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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. 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. To enable this type of backup, see Setting Up Clients to Use Subfile Backup.
Assume that on a Monday, a user requests an incremental backup of a file called CUST.TXT. The user makes daily updates to the CUST.TXT file and requests subsequent backups.
The following table describes how Tivoli Storage Manager handles backups of
file CUST.TXT.
| Version | Day of subsequent backup | What Tivoli Storage Manager backs up |
|---|---|---|
| One | Monday | The entire CUST.TXT file (the base file) |
| Two | Tuesday | A subfile of CUST.TXT. The server compares the file backed up on Monday with the file that needs to be backed up on Tuesday. A subfile containing the changes between the two files is sent to the server for the backup. |
| Three | Wednesday | A subfile of CUST.TXT. Tivoli Storage Manager compares the file backed up on Monday with the file that needs to be backed up on Wednesday. A subfile containing the changes between the two files is sent to the server for the backup. |
To enable subfile backup, do the following:
set subfile client
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 Creating Client Option Sets from the Server for how to set up and use client option sets.
See Tivoli Storage Manager for Windows: Backup-Archive Installation and User's Guide for more information about the options.
The following sections describe how Tivoli Storage Manager manages subfiles that are restored, exported, imported, or added to a backup set.
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.
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.
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. For more information on how the server manages file expiration, see Running Expiration Processing to Delete Expired Files.
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.
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.