Rules are the basic elements of a VM storage policy. Each individual rule is a statement that describes a single requirement for virtual machine storage and data services.
Typically, rules can be in one of the following categories: placement rules, including capability-based and tag-based, and data services rules. Within a storage policy, individual rules are organized into collections of rules, or rule sets.
Placement Rules: Capability-Based
Placement rules specify a particular storage requirement for the VM and enable SPBM to distinguish compatible datastores among all datastores in the inventory. These rules also describe how the virtual machine storage objects are allocated within the datastore to receive the required level of service. For example, the rules can list Virtual Volumes as a destination and define the maximum recovery point objective (RPO) for the Virtual Volumes objects. When you provision the virtual machine, these rules guide the decision that SPBM makes about the virtual machine placement. SPBM finds the Virtual Volumes datastores that can match the rules and satisfy the storage requirements of the virtual machine.
Placement Rules: Tag-Based
Tag-based rules reference datastore tags. These rules can define your VM placement, for example, request as a target all datastores with the VMFS-Gold tag. You can also use the tag-based rules to fine-tune your VM placement request further. For example, exclude datastores with the Palo Alto tag from the list of your Virtual Volumes datastores.
Data Service Rules
Unlike the placement rules, the data service rules do not define storage placement and storage requirements for the virtual machine. Instead, these rules activate specific data services for the virtual machine, for example, caching and replication. Storage systems or other entities can provide these services. They can also be installed on your hosts and vCenter Server. You include the data service rules in the storage policy components. If the data service is provided by a storage system, you add the components to the regular rule sets. If the data service is provided by I/O filters, the component is added to the common rule set.