Lossy compression is used to compress image and video files, as much of the data is not needed at all. For example, if you have a video at 60 frames per second, then it can be compressed down to 24 frames per second as the human brain can't tell the difference between 60 and 24 fps. This is lossy compression as data is deleted.

The compression used on backups is lossless. It checks blocks of data and if it sees a recuring pattern it replaces it with a single instance of that pattern. For example, in a text document, the word "the" may appear 20 times. The backup program takes the word "the" and in place of the other 19 "the"s it will leave a symbol or reference for it so that 60 characters (3x20) can be compressed to 6 for example. When the document is uncompressed, it converts the symbol's into the word "the".

A comment on: Should I Compress and Encrypt my backups?

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No, backup compression is not lossy - it's can't be and still preserve the integrity of the backup. The "level" of compression typically controls how much analysis is done, or how big a buffer to use, or other variables that can make the compression more effective typically at the cost of speed. But a backup's compression is NEVER lossy.

23-Dec-2009

A comment on: Should I Compress and Encrypt my backups?

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