Administrator's Guide

How ADSM Uses Sequential Access Devices

Each ADSM library is a collection of drives. A device class, which governs how data is stored, is associated with one library. When you define a storage pool, you associate the pool with a device class. Volumes are associated with pools. Figure 11 shows these relationships.

Figure 11. Relationships between Storage and Device Objects

When the ADSM server determines that data is to be stored into or retrieved from a storage pool, it performs the following procedure:

1. Selects a volume from the given storage pool. The selection is based on the type of operation:

   Retrieval

   The name of the volume is stored in the server database.

   Store

   If a defined volume in the storage pool can be used for the data being stored, the server chooses this volume name.

   If no defined volumes in the storage pool can be used for the data, and if the MAXSCRATCH parameter of the storage pool permits it, the server may try a scratch mount.

2. Determines the name of the library containing the drives that can be used for the operation by checking the device class associated with the storage pool.

   • The server evaluates the status of each drive in the library until an available drive is found or until all drives have been checked. Drive status can be:

     • The drive is offline.

     • The drive is busy and cannot be used for this mount.

     • The drive is in an error state and cannot be used for this mount.

     • The drive is available and can be used for this mount.

3. Performs the volume mount operation:

   • If the library is manually operated, the server displays request messages for a mount operator, asks that the desired volume, or a scratch volume, be mounted in the selected drive.

   • If the library is automated, the server directs a robotic device to move the volume from a storage slot into the selected drive. No manual intervention is required.

     If a scratch mount is requested, the server checks the library's volume inventory to see if there is a volume with a status code of SCRATCH. The volume inventory is established and managed by using the commands described in "Managing Storage Volumes in Automated Libraries". Volume status codes are described in "Private and Scratch Volumes in Automated Libraries". If a scratch volume is found, its volume status code is changed to PRIVATE and it is mounted in the drive. Eventually, it is automatically defined as part of the original storage pool. However, if the library's volume inventory does not contain any volumes with a status code of SCRATCH, the mount request fails.

4. Dismounts the volume from the drive when it has finished accessing the volume and the mount retention period has elapsed.

   • If the library is manually operated, the server ejects the volume from the drive so that a mount operator can place it in an appropriate storage location.

   • If the library is automated, the server interacts with a robotic device to move the volume from the drive back to its original storage slot in the library.