Allocation Policies

To make full use of multi-volume support features, VxFS provides support for allocation policies that allow files or groups of files to be assigned to specified volumes within the volume set.

A policy specifies a list of volumes and the order in which to attempt allocations. A policy can be assigned to a file, to a file system, or to a Storage Checkpoint created from a file system. When policies are assigned to objects in the file system, you must specify how the policy maps to both metadata and file data. For example, if a policy is assigned to a single file, the file system must know where to place both the file data and metadata. If no policies are specified, the file system places data randomly.

The following example shows how allocation policies work. Assume that there are two volumes from different classes of storage:

Create a file system on the myvset volume set and mount it:

Use the following fsapadm commands to define two allocation policies called "datapolicy" and "metadatapolicy" to refer to the vol1 and vol2 volumes:

Assign these policies at the file system level. The data policy must be specified before the metadata policy:

The assignment of the policies on a file system-wide basis ensures that any metadata allocated is stored on the device with the policy "metadatapolicy" (vol2) and all user data is be stored on vol1 with the associated "datapolicy" policy.

The effect of creating a number of files is shown in the following script:

Before the script completes, it runs out of space even though space is still available on the vol2 volume:

To allocate user data from the vol1 volume and then use vol2 if space runs out, assign the allocation policy as follows:

You must have system administrator privileges to create, remove, change policies, or set file system or Storage Checkpoint level policies. Users can assign a pre-existing policy to their files if the policy allows that. Policies can be inherited for new files.