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A placement class is a Dynamic Storage Tiering attribute of a given volume in a volume set of a multi-volume file system. This attribute is a character string, and is known as a volume tag. A volume may have different tags, one of which could be the placment class. The placement class tag makes a volume distinguishable by DST.
Volume tags are organized as hierarchical name spaces in which the levels of the hierarchy are separated by periods. By convention, the uppermost level in the volume tag hierarchy denotes the Storage Foundation component or application that uses a tag, and the second level denotes the tag's purpose. DST recognizes volume tags of the form vxfs.placement_class.class_name. The prefix vxfs identifies a tag as being associated with VxFS. placement_class identifies the tag as a file placement class used by DST. class_name represents the name of the file placement class to which the tagged volume belongs. For example, a volume with the tag vxfs.placement_class.tier1 belongs to placement class tier1. Administrators use the vxvoladm command to associate tags with volumes.
See the vxadm(1M) manual page.
VxFS policy rules specify file placement in terms of placement classes rather than in terms of individual volumes. All volumes that belong to a particular placement class are interchangeable with respect to file creation and relocation operations. Specifying file placement in terms of placement classes rather than in terms of specific volumes simplifies the administration of multi-tier storage.
The administration of multi-tier storage is simplified in the following ways:
Adding or removing volumes does not require a file placement policy change. If a volume with a tag value of vxfs.placement_class.tier2 is added to a file system's volume set, all policies that refer to tier2 immediately apply to the newly added volume with no administrative action. Similarly, volumes can be evacuated, that is, have data removed from them, and be removed from a file system without a policy change. The active policy continues to apply to the file system's remaining volumes.
File placement policies are not specific to individual file systems. A file placement policy can be assigned to any file system whose volume set includes volumes tagged with the tag values (placement classes) named in the policy. This property makes it possible for data centers with large numbers of servers to define standard placement policies and apply them uniformly to all servers with a single administrative action.