VxFS uses multi-tier online storage by way of the Dynamic Storage Tiering (DST) feature, which functions on top of multi-volume file systems. Multi-volume file systems are file systems that occupy two or more virtual volumes. The collection of volumes is known as a volume set. A volume set is made up of disks or disk array LUNs belonging to a single Veritas Volume Manager (VxVM) disk group. A multi-volume file system presents a single name space, making the existence of multiple volumes transparent to users and applications. Each volume retains a separate identity for administrative purposes, making it possible to control the locations to which individual files are directed.

See About multi-volume support.

DST allows administrators of multi-volume VxFS file systems to manage the placement of files on individual volumes in a volume set by defining placement policies. Placement policies control both initial file location and the circumstances under which existing files are relocated. These placement policies cause the files to which they apply to be created and extended on specific subsets of a file system's volume set, known as placement classes. The files are relocated to volumes in other placement classes when they meet the specified naming, timing, access rate, and storage capacity-related conditions.

You make a VxVM volume part of a placement class by associating a volume tag with it. For file placement purposes, VxFS treats all of the volumes in a placement class as equivalent, and balances space allocation across them. A volume may have more than one tag associated with it. If a volume has multiple tags, the volume belongs to multiple placement classes and is subject to allocation and relocation policies that relate to any of the placement classes. Multiple tagging should be used carefully.

See Placement classes.

VxFS imposes no capacity, performance, availability, or other constraints on placement classes. Any volume may be added to any placement class, no matter what type the volume has nor what types other volumes in the class have. However, a good practice is to place volumes of similar I/O performance and availability in the same placement class.

The Using Dynamic Storage Tiering Symantec Yellow Book provides additional information regarding the Dynamic Storage Tiering feature, including the value of DST and best practices for using DST. You can download Using Dynamic Storage Tiering from the following Web page:

http://www.symantec.com/enterprise/yellowbooks/index.jsp