Overview

Using compression in your TSM client can save storage space for your backups saving you money. Compression happens on the client side before being sent to the TSM server also sending less data across the network. The compression algorithm does use additional CPU time and may cause the backup to take slightly longer, but it will not be noticed on most systems.

Settings

Compression is an option in your dsm.opt or dsm.sys file (depending on platform). It is usually on by default in most config files, but if the option is not present compression will be disabled.

NOTE: If you make any configuration changes, the scheduler will need to be restarted for any changes to take affect.

• To enable compression:

		Compression      Yes

• To disable compression:

		Compression      No

Negative Compression

Data that is already compressed cannot be re-compressed. Compressing a file that's already compressed can actually cause the file to grow. In TSM, this behavior is shown as negative compression. If you have client compression enabled and you are backing up a lot of compressed data you may end up storing more data than you actually have. You can check for this in the backup summary in your dsmsched.log:

2012-02-06 01:48:37 --- SCHEDULEREC STATUS BEGIN
2012-02-06 01:48:37 Total number of objects inspected:  108,725

2012-02-06 01:48:37 Total number of objects updated:          0
2012-02-06 01:48:37 Total number of objects rebound:          0
2012-02-06 01:48:37 Total number of objects deleted:          0
2012-02-06 01:48:37 Total number of objects expired:          1
2012-02-06 01:48:37 Total number of objects failed:           0
2012-02-06 01:48:37 Total number of bytes transferred:   489.06 MB
2012-02-06 01:48:37 Data transfer time:                    9.93 sec
2012-02-06 01:48:37 Network data transfer rate:        50,396.29 KB/sec
2012-02-06 01:48:37 Aggregate data transfer rate:        406.87 KB/sec
2012-02-06 01:48:37 Objects compressed by:                  -19%
2012-02-06 01:48:37 Elapsed processing time:           00:20:30
2012-02-06 01:48:37 --- SCHEDULEREC STATUS END

In a normal backup compression will be a positive number:

2012-02-06 00:30:13 Objects compressed by:                   26%

If you're seeing negative compression on a regular basis you may benefit from turning compression off. There are three approaches:

1. Disable compression completely using the "COMPRESSION NO" option in dsm.opt/dsm.sys.
2. Use the EXCLUDE.COMPRESSION option to disable compression on specific files or file types in the dsm.opt/dsm.sys or inclexcl file.
3. Use the "COMPRESSALWAYS NO" option in dsm.opt/dsm.sys.

Over time, the data stored in the TSM server will be replaced with uncompressed data, and you may save a significant amount on your bill. Unfortunately, data that's already stored will not become uncompressed just by changing the client option. There are two ways to address this:

1. Wait. Over time old data will be replaced with new uncompressed data.
2. Delete. Delete compressed files from the backup server and re-backup your data.

If you want to pro actively clean up the problem files, the best approach is to find the files that are already compressed and delete them. Many of the files you have backed up are not compressed files and would benefit from using compression, so targeting only the compressed files will be the most effective strategy.

What files are compressed?

Here are some examples of files that are already compressed:

• Logs
  • On many systems old logs are compressed to save space
• Images
  • JPEG
  • PNG
  • GIF
• Video files
  • MPEG
  • Quicktime
  • AVI
• Audio
  • MP3
  • WMA
• Files
  • .zip
  • .bz2
  • .gzip
  • .gz
  • .rar

Excluding specific files or file types

TSM provides an option for excluding certain files from compression. You can add lines to your inclexcl file or dsm.opt depending on your platform:

exclude.compression /Users/jones/proj1/file.zip
exclude.compression /Users/jones/proj2/*
exclude.compression /.../*.zip
exclude.compression /.../*.gz

Using COMPRESSALWAYS

The COMPRESSALWAYS option controls whether TSM will continue compressing an object it grows during compression. TSM monitors the file size and detects if the file grows due to compression. The default, if not specified, is YES. If COMPRESSALWAYS is set to YES and it detects growth during compression, it will continue to compress the file and negative compression will be the result. If NO is specified, it will stop sending the object and resend it uncompressed.

WARNING: This may cause a lot of extra data to be sent during a backup and may significantly increase your backup times.