Sunday, April 19, 2009

Delaying the Re-use of Tape Storage Pools

The REUSEDELAY attribute of a sequential access (tape or file disk pools) storage pool the number of days that must elapse before a volume can be reused or returned to scratch status, after all files have been expired, deleted, or moved from the volume.

When you delay reuse of such volumes and they no longer contain any files, they enter the pending state. Volumes remain in the pending state for as long as specified with the REUSEDELAY parameter for the storage pool to which the volume belongs. Server internals will take care of finally deleting the Pending Volume from the storage pool when its time is up.

Delaying reuse of volumes can be helpful under certain conditions for disaster recovery. When TSM expires, deletes, or moves files from a volume, the files are not actually erased from the volumes: the database references to these files are removed. Thus the file data may still exist on sequential volumes if the volumes are not immediately reused.

If a disaster forces you to restore the TSM database using a database backup that is old or is not the most recent backup, some files may not be recoverable because TSM cannot find them on current volumes. However, the files may exist on volumes that are in pending state. You may be able to use the volumes in pending state to recover data by doing the following:

1. Restore the database to a point-in-time prior to file expiration.
2. Use a primary or copy storage pool volume that has not been rewritten and contains the expired file at the time of database backup.

If you back up your primary storage pools, set the REUSEDELAY parameter for the primary storage pools to 0 to efficiently reuse primary scratch volumes. For your copy storage pools, you should delay reuse of volumes for as long as you keep your oldest database backup. No useful purpose is served by setting REUSEDELAY to a value dramatically larger than the Retention period for Database backups.

Volumes in a storage pool with a non-zero REUSEDELAY may not remain in the storage pool for the REUSEDELAY period if access is set to destroyed. If REUSEDELAY is set to zero (zero is the default), this problem does not apply. Volumes which are in a destroyed state will be immediately deleted from the storage pool and set to scratch once they have been restored or deleted. Try to avoid updating a volume's access to DESTROYED, use UNAVAILABLE instead.

The TSM database retention period is specified using the SET DRMDBBACKUPEXPIREDAYS. By specifying this value to the REUSEDELAY period in the copy pool definition ensures that the database can be restored to an earlier level and database references to files in the storage pool are still valid.

For the PDF version of this document, send a blank email, with subject line "Delaying the Re-use of Sequential Access Volumes", to TSM Assist

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