Veritas File System 5.0 Administrator's Guide (September 2006)
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.
Tagging volumes as placement classes
The following example tags the vsavola volume as placement class tier1, vsavolb
as placement class tier2, vsavolc as placement class tier3, and vsavold as
placement class tier4 using the vxadm command.
137Dynamic Storage Tiering
Placement classes