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Another feature of Storage DRS is its abaility to balance VMs across datastores in the datastore cluster based on I/O metrics, specifically based on latency.

First, let us see how Storage DRS (SDRS) is able to capture this information.

SDRS uses Storage I/O Control (SIOC) to evaluate datastore capabilities & capture latency information regarding all the datastores in the datastore cluster. SIOC was first introduced in vSphere 4.1. Its purpose is to ensure that no single VM uses all the bandwidth of a particular datastore, and it modifies the queue depth to the datastores on each ESX to achieve this.

In SDRS, its implementation is different. SIOC (on behalf of SDRS) checks the capabilities of the datastores in a datastore cluster by injecting various I/O loads. Once this information is normalized, SDRS will have a good indication of the types of workloads that a datastore can handle. This information is used in initial placement and load balancing decisions.

SDRS continuously uses SIOC to monitor how long it takes an I/O to do a round trip – this is the latency. This information about the datastore is passed back to Storage DRS. If the latency value for a particular datastore is above the threshold value (default 15ms) for a significant percentage of time over an observation period (default 16 hours), SDRS will try to rebalance the VMs across the datastores in the datastore cluster so that the latency value returns below the threshold. This may involve a single or multiple Storage vMotion operations. In fact, even if SDRS is unable to bring the latency below the defined threshold value, it may still move VMs between datastores to balance the latency.

And since we now support Storage I/O Control on NFS in vSphere 5.0, we can also have NFS datastore clusters in SDRS.

If the datastore cluster is set to manual mode, SDRS will raise an alarm to bring to the administrators attention that a recommendation has been made. By looking at the SDRS tab, the administrator can then see the recommendations made by SDRS in order to balance the I/O load. An example of a recommendation is shown here:

Sdrs-7
What is very cool about the recommendation is that it gives the administration insight into what the latency measurements are on the source and destination datastores. The administrator can then refer to this information before deciding on whether to migrate the VM or not.

Storage DRS provides customers with a way of automatically load-balancing their datastores, avoiding hot-spots on your storage.