Paul Mather wrote:

> By definition, random data are not compressible.  It's my understanding that 
> the "compressed capacity" of tapes is based explicitly on an expected 2:1 
> compression ratio for source data (and this is usually cited somewhere in the 
> small print).  That is a reasonable estimate for text.  Other data may 
> compress better or worse.  Already-compressed or encrypted data will be 
> incompressible to the tape drive.  In other words, "compressed capacity" is 
> heavily dependent on your source data.

Agreed.

Looking at our library of around 100 LTO4 tapes, (800GB 
uncompressed/1600GB uncompressed), using hardware compression, the 
average full tape content is about 1000GB. >90% of the content is 
standard and high definition video frames - largely random.

However there are some tapes with over 2TB on them, containing 2 bit 
images where large areas of the frame are the same pixel value.

Regards,

Richard

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