 shows two symsg commands, the first in red explicitly states both an SRP and service level, making it a “FAST managed” storage group. The second in blue specifies neither, and therefore is not “FAST managed”. By default, the second storage group will be in the default SRP and have a service level of Optimized, however the storage group will show an SRP of NONE and a service level of NONE. Assigning storage groups like this allows a single device to be in two storage groups at once since a device can only be managed by one FAST storage group.
Figure 128. FAST-managed vs managed by FAST storage group in Solutions Enabler
The following points apply to storage groups on arrays running on PowerMax:
Unisphere for PowerMax makes provisioning different storage groups with different service levels a simple process. In the example in Figure 129, a single parent storage group is provisioned with three child storage groups. Each child group has a different service level. Each storage group also has data reduction enabled, though it is possible to disable it at the storage group level for any or all of the children by using the edit function.
Figure 129. Service level storage group provisioning
Service levels on the PowerMax provide a more advanced way to ensure performance requirements are met for particular applications. For some customers, all applications are of equal importance and using service levels may be unnecessary and therefore the best course of action is to use the Optimized mode, allowing the PowerMax to provide the best possible response it can for all devices. For other customers, applications are tiered by design and therefore using service levels meets that need perfectly. Of course, Optimized is still available even if other service levels are used, but as mentioned previously, Optimized IOs will not be throttled in deference to any service level. If using service levels to ensure performance for particular applications, therefore, it might be beneficial to place applications without any requirement in a Bronze service level, understanding that those storage groups might be throttled to ensure performance for the more important applications.
In general Dell recommends using either Optimized for the entire array or service levels, but not both, for the above reasons.