Re: [Bacula-users] Quantum Scalar i500 slow write speed

[Richard S. <ri...@sa...>]

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