Throttles

Throttling is a mechanism to control amount of resources that are used when the system is processing I/Os on supported objects. The system supports throttles on hosts, host clusters, volumes, copy offload operations, and storage pools. If a throttle limit is defined, the system either processes the I/O for that object, or delays the processing of the I/O to free resources for more critical I/O operations.

When you configure throttles on the system, keep in mind the following guidelines:

The throttle limit is a per node limit. For example, if a throttle limit is set for a volume at 100 IOPS, each node on the system that has access to the volume allows 100 IOPS for that volume. Any I/O operation that exceeds the throttle limit are queued at the receiving nodes.
The multipath policies on the host determine how many nodes receive I/O operations, and therefore the effective throttle limit.
If more than one throttle applies to an I/O operation, the lowest and most stringent throttle is used.

For each type of throttle, you can either IOPS or bandwidth limit or both. The throttle limit controls whether the processing of the host I/O continues or if it is delayed for later processing. Several reasons can exist to delay processing for specific host and host clusters. For example, if a host or host cluster has I/O intensive workloads, such as data mining systems, you can create a throttle limit that automatically delays the processing of I/O operations that exceed the configured throttle. Each host in the system may have a throttle defined or you can define throttle limits for a host cluster. In case of a host cluster throttle, all the hosts in the host cluster share the throttle limit.

Note: If you are creating a host cluster, any hosts that are added cannot have throttles configured. Throttles can be applied to the entire host cluster.

The system also supports throttles to delay processing of I/O operations for volumes. If storage systems provide storage to a wide variety of applications, then production volumes with more critical I/O can be competing with volumes that have lower priority operations. For example, volumes that are used for backup or archive operations can have I/O intensive workloads, potentially taking bandwidth from production volumes. Volume throttle can be used to limit I/Os for these types volumes so that I/O operations for production volume are not affected.

You can also create throttles for systems that have copy offload features enabled, such as offloaded data transfer (ODX) on Microsoft Windows Server 2012 or for XCOPY/WRITESAME features on VMware hosts. Copy offload frees up hosts and speeds the copy process by offloading the operations of certain types of hosts to a storage system. For systems with these features enabled, administrators can define throttles to delay processing for copy offloads to free bandwidth for other more critical operations. When a throttle for copy offload is defined, the throttle applies across the entire system. Like other throttles on the system, you can set IOPS throttles, bandwidth throttles, or both; however, bandwidth throttles are more effective for copy offload operations.

Throttles can be defined for storage pools to control I/O operations on back-end storage systems. Storage pool throttles can be used to avoid overwhelming the back-end storage and be used with virtual volumes. Since virtual volumes use child pools, and throttle limit for the child pool can control the I/O operations from that virtual volume. Parent and child pool throttles are independent of each other. A child pool can have higher throttle limits than its parent pool.

Throttles for volumes, hosts, and host clusters can be configured in both the management GUI and the command-line interface (CLI). Throttles for copy offload and storage pools can be configure in the CLI only.