This feature delivers the DRS benefits of resource aggregation, automated initial placement, and bottleneck avoidance to storage. You can group and manage similar datastores as a single load-balanced storage resource called a datastore cluster. Storage DRS makes VMDK placement and migration recommendations to avoid I/O and space utilization bottlenecks on the datastores in the cluster. Storage DRS takes care of the initial placement of virtual machines and VMDK files. This placement is based on Space and I/O capacity. Storage DRS will select the best datastore to place this virtual machine or virtual disk in the selected Datastore Cluster. When Storage DRS is set to fully automatic, it will do automated load balancing actions. Of course this can be configured as manual as well and that is actually the default today. Load balancing again is based on space and I/O capacity. If and when required Storage DRS will make recommendations based space and I/O capacity. It will however only do this when a specific threshold is reached.
A datastore cluster is a collection of datastores aggregated into a single unit of consumption for an administrators. When a datastore cluster is created, Storage DRS can manage the storage resources comparable to how DRS manages compute resources in a cluster. As with a cluster of hosts, a datastore clusters is used to aggregate storage resources, enabling smart and rapid placement of new virtual machines and virtual disk drives and load balancing of existing workloads. When you create a VM you will be able to select a Datastore Cluster as opposed to individual datastores. Storage DRS provides initial placement recommendations to datastores in a Storage DRS-enabled datastore cluster based on I/O and space capacity.
During the provisioning of a virtual machine, a datastore cluster can be selected as the target destination for this virtual machine or virtual machine disk after which a recommendation for initial placement is done based on I/O and space capacity. Initial Placement in a manual provisioning process has proven to be very complex in most environments and as such important provisioning factors like current I/O load or space utilization are often ignored. Storage DRS ensures initial placement recommendations are made in accordance with space constraints and with respect to the goals of space and I/O load balancing. Although people are really excited about automated load balancing, it is Initial Placement where most people will start off with and where most people will benefit from the most as it will reduce operational overhead associated with the provisioning of virtual machines.
Ongoing balancing recommendations are made when one or more datastores in a datastore cluster exceeds the user-configurable space utilization or I/O latency thresholds. These thresholds are typically defined during the configuration of the datastore cluster. Storage DRS utilizes vCenter Server’s datastore utilization reporting mechanism to make recommendations whenever the configured utilized space threshold is exceeded. I/O load is evaluated by default every 8 hours currently with a default latency threshold of 15ms. Only when this I/O latency threshold is exceeded Storage DRS will calculate all possible moves to balance the load accordingly while considering the cost and the benefit of the migration. If the benefit doesn’t last for at least 24 hours, Storage DRS will not make the recommendation.