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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 to manage storage resources.
When a datastore cluster is created, Storage DRS can manage the storage resources comparably 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, enabling smart and rapid placement of new virtual machines and virtual disk drives as well as load balancing of existing workloads. It is important to note that Storage DRS does not have to be enabled on a datastore cluster, it is simply an option. When Storage DRS is not enabled, a datastore cluster is essentially just 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 happens when the virtual machine is being created or cloned, when a virtual machine disk is being migrated to another datastore cluster, or when you add a disk to an existing virtual machine. Initial placement recommendations are made in accordance with space constraints and with respect to the goals of space and IO load balancing. Figure 140 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 IO 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 IO latency thresholds. When Storage DRS is invoked, it checks each datastore's space utilization and IO latency values against the threshold.
For IO latency, Storage DRS uses the 90th percentile IO latency 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 utilized space 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. 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.
Storage DRS affinity rules enable control over which virtual disks should or should not be placed on the same datastore within a datastore cluster. By default, a virtual machine's virtual disks are kept together on the same datastore. Storage DRS offers three types of affinity rules:
In addition, Storage DRS offers Datastore Maintenance Mode, which automatically evacuates all registered virtual machines and virtual disk drives from the selected datastore to the remaining datastores in the datastore cluster. This is shown in Figure 141.
Datastores and hosts that are associated with a datastore cluster must meet certain requirements to use datastore cluster features successfully. Follow these guidelines when you create a datastore cluster: