When defining or updating disk storage pools, you can enable or disable cache.
When cache is disabled and migration occurs, the server migrates the files to the next storage pool and erases the files from the disk storage pool. By default, the system disables caching for each disk storage pool because of the potential effects of cache on backup performance.
You can enable cache by specifying CACHE=YES when you define or update a storage pool. When cache is enabled, the migration process leaves behind duplicate copies of files on disk after the server migrates these files to the next storage pool in the storage hierarchy. The copies remain in the disk storage pool, but in a cached state, so that subsequent retrieval requests can be satisfied quickly. However, if space is needed to store new data in the disk storage pool, cached files are erased and the space they occupied is used for the new data.
The advantage of using cache for a disk storage pool is that cache can improve how quickly the server retrieves some files. When you use cache, a copy of the file remains on disk storage after the server migrates the primary file to another storage pool. You may want to consider using a disk storage pool with cache enabled for storing space-managed files that are frequently accessed by clients.
However, using cache has some important disadvantages:
For the best performance for client backup operations to disk storage pools, do not use cache.
If you leave cache disabled, you may want to consider higher migration thresholds for the disk storage pool. A higher migration threshold keeps files on disk longer because migration occurs less frequently.
When space is needed, the server reclaims space occupied by cached files. Files that have the oldest retrieval date and occupy the largest amount of disk space are overwritten first. For example, assume that two files, File A and File B, are cached files that are the same size. If File A was last retrieved on 05/16/99 and File B was last retrieved on 06/19/99, then File A is deleted to reclaim space first.
You can change whether the server tracks the retrieval date for a file with the server option, NORETRIEVEDATE. When you include this option in the server options file, the server does not update the retrieval date for files. As a result, the server may remove copies of files in cache even though clients retrieved the files recently.
The space utilization statistic for the pool (Pct Util) includes the space used by any cached copies of files in the storage pool. The migratable data statistic (Pct Migr) does not include space occupied by cached copies of files. The server uses the statistic on migratable data (Pct Migr) to compare with migration threshold parameters to determine when migration should begin or end. For more information on storage pool statistics, see Monitoring Storage Pools and Volumes.