This technique (appending to a tape) is a very unstable technique. The EOD option in the mt man page is ambiguous, stating that it seeks to the "end of data". This may mean: seek to the first tapemark (a separator created by closing a data transfer on the tape) or it may seek to the special marker created by the tape drive after writing is complete and the tape rewound or otherwise moved back towards the beginning. The mt command indicates that EOD is for DDS and QIC tape drives only and does not mention DLT or Ultrium.
Now all this is to preface the technique being used to append data. It might work but you'll have to map the appended archives very carefully. DO NOT use this technique for production until you've thoroughly tested the technique. The best way is to save a very short series of files, perhaps 3-4 text files, then append to the backup 3-4 times. Now test what is on the tape by doing a directory list using the no-rewind device. You may see that each backup set is followed by a null data block which is the separator between archives. Thusyou would have to position the tape forward one additional file mark (mt ... fsf).
For several decades, sysadmins have been trying to save tapes by appending backups to long tapes with disasterous results. While it may work for a while, a single mistake (ie, a user accidently writes to the beginning of multi-backup tape) will destroy *everything* on the tape. Modern tape drives (DDS, DLT, Ultrium, etc) write a hardware-generated code at the end of writing, regardless of where the tape is positioned. This is done to prevent despooling (run off the end of the tape) but is has a chilling effect on all backups, especially multiple appends: there is no way to position beyond the hardware marker!!
So the 20 backups appended to the tape are destroyed when someone forgets to position the tape and writes to the beginning. There is no command that will position the tape past this mistake. (actually, data recovery companies have special tape drives and programs to recover data, but they are very expensive services).
So to answer your last question, pax (and tar and cpio) can all be used to backup your data files. As manetioned, there are limits to the maximum filesize that can be saved (2Gb or 8Gb for patched tar). fbackup is the preferred non-commercial tool. fbackup explicitly prevents multiple backups on the same tape by always issuing a rewind from within the program.
Bill Hassell, sysadmin
