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Why We Don't Quote Compression
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| Some History on the 2 to 1 compression lie. |
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Tape vendors are fond of quoting 2 sizes for their tape capacity such as 20/40GB or 40/80GB. What these numbers specify is the native data size of the tape and the amount of compressed data that would fit on the tape assuming a 2 to 1 compression ratio. The problem is that in the real world the amount of compression you can get is highly dependant on the nature of the data. Most people that have tried compression have discovered that somewhere around 1.5 to 1.7 to 1 compression is more typical. It's rare to achieve 2 to 1. I believe the marketing folks at some tape company in the dim recesses of history rounded 1.7:1 up to 2:1 for the purpose of simplicity (and of making their products look better!). Of course, all the competition had to follow suit. Tape vendors have been lying about their compression ratios for so long we've all taken it for granted. But now Sony is taking this a step further by claiming 2.6 to 1 compression with no apparent difference in the algorithm that is being used. The compression that has been used for quite a few tape products is called ALDC. Sony doesn't say much about why all of the sudden they believe that they can expand their 2 to 1 compression lie to an even more outrageous 2.6 to 1 compression lie.
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| Bogus claim on Sony's web site: |
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"What is Adaptive Lossless Data Compression?"
Adaptive Lossless Data Compression (ALDC) is a compression technology co-developed with IBM. ALDC provides greater data compression efficiency compared to older technologies commonly used by competitors, such as IDRC or DLZ algorithms. Enhanced for the AIT format, ALDC achieves compression ratios up to 2.6:1.
(www.TapeLibrary.com)
Well that sounds nice. The operative words here are "up to" 2.6:1. That's like saying my volkswagon can go "up to" 120 miles per hour (downhill with a stiff tail wind - right before it crashes). The problem is that I can find absolutely no mathematical basis for the claim of 2.6 to 1 compression ratios for the "new" ADLC format. The graph below (fig 7)from IBM's own white paper on ALDC confirms compression ratios on real world data of 1.7 to 1 – about the same as it's always been.
(www.research.ibm.com)
Bottom line? Multiple AIT-3 Customers have reported "real" compression less than 1.7 to 1. Sony claims it's 100GB/260GB AIT-3 tape can store 260GB. As far as I can tell, those claims are completely bogus. You'll be lucky to compress 170GB of data onto an AIT-3 tape.
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| Think We're The Only Skeptics? |
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"Note that Sony touts a 2.6:1 compression ratio for its AIT drives, as opposed to the 2:1 compression ratio for every other tape format out there. Sony attributes this increased compression to its ALDC (Adaptive Lossless Data Compression) technology. While we don't doubt that ALDC can provide improvements, compression ratios can be drastically affected by the nature of the data being backed up. For example, feeding precompressed data to the tape drive can reduce compression ratios considerably."
(www.nwc.com)
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