Different types of TSM incremental backups and their advantages and disadvantages


There are many backup types available in TSM Backup-Archive Client, You need to choose appropriate backup technique suitable for your environment according to your available resources and requirements. Below are some of the commonly used backup techniques and their advantages and disadvantages. 

Progressive Incremental Backup

Progressive incremental backup is the standard method of backup that is used by Tivoli Storage Manager. It is commonly known as incremental backup, Incremental backup processing backs up only those files that changed since the last full or incremental backup, unless the files are excluded from backup. If incremental backups does not complete in expected time frame, you can opt for other backup techniques. 

How it works ?
The following processes happens when you run an incremental backup
  • The client queries the Tivoli Storage Manager server for active backup version metadata.
  • The server returns a list of active backup versions for the entire file system.
  • The client scans and compares the list with the local file system to determine which files are new or changed since the last backup.
  • The client backs up the new and changed files.
When to use this backup technique ?
This is the default backup method and use incremental backup if you have enough system memory resources, backup window duration, or other operational issues. 

Advantages
  • This method is the most comprehensive & by default recommended backup method for Tivoli Storage Manager.
  • No redundant backups are made. You back up only what is changed.
  • There is less network utilization because unchanged files do not have to be sent over the network.
  • This method is a form of single-instance storage because a file is not backed up again if it does not change. Incremental backups are more efficient and save space on the server storage pools.
  • Files are easier to restore because you do not have to restore the base backup version first and apply incremental or differential changes.
Disadvantages
  • The client system might run out of memory if the number of active backup versions is too large.
  • The time that it takes to scan file systems that contain millions of files can exceed the duration of the backup window.

Incremental-by-date Backup

This backup method backs up new and changed files that have a modification date later than the date of the last incremental backup that is stored at the server, unless the files are excluded from backup.

How it works ?
  • When you run an incremental-by-date backup. The client queries the server for the most recent backup of the entire file system.
  • The server returns the time stamp of the most recent backup of the entire file system.
  • The client scans and compares the list from the server with the local file system and backs up the new and changed files that are based on the time stamp of the most recent backup.
When to use ?
  • When the scheduled backups are not completed within the allotted time.
  • The changes to the file system are additive or changing, but not deleted.
  • You also run weekly (or periodic) full incremental backups.
Advantages
  • This method reduces the time that it takes to determine which files changed.
  • This method removes the processing time on the server that is used to query the database for changed files.
  • This method removes the network traffic that is used to communicate the query results.
Disadvantages
  • This method reduces the flexibility of the scope of the backup operation. You must back up the entire file system.
  • The files are not backed up if the changes do not affect the date (for example, attribute, mode, ACL, rename, copy, move, and security changes).
  • The deleted files are not expired on the server.
  • Policy rebinding does not take place.
  • The entire file system must be scanned.
  • This method cannot be used if the client and server clocks are set to different times or are not in the same time zone.

Adaptive Subfile Backup

If you plan to back up your files over a network device with limited bandwidth, you can reduce network traffic by using adaptive subfile backup technique.

How it works ?
Adaptive subfile backup processing backs up the changed portion of a file on Windows clients.

When to use  Adaptive Subfile backup ?
  • The network is limited or constrained.
  • The file sizes are small (less than 2 GB in size).
Advantages
  • Faster throughput.
  • Reduced storage pool consumption
Disadvantages
  • This method uses a large amount of local cache space.
  • Some processing time is required during the backup.
  • The restore operations can take longer because of the base file and delta files are restored.
  • The client can run out of disk space during the restore if disk space is constrained because of how files are reconstructed from the base files and the delta files.

Configuring Multiple session backup

The backup-archive client can run concurrent sessions to back up and restore data to help improve performance. During incremental backup processing, the client can process multiple objects in parallel by opening more than one session with the Tivoli Storage Manager server.

How it works ?
Multiple sessions are used when you specify multiple file specifications on a backup, restore, archive, or retrieve command. For example, you can start a multiple session backup with the following command

On the AIX, HP-UX, Linux, Mac OS X, or Solaris clients 
incr /fs1/data1   /fs2/data2

On the Windows client
incr c: d:

The resourceutilization option is used to regulate the level of resources that the Tivoli Storage Manager server and client can use during processing. The default is to use a maximum of two sessions, one session to query the server and one session to send file data.

Also Read: Quick Steps to configure TSM for Database (TDPOracle & TDPSQL) and TSM for Mail (TDPExchange & TDPDomino) 

When to use multi-session backups ?
Use multiple backup sessions when you want to increase client performance, and you have sufficient client and server resources and processing capacity. For example, the server and client hardware must have sufficient memory, storage, and processor capacity to support multiple sessions. The network bandwidth must also be sufficient to handle the increased amount of data that flows across the network.

Advantages
Using more than one backup session can often lead to overall improvements in throughput.

Disadvantages
  • During a multiple session backup operation, files from one file specification might be stored on multiple tapes on the server and interspersed with files from different file specifications. This arrangement can decrease restore performance.
  • To avoid the performance degradation in restore operations, set the collocatebyfilespec option to yes. This setting eliminates the interspersing of files from different file specifications by limiting the client to one server session for each file specification. Therefore, if the data is stored to tape, the files for each file specification are stored together on one tape, unless another tape is required for more capacity.
  • The client might produce multiple accounting records.
  • The server might not start enough concurrent sessions. To avoid this situation, the maxsessions server parameter must be reviewed and possibly changed.
  • A query node command might not summarize the client activity.




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