What is TSM adaptive subfile backup ?

A basic problem that remote and mobile users face today is connecting to storage management services by using modems with limited bandwidth or poor line quality. This creates a need for users to minimize the amount of data they send over the network, as well as the time that they are connected to the network. To help address this problem, you can use subfile backups. 

Adaptive Subfile Backup Overview

When a client's file has been previously backed up, any subsequent backups are typically made of the portion of the client's file that has changed (a subfile), rather than the entire file. A base file is represented by a backup of the entire file and is the file on which subfiles are dependent. If the changes to a file are extensive, a user can request a backup on the entire file. A new base file is established on which subsequent subfile backups are dependent. This type of backup makes it possible for mobile users to reduce connection time, network traffic, and the time it takes to do a backup.

The subfile backup pertains to the sections of the files that have changed. To enable subfile backup, perform the following tasks: 

How to setup subfile backup on the TSM Server
You must set up the server to allow clients to back up subfiles. Issue the SET SUBFILE command:
set subfile <clientname>

How to setup subfile backup on the TSM Clients
The SUBFILEBACKUP, SUBFILECACHEPATH, and SUBFILECACHESIZE options must be set in the client's options file (dsm.opt). You can control these options from the server by including them in client option sets. For example, you can disable subfile backup for individual client nodes by setting SUBFILEBACKUP=NO in the client option set associated with the client node. See Backup-Archive Clients Installation and User's Guide for more information about the options.

The subfilebackup option in dsm.opt specifies whether to enable adaptive subfile backup for this client
subfilebackup YES/NO

The subfilecachepath option specifies the path where the client cache resides for adaptive subfile backup processing. If you do not specify a path, Tivoli Storage Manager creates a path called \cache under the directory where the Tivoli Storage Manager executables reside.
subfilecachepath c:\temp\tsmcache

The subfilecachesize option specifies the client cache size for adaptive subfile backup. If the cache size is too small, base files for some files will not be cached and subfile processing will not apply for them. However, setting the value too large can take up more disk space than can be spared. It Specifies the size, in megabytes, of the client cache for adaptive subfile backup processing. 
subfilecachesize 10

How subfile backups are managed ?

Tivoli Storage Manager manages subfiles that are restored, exported, imported, or added to a backup set.

Also Read: Different types of Incremental Backups

Restoring subfiles
When a client issues a request to restore subfiles, Tivoli Storage Manager restores subfiles along with the corresponding base file back to the client. This process is transparent to the client. That is, the client does not have to determine whether all subfiles and corresponding base file were restored during the restore operation. You can define (move) a backup set that contains subfiles to an earlier version of a server that is not enabled for subfile backup. That server can restore the backup set containing the subfiles to a client not able to restore subfiles. However, this process is not recommended as it could result in a data integrity problem.

Exporting and importing subfiles
When subfiles are exported during an export operation, Tivoli Storage Manager also exports the corresponding base file to volumes you specify. When the base file and its dependent subfiles are imported from the volumes to a target server and import processing is canceled while the base file and subfiles are being imported, the server automatically deletes any incomplete base files and subfiles that were stored on the target server.

Expiration processing of base files and subfiles
Because subfiles are useless without the corresponding base file, the server processes base files eligible for expiration differently. For example, when expiration processing runs, Tivoli Storage Manager recognizes a base file as eligible for expiration but does not delete the file until all its dependent subfiles have expired. Adding subfiles to backup sets When a subfile is added to a backup set, Tivoli Storage Manager includes its corresponding base file with the backup set. If the base file and dependent subfiles are stored on separate volumes when a backup set is created, additional volume mounts may be required to create the backup set.

Also Read: Different types of Image Backups

Deleting base files
If a base file is deleted as a result of processing a DELETE VOLUME command, the server recognizes its dependent subfiles and deletes them from the server as well. Subfiles without the corresponding base file are incomplete and useless  to the user.

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