Veritas™ File System 5.0.1 Administrator's Guide

Placement classes
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 can have different tags, one of which can be
the placement class. The placement class tag makes a volume distinguishable by
DST.
Volume tags are organized as hierarchical name spaces in which periods separate
the levels of the hierarchy . 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 that DST uses. 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 vxvoladm(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.
145Dynamic Storage Tiering
Placement classes