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A datastore cluster is a collection of datastores with shared resources and a shared management interface. Datastore clusters are to datastores what clusters are to hosts. When you create a datastore cluster, you can use vSphere Storage DRS (SDRS or Storage DRS) to manage storage resources.
When a datastore cluster is created, Storage DRS can manage the storage resources similarly to how vSphere DRS manages compute resources in a cluster. As with a cluster of hosts, a datastore cluster is used to aggregate storage resources. This method enables smart and rapid placement of new virtual machines, and virtual disk drives as well as load balancing of existing workloads. Storage DRS does not have to be enabled on a datastore cluster, it is merely an option. When Storage DRS is not enabled, a datastore cluster is essentially a folder to group datastores together.
Storage DRS provides initial placement and ongoing balancing recommendations to datastores in a Storage DRS-enabled datastore cluster. Initial placement occurs when Storage DRS selects a datastore within a datastore cluster on which to place a virtual machine disk. This placement happens when:
Initial placement recommendations are made in accordance with space constraints and considering the goals of space and I/O load balancing. Figure 125 demonstrates the options available for either manual or automated control of different features for a datastore cluster.
These goals aim to minimize the risk of over-provisioning one datastore, storage I/O bottlenecks, and performance impact on virtual machines. It is particularly useful when VMs use thin VMDKs.
Storage DRS is invoked at the configured frequency (by default, every eight hours) or when one or more datastores in a datastore cluster exceeds the user-configurable space utilization or I/O latency thresholds. When Storage DRS is invoked, it checks the space utilization and I/O latency values of each datastore against the threshold.
For I/O latency, Storage DRS uses the 90th percentile I/O latency that is measured over the course of a day to compare against the threshold. Storage DRS applies the datastore utilization reporting mechanism of VMware vCenter Server, to make recommendations whenever the configured used space threshold is exceeded. Storage DRS calculates all possible moves, to balance the load accordingly while considering the cost and the benefit of the migration. Storage DRS considers moving virtual machines that are powered off or powered on for space balancing. Storage DRS includes powered-off virtual machines with snapshots in these considerations.