Lousy performance on HP DLT tape

HP SureStore DLT1 VS80 on a Adaptec 19160B Ultra160 SCSI adapter will
give me the following performance when doing a full backup:

----- s n i p -----
Client Identifier:             tux.prisma-ob.com

Exit Status:                   0

Start-Time:                    Sat Feb 14 14:21:03 2004
End-Time:                      Mon Feb 16 02:34:30 2004
==> 123207 sec

Backup Server:                 tux
Backup Port:                   2988
First Cartridge used:          2
First Tapefile accessed:       1
Last Cartridge used:           2
Last Tapefile accessed:        326
Used Cartridges:               2

Stored filesystem entries:     137658
Bytes written to media:        16234770394
Sum of filesizes:              16219825793
Sum of compressed filesizes:   0
Sum of uncompressed filesizes: 16219825793
==> 131768 bytes/sec
----- s n i p -----

On the other hand, the HP test program (hptapeperf) will tell
me that it can write to the tape at 2.1Mbyte per second, which
sounds quite reasonable. But the question is, why on earth
is AFBackup so slow?! The backup is done on the same host that
the tape is 'mounted' on, so no network lagging to talk about...


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