Advantages and Disadvantages of using cache in TSM disk storage pools

 IBM Tivoli Storage Manager has a feature of caching the data in the disk storage pool even after the data has been migrated to next storagepool. Using cache in the TSM environment  disk storage pools has both advantages and disadvantages. 

What is chache ?

When cache is enabled, the migration process leaves behind duplicate copies of files after the server migrates these files to the next storage pool in the storage hierarchy. Using cache can improve the speed with which the server retrieves some files. Consider enabling cache for space-managed files that are frequently accessed by clients. 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.

For the cache enable storage pools, PCT_UTILIZED is the comination of both cache data and original client data. For the cache disabled storage pools, PCT_UTILIZED reflects only  the original client data. 

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To enable cache, specify CACHE=YES when defining or updating a storage pool.

Example: update stgpool diskstgpool cache=yes

How the server removes cached files ?

When space is needed, the server reclaims space occupied by cached files. Files that have the oldest retrieval date 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 05/16/08 and File B was last retrieved on 06/19/08, then File A is deleted to reclaim space first.

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If you do not want the server to update the retrieval date for files when a client restores or retrieves the file, specify the server option NORETRIEVEDATE in the server options file. If you specify this option, the server removes copies of files in cache regardless how recently the files were retrieved.

Advantages and Disadvantages of Chache in Storage Pools

  • Using cache can increase the time required for client backup operations to complete. Performance is affected because, as part of the backup operation, the server must erase cached files to make room for storing new files. The effect can be severe when the server is storing a very large file and must erase cached files. For the best performance for client backup operations to disk storage pools, do not use cache.
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  • Using cache can require more space for the server database. When you use cache, more database space is needed because the server has to keep track of both the cached copy of the file and the new copy in the next storage pool.
  • If you want to use caching, you cannot also enable shredding for that disk storage pool.
  • 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.
  •  If you leave cache disabled, consider higher migration thresholds for the disk storage pool. A higher migration threshold keeps files on disk longer because migration occurs less frequently.
  • If fast restores of active client data is your objective, you can also use active-data pools, which are storage pools containing only active versions of client backup data. 

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