The only thing you are guarenteed on a DDS3 tape is 12Gb which is the uncompressed capacity of the tape. To turn on compression, you use a device file that has the text: "best density" in the lssf command:
lssf /dev/rmt/*
Using this device file will turn on the drive's compression which is about equal to any external compression utility. A tar file is not compressed, it is simply a concatenation of the orginal files with headers between each file, so there is nothing you can determine about the compressability of the data. You can run gzip or compress on one of your tar files to see what the difference is:
ll myfile.tar
compress myfile.tar
ll myfile.tar.Z
The difference is roughly how much compression will be used when you backup that file to tape. Average data compression on a basic HP-UX installation is just under 2:1 which means you can store just under 24Gb on the DDS3 tape. If you are backing up a commercial database that is only partially used, your compression ratio may be 10:1 or even 20:1 (in other words, you can store 120Gb on a DDS3 tape). But that is because the empty database typically contains long strings of nulls or is a sparse file. There really isn't 120Gb of data, just codes that indicate long strings of the same pattern.
Noted also that except for fbackup, when you restore a sparse file (where many records were never defined), all the missing holes in the restored file will be filled with nulls, thus occupying more space on the disk. This is normal and expected since if all the records are eventually used, the file would still be the same size. fbackup has an option (-s) which compresses long strings of nulls into directory entries that mark these blocks as undefined.
So all of this is to prepare you for the fact that the *only* amount of data that can be stored on any tape drive is the native capacity which is sometimes difficult to determine. The tape industry adopted the 'compressed model' more than a decade ago, calling a 12Gb DDS3 a DAT24, implying that it stores 24Gb. However, the fine print says: "assuming 2:1 compression". In Unix land, we say: "Your mileage may vary"
Here is a quick reference chart:
Tape type Length Native capacity
DDS1________60m____1.3Gb
DDS1________90m____2.0Gb
DDS2_______120m____4.0Gb
DDS3_______125m____12.0Gb
DDS4_______150m____20.0Gb
DDS72______180m____36.0Gb
More details:
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=lpg50457
Bill Hassell, sysadmin
