The DDS4 capacity is 20Gb - period. There is no way to increase the density. So your 15Gb file takes most of the tape. Now marketing (across the board for all type of tape drives, any manufacturer) decided many years ago that the hardware compression feature should be marketed but early on, testing was done on commercial operating systems with lots of highly compressible data. Thus, your 20Gb DDS4 drive would have been rated at 40Gb to 160Gb. But these numbers are meaningless as they depend on the hardware compression algorithm and the actual data used.
By creating a file with 100 million spaces and another file with 100 million random characters, the tape drive will seem to store 100Gb or about 20Gb, depending on which file you store on the tape. However, the bit patterns stored on the tape are still just 20Gb. If the file is compressible, then the data is encoded for more efficiency. In your case, you have defeated the compression hardware by supplying already compressed data to the tape drive. In your tar|gzip combination, you did not send 40Gb to the tape. Instead, you sent perhaps 40Gb to gzip and the output of gzip was significantly less, but the tape can't compress already compressed data.
So for a truly efficient operation, you can drop gzip and let the hardware do the compression. To see exactly how compressed the data is, run the tapeinfo utility which is available from:
ftp://contrib:9unsupp8@hprc.external.hp.com/sysadmin/programs/tapeinfo/
NOTE: there are two tapeinfo programs within HP. The standard program is part of the online diagnostics, but this one is a contributed program from the tape drive labs. Be sure you are executing the correct version.
Bill Hassell, sysadmin
