ADSM provides automatic migration to maintain free space in a primary storage pool. For example, ADSM can migrate data stored on a random access disk storage pool to a less expensive sequential access storage pool when the migration threshold parameter you set is exceeded.
When you define or update a storage pool, set migration thresholds to specify when the server should begin migrating, or moving, data to the next storage pool in the storage hierarchy. This process helps to ensure that there is sufficient free space in the storage pools at the top of the hierarchy, where faster devices can provide the most benefit to clients.
You can use the defaults for the migration thresholds, or you can change the threshold values to identify the maximum and minimum amount of space for a storage pool. See "Defining a Primary Storage Pool" for more information about migration thresholds.
Before you define migration thresholds, you should understand how the server determines when to migrate files, and how it chooses which files to migrate. Then you can determine migration thresholds for both disk and sequential access storage pools.
For disk storage pools, migration thresholds can be set lower when cache is enabled. See "The Use of Cache on Disk Storage Pools" for information about setting the CACHE parameter.
When the high migration threshold is reached in a storage pool, ADSM migrates files from the pool to the next storage pool. ADSM first identifies which client node has backed up or migrated the largest single file space or has archived files that occupy the most space. When the server identifies the client node based on these criteria, the server migrates all files from every file space belonging to that client.
After the server migrates the files for the first client node to the next storage pool, the server checks the low migration threshold for the storage pool. If the amount of space used in the storage pool is now below the low migration threshold, migration ends. If not, the server chooses another client node by using the same criteria as described above, and the migration process continues.
If multiple migration processes are running (controlled by the MIGPROCESS parameter of the DEFINE STGPOOL command), the server may choose the files from more than one node for migration at the same time.
For example, Table 16 displays information contained in the database that is used by the server to
determine which files to migrate. This example assumes no space-managed
files are stored in the storage pool.
Table 16. Database Information on Files Stored in DISKPOOL
| Client Node | Backed-Up File Spaces | Archived Files (All Client File Spaces) |
|---|---|---|
| TOMC | TOMC/C = 200MB
TOMC/D = 100MB | 55MB |
| CAROL | CAROL = 50MB | 5MB |
| PEASE | PEASE/home = 150MB
PEASE/temp = 175MB | 40MB |
Figure 17. The Migration Process and Migration Thresholds
Figure 17 shows what happens when the high migration threshold defined for the disk storage pool DISKPOOL is exceeded. When the amount of migratable data in DISKPOOL reaches 80%, the server performs the following tasks:
The server migrates all of the data from both file spaces belonging to node TOMC, even if the occupancy of the storage pool drops below the low migration threshold before the second file space has been migrated.
If the cache option is enabled, files that are migrated remain on disk storage (that is, the files are cached) until space is needed for new files. For more information about using cache, see "The Use of Cache on Disk Storage Pools".
In this example, the server migrates all files that belong to the client node named PEASE to the TAPEPOOL storage pool.
Setting migration thresholds for disk storage pools ensures sufficient free space on faster speed devices, which can lead to better ADSM performance. Choosing thresholds appropriate for your situation takes some experimenting, and you can start by using the default values. You need to ensure that migration occurs frequently enough to maintain some free space but not so frequently that the device is unavailable for other use.
To choose the high-migration threshold, consider:
If you set the high-migration threshold too high, the pool may be just under the high threshold, but not have enough space to store an additional, typical client file. Or, with a high threshold of 100%, the pool may become full and a migration process must start before clients can back up any additional data to the disk storage pool. In either case, the server stores client files directly to tape until migration completes, resulting in slower performance.
If you set the high-migration threshold too low, migration runs more frequently and can interfere with other operations.
Keeping the high-migration threshold at a single value means that migration processing could start at any time of day, whenever that threshold is exceeded. You can control when migration occurs by using administrative command schedules to change the threshold. For example, set the high-migration threshold to 95% during the night when clients run their backup operations. Lower the high-migration threshold to 50% during the time of day when you want migration to occur. By scheduling when migration occurs, you can choose a time when your tape drives and mount operators are available for the operation.
To choose the low-migration threshold, consider:
If your disk space is limited, try setting the threshold so that migration frees enough space for the pool to handle the amount of client data that is typically stored every day. Migration then runs about every day, or you can force it to run every day by lowering the high-migration threshold at a time you choose.
If you do not use cache, you may want to keep the low threshold at a higher number so that more data stays on the disk.
You may need to balance the costs of larger disk storage pools with the costs of running migration (drives, tapes, and either operators or automated libraries).
Caching is a method of providing immediate access to files on disk storage, even if the server has migrated files to a tape storage pool. However, cached files are removed from disk when the space they occupy is required. The file then must be obtained from the storage pool to which it was migrated.
| Note: | The use of cache has some disadvantages. See "The Use of Cache on Disk Storage Pools". |
To ensure that files remain on disk storage and do not migrate to other storage pools, use one of the following methods:
A disadvantage of using this method is that if the file exceeds the space available in the storage pool, the operation to store the file fails.
When you set the high migration threshold to 100%, files will not migrate at all. You can still define the next storage pool in the storage hierarchy, and set the maximum file size so that large files are stored in the next storage pool in the hierarchy.
A disadvantage of setting the high threshold to 100% is that once the pool becomes full, client files are stored directly to tape instead of to disk. Performance may be affected as a result.
You can define or update migration thresholds for sequential access storage pools. However, you probably will not perform this type of migration on a regular basis. An operation such as tape-to-tape migration has limited benefits compared to disk-to-tape migration, and requires at least two tape drives.
You may find it necessary to migrate data from one sequential access storage pool to another. For example, you install a tape drive that uses a different type of tape and want to move data to that tape. In this case, migration from a sequential access storage pool may be appropriate.
Migration from sequential storage pools is performed by volume, to minimize the number of mounts for source volumes. Sequential volumes selected for migration are those that were least recently referenced.
When defining migration criteria for sequential access storage pools, consider:
If you decide to migrate data from one sequential access storage pool to another, ensure that:
For information about setting an access mode for sequential access storage pools, see "Defining a Primary Storage Pool".
When you enable collocation for a storage pool, ADSM attempts to keep all files belonging to a client node or a client file space on a minimal number of volumes. For information about collocation for sequential access storage pools, see "Collocation on Sequential Access Storage Pools".
If you want to limit migration from a sequential access storage pool to another storage pool, set the high-migration threshold to a high percentage, such as 95%.
For information about setting a reclamation threshold for tape storage pools, see "Reclaiming Space in Sequential Access Storage Pools".
There is no straightforward way to selectively migrate data for a specific node from one sequential storage pool to another. If you know the volumes on which a particular node's data is stored, you can use the MOVE DATA command to move all files from selected volumes to the new storage pool.
Copy storage pools are not part of the storage migration hierarchy. Files are not migrated to or from copy storage pools. The only way to store files in copy storage pools is by using the BACKUP STGPOOL command.
Migration of files between primary storage pools does not affect copy storage pool files. Copy storage pool files do not move when primary storage pool files move.
For example, suppose a copy of a file is made while it is in a disk storage pool. The file then migrates to a primary tape storage pool. If you then back up the primary tape storage pool to the same copy storage pool, a new copy of the file is not needed. ADSM knows it already has a valid copy of the file.