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Dell PowerStore Configuring Volumes

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Quality of Service (QoS) policies

You can apply QoS policies to set maximum limits on I/O for volumes and volume groups. These policies to ensure that critical applications get priority over other workloads and provide predictable performance for each application.

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.

QoS policies are interoperable with existing performance policies.

You can apply QoS policies to the following:

  • Volumes: Set an IOPS or bandwidth limit on a single volume.
  • Volume groups: Set an IOPS or bandwidth limit to be shared with all volumes in a volume group. If a policy is assigned to a volume group, you cannot assign a policy to an individual volume.
    NOTE:If you remove a volume from a volume group, that removed volume does not inherit a QoS policy that has been assigned to the group.

You can apply a QoS policy to a resource at any time. However, you can apply only one QoS policy to a volume or volume group.

NOTE:When you delete a volume from a volume group, that volume does not inherit the QoS policy that is assigned to the volume group.

QoS policies allow you to limit the following:

  • Maximum IOPS: This value specifies the maximum number of I/O operations per second.
  • Maximum bandwidth: This value specifies the maximum number of KB per second.
NOTE:You can set limits for the maximum IOPS or maximum bandwidth, or both. If both are selected, the system limits traffic according to which limit is reached first.

Supported limits

You can assign up to 100 QoS policies for each cluster.

Considerations

QoS is a performance-limiting feature. If an I/O rule for a resource has a limit that is too low, performance issues can result.

I/O limit rules

There are two types of limits that you can apply with I/O rules:

  • Absolute limit: This limit is defined as the maximum number of IOPS a resource is allowed to complete regardless of its size.
  • Density-based limit: This limit is based on the size of the resource and changes in proportion to the size of the provisioned resource.
    NOTE:For density-based limits, the max IOPS and max bandwidth are based on the amount of provisioned GB that the resource has.

Burst

You can select a burst setting for a QoS policy. The Burst option allows traffic to exceed the maximum IOPS or bandwidth limit by a percentage of that limit for a few seconds. Valid percentage values are 1 to 100. A burst setting of 0 means that the feature is disabled.

To use the burst setting, you must accumulate burst credits. You accumulate burst credits when the resource operates below the I/O limit. The resource can keep exceeding the limit until all credits are used.

For example, assume you have a 10,000 IOPS limit and a 20% burst setting. The resource operates below its IOPS limit for a certain period, allowing you to accumulate 2,000 credits. If IOPS exceeds the 10,000 limit, the burst setting enables the resource to use those credits to allow increased traffic.

NOTE:A host can consume the available credits even if no burst value has been set.

When a volume or volume group is associated with a QoS policy that has a density-based I/O limit, the burst limits are updated if the volume is resized or the volume group has members that are added or removed.

NOTE:If a resource consistently tries to exceed a defined limit, the burst setting is not applied.

Migration

If a volume or volume group is migrated, any associated QoS policy is migrated along with that resource.

Metro volumes

If you assign a QoS policy to a metro volume, that policy is not automatically copied to the peer cluster. You can assign QoS policies independently to metro volumes on the source or destination.

If you want to assign a QoS policy to a metro volume, it is recommended that you apply the policy to the local and remote sides.

You can unassign a QoS policy at either end of a metro volume.

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