When to choose tape to tape migration in TSM


IBM Tivoli Storage Manager (TSM) supports both Disk-Tape Migration and Tape-Tape Migration. The same process which we normally use for migrating data from Disk to tape can also be used for Tape to Tape migration. You can set up migration thresholds for sequential-access storage pools. Migrating data from one sequential-access storage pool to another might be appropriate in some cases, for example, when you install a tape drive that uses a different type of tape and want to move data to that tape. A tape-to-tape migration has limited benefits compared to disk-to-tape migration, and requires at least two tape drives. 

Tape to Tape Migration in TSM:

You can migrate data from a sequential-access storage pool only to another sequential-access storage pool. You cannot migrate data from a sequential-access storage pool to a disk storage pool. If you need to move data from a sequential-access storage pool to a disk storage pool, use the MOVE DATA command.

To control the migration process, set migration thresholds and migration delays for each storage pool using the DEFINE STGPOOL and UPDATE STGPOOL commands. You can also specify multiple concurrent migration processes to better use your available tape drives or FILE volumes. Using the MIGRATE STGPOOL command, you can control the duration of the migration process and whether reclamation is attempted prior to migration.

How the server migrates files from sequential-access storage pools ?

The server migrates files by volume from sequential-access storage pools. Volumes that exceed the reclamation threshold are migrated first. Files in the least frequently referenced volumes are migrated next. Before files are migrated, the server checks the migration delay for the storage pool.

Also Read: TSM Storage Pool Concepts (V7 Revised)

For tape and optical storage pools, the server begins the migration process when the ratio of volumes containing data to the total number of volumes in the storage pool, including scratch volumes, reaches the high migration threshold. For sequential-access disk (FILE) storage pools, the server starts the migration process when the ratio of data in a storage pool to the pool's total estimated data capacity reaches the high migration threshold. The calculation of data capacity includes the capacity of all the scratch volumes specified for the pool.

When migrating files by volume from sequential-access storage pools, including sequential-access disk storage pools associated with a FILE device class, the server performs the following procedure:
  • The server first reclaims volumes that have exceeded the reclamation threshold. Reclamation is a server process of consolidating files from several volumes onto one volume. 
  • After reclamation processing, the server compares the space used in the storage pool to the low migration threshold.
  • If the space used is now below the low migration threshold, the server stops processing. If the space used is still above the low migration threshold, the server determines which volume is the least recently referenced volume.
  • If the amount of time a file has been in the storage pool exceeds the amount of time specified as the migration delay for the storage pool, the file is eligible for migration. The server selects the volume for migration only when all files on the volume are eligible for migration.
  • The server repeats steps 3 and 4 until the storage pool reaches the low migration threshold.
Because migration delay can prevent volumes from being migrated, the server can migrate files from all eligible volumes but still find that the storage pool is above the low migration threshold. If you set migration delay for a pool, you need to decide what is more important: either ensuring that files stay in the storage pool for as long as the migration delay, or ensuring there is enough space in the storage pool for new files. 

Also Read: How to configure TSM Server Administrative Schedules ?

For each storage pool that has a migration delay set, you can choose what happens as the server tries to move enough files out of the storage pool to reach the low migration threshold. If the server cannot reach the low migration threshold by migrating only volumes that meet the migration delay requirement, you can choose one of the following: 
  • Allow the server to migrate volumes from the storage pool even if they do not meet the migration delay criteria (MIGCONTINUE=YES). This is the default. Allowing migration to continue ensures that space is made available in the storage pool for new files that need to be stored there.
  • Have the server stop migration without reaching the low migration threshold (MIGCONTINUE=NO). Stopping migration ensures that volumes are not migrated for the time you specified with the migration delay. The administrator must ensure that there is always enough space available in the storage pool to hold the data for the required number of days.

Migration criteria for sequential-access storage pool

If you are planning to use migration for sequential-access storage pools, you need to consider a number of factors, including the time required to mount tapes into drives and whether collocation is enabled.

Also Read: How TSM Server determines the eligibility of files during different types of backup ?


When defining migration criteria for sequential-access storage pools, consider: 
1. The capacity of the volumes in the storage pool
2. The time required to migrate data to the next storage pool
3. The speed of the devices that the storage pool uses
4. The time required to mount media, such as tape volumes, into drives
5. Whether operator presence is required
6. The number of concurrent migration processes
If you decide to migrate data from one sequential-access storage pool to another, ensure that: 
  • Two drives (mount points) are available, one in each storage pool.
  • The access mode for the next storage pool in the storage hierarchy is set to read/write.

  • Collocation is set the same in both storage pools. For example, if collocation is set to NODE in the first storage pool, then collocation should be set to NODE in the next storage pool. 
  • 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%.
There is no straightforward way to selectively migrate data for a specific node from one sequential storage pool to another. You can use the MOVE NODEDATA command to move file spaces for a node from one storage pool to another.




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