When considering media requirements for your tape-based backup operations, you must always use the Native Capacity rating for all tape technologies. Most tape device and media manufacturers market their technologies based upon a mythical 2:1 compression value — meaning that for every two bytes of data in a file on your disk, only one byte of space is required on the tape. Unfortunately, this 2:1 value is almost impossible to achieve unless you are working with a specially defined set of files where the contents contain enough repetitive data elements to allow the compression algorithms to combine them to conserve space on the storage media. For even the best of day to day environments, that number is impossible to achieve.
In a normal office environment where your file data consists mainly of email, word processing documents, spreadsheets, and other text-based files, you may see your results reach 1.4:1 (meaning that the native, uncompressed 800GB capacity of an LTO-4 tape will appear to hold 1.1TB). However, if you are working with film or music content, your data has already been compressed by your file type definitions (BWAV, DXF, MOV, etc.) and you will only see 1:1 for the devices' capacity (LTO-4 = 800GB/tape, LTO-5 = 1.5TB/tape, LTO-6 = 2.5TB/tape).
This holds true for both BRU's software-based compression when writing to disk and all tape drive's hardware-based compression when writing to tape.
A list of all tape technologies and the native capacity and performance figures can be found in our Tape Drive Performance Figures article #95.