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How Much Space Remains on a Tape?

Views: 11453
Votes: 2
Posted: 08 Dec, 2011

This question is probably one of the top ten questions that we are most frequently asked by tape drive owners: How can I tell how much space is left on a tape?

The simple and straight forward answer: you can't.

The reason for this is because all BRU products permit the drive to perform compression (please see KB #185 on tape drive compression for additional information) during backup operations. This means that the data that resides on the tape is not necessarily the same size as it exists on disk. This is especially true for text-based files.

Over the course of several hundred GigaBytes of data, the total amount of data that may have been backed may have been compressed to some degree.  Therefore, while the total amount of data backed up is 500GB, the actual spaced used on the tape might be 460GB, a 40GB reduction. Of course, your milage will vary.

The compression ratio cannot be determined, through any means, prior to the tape drive obtaining the data.  Most tape drives have a maximum compression ration of 2:1. There are some drives, however, that state the drive compression is higher, such as 2.6:1.  This means that a 1MB file (1,024KB) can be reduced to a mere 393 KB (possibly smaller under certain circumstances).

With the compression being the primary factor, there are some other factors that affect the ability to determine how much space is left on a tape, they are:

  • Tape length — The physical length of media in the tape cartridge plays a role in the amount of data as well.
  • Re-Writes — Tape drives have read-after-write-verify, if the block is found to be invalid, the tape drive re-writes this block automatically. Therefore, if the tape, or the tape drive, is encountering these re-writes, those take up space since the block is re-written, not overwritten.
  • Type of Data — The type of data being backed up also plays a significant role because some data is more compressible than other types.  For example, pictures, audio, movies, and other multimedia files are not all that compressible by the tape drive; whereas text-based files can be quite compressible (sometimes more than the 2:1 or 2.6:1 claim).  Mixing these types of files for a backup job will certainly cause unpredictable results as to how much space the data will actually use on a tape.

 With all of these factors, it's best practice to expect each tape to only hold it's uncompressed maximum. All additional capacity achieved should be considered a "bonus" when backing up to tape.  For example, LTO -5 tapes have a native (uncompressed) capacity of 1.5TB.  If any amount over 1.5TB is obtained, it means that the tape drive was able to compress some of the data that it received and, as a result, it was able to increase the amount of data that could fit on that tape.

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