A scratch volume is a labeled volume that is empty or contains no valid data, and can be used to satisfy any request to mount a scratch volume. A private volume is a volume that is in use or owned by an application, and may contain valid data. Volumes that you define to ADSM are private volumes. A private volume is used to satisfy only a request to mount that volume by name. For each storage pool, you must decide whether to use scratch volumes.
If you use scratch volumes, ADSM uses volumes as needed, and returns the volumes to scratch when they become empty (for example, when all data on the volume expires). If you do not use scratch volumes, you must define each volume you want ADSM to use. Volumes that you define to ADSM are private volumes, and do not return to scratch when they become empty.
For more information on defining volumes, see "Preparing Removable Media for ADSM".
For each automated library, ADSM tracks in its volume inventory for the library whether a volume has scratch or private status. If you allow scratch volumes to be used for a storage pool, ADSM will choose a scratch volume from the scratch volumes that are checked in for the automated library.
When ADSM uses a scratch volume, ADSM changes the volume's status to private by defining it. ADSM tracks whether defined volumes were originally scratch volumes. Volumes that were originally scratch volumes return to scratch status when they become empty. You lose the usage statistics on the volumes when the status of the volumes is changed.
For information on changing the status of a volume in an automated library, see "Changing the Status of a Volume in a Library".
ADSM maintains a volume inventory for each automated library that you define. The inventory for a library includes only those volumes that you have checked in to that library.
The list of volumes that are checked in to a library is not necessarily identical to the list of volumes in the storage pools associated with the library. A volume may be checked in to the library but not in a storage pool (a scratch volume). A volume may be defined to a storage pool associated with the library (a private volume), but not in the library's volume inventory.
For more information on how to check in volumes, see "Informing the Server about New Volumes in a Library".
You may want to use the private status for volumes if you carefully regulate which volumes are used by individual storage pools in your environment. You must define the volumes (DEFINE VOLUME command) for each storage pool. To mount a private volume, you must provide the volume name. If you are doing database backup, dump, or load, or import or export operations, you must list the volumes to use if you want to use private volumes.
When the ADSM server needs a new volume for a drive in an automated library, the server can choose any volume in the library whose status code indicates that it is a scratch volume. The scratch volume is selected only when the MAXSCRATCH value is greater than zero. After the volume is mounted, its status code is changed to private and the volume is automatically defined as part of the storage pool for which the mount request was made. ADSM tracks in the database that this defined volume was originally a scratch volume. When that volume is deleted from the storage pool (for example, all the data it contains expires), the volume returns to scratch status and can be reused by the same or a different storage pool that uses the library.
One of the benefits of using scratch volumes is that different storage pools that share the same automated library can dynamically acquire volumes from the library's pool of scratch volumes. The volumes need not be preallocated to the different storage pools.
Another benefit of using scratch volumes, even if only a single storage pool is associated with an automated library, is that you need not explicitly define all of the volumes for the storage pool using DEFINE VOLUME commands. Volumes are automatically added to and deleted from the storage pool by the server.
| Note: | One disadvantage of using scratch volumes is that your volume usage information (for determining when the media has reached its end of life) is deleted when the private volume is returned to the scratch volume pool. |
If a scratch volume is used for a database backup or export operation, ADSM changes the volume's status code to private. The volume returns to the scratch pool only when an ADSM administrator determines that the volume's data is no longer needed, and uses the UPDATE LIBVOLUME command to change the status of the volume to scratch.