Yosemite Server Backup, version 8.1 and above
Most tape storage devices advertise a 2:1 or greater compression ratio. This is based on tests done under optimal circumstances for compression. Most data within a real world environment has built in compression added into the file type. Common file types such as JPG, and MS Office Files that end in x, are encoded in a compressed format. Other types of data can compress very well. You will commonly see higher compression rates on Database Log files (ldf, log, etc) and text files.
Compression is completely dependent on the type of data being backed up. If you are backing up a dataset that is primarily in a compressed format (jpg, zip, docx, xlsx, mp3, mpg, etc), you will most likely receive less than a 2:1 ratio.
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