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Block storage resources are accessed through Fibre Channel, NVMe over Fibre Channel, iSCSI, or NVMe over TCP protocols. A host should only access a block resource using one of these protocols at a time. It is not supported for the same host to access the same block resource using more than one protocol.
PowerStore block storage resources are accessed using ALUA/ANA active/optimized or active/non-optimized paths between the host and the two nodes within the PowerStore appliance. I/O is normally sent on an active/optimized path. PowerStore automatically chooses one of the nodes for the active/optimized path, when the volume is mapped to the host, to maintain a balanced workload across the nodes. This characteristic is called node affinity and can be viewed in PowerStore Manager and modified with PSTCLI or REST. These changes take effect immediately and are nondisruptive if the host is correctly configured for multipathing.
Introduced in PowerStoreOS 2.1, the node affinity of block storage resources will be dynamically rebalanced between nodes to maintain relatively consistent utilization, latency, and performance between both nodes of an appliance.
This feature works on block resources that have not had the node affinity manually set by means of PSTCLI or REST. If the node affinity was manually set, the volume must be unmapped and then remapped to the host, which will reset the affinity back to “system selected”. This impacts multipathing only; the system does not need to trespass any volume between nodes.
In PowerStoreOS 4.0, dynamic node affinity is expanded to include virtual volumes (vVols). Data and config vVols are eligible for rebalancing, while unbound vVols are ignored. When an imbalance is detected, the node affinity of volumes is changed first. If additional balancing is required after volumes are balanced, vVols are changed next.
This feature is designed to be transparent and seamless to clients. The goal is to achieve an even throughput, bandwidth, and CPU utilization balance across both nodes within an appliance, regardless of resource type. The system makes changes automatically without requiring any admin interaction. This provides balanced hardware utilization, optimizes performance, and adapts dynamically as workloads change.
All block storage resources in a PowerStore system have a defined performance policy. By default, this policy is set to Medium. The performance policy does not have any impact on system behavior unless some volumes have been set to Low Performance Policy, and other volumes are set to Medium or High. During times of system resource contention, PowerStore devotes fewer compute resources to volumes with Low Performance Policy. Reserve the Low policy for volumes that have less critical performance needs.
In PowerStoreOS version 4.0, Quality of Service (QoS) can be configured for volumes and volume groups. A QoS policy, which includes an I/O limit rule, sets the maximum limits on I/O for volumes and volume groups. These policies are used to ensure that critical applications get priority over other workloads and provide predictable performance for each application. QoS policies are interoperable with existing performance policies.
As this feature is an I/O limiting feature, host performance can be impacted. The impact on host I/O depends directly on the amount of I/O to the resource and the limit configured. Having a limit too low can cause performance-related issues, which can be seen as longer than usual response times and queued I/Os on the host. If the performance impact is too severe, the QoS limit may need to be adjusted.
Maximum limits are enforced only from I/O that arrives from an external host. These limits are not enforced on internal synchronous or asynchronous replication or migration I/O. QoS policies are also not replicated to a remote system. When replication is configured, it is suggested to configure the same QoS limits on each end of the replication session. Having different limits in a metro volume configuration may change the pattern of I/O, as a host may prefer paths that have a higher QoS limit.
Dell Technologies recommends setting QoS on workloads that might monopolize system resources and starve other applications of their required performance. Consider some of the following opportunities to utilize PowerStore QoS: