Slow writing speed on LTO4 SAS tape

I have a LTO4 SAS tape that's supposed to write at 120 MBytes/s, yet all
I can get it barely 20something MBytes/s.  Tried all the writing
methods:
tar --create --file /dev/st0
cat file > /dev/st0
dd if=file of=/dev/st0 bs=1M

The best speed is with dd, around 27 MBytes/s

The SAS controller is Dell PERC 5/E,
0000:0d:08.0 SCSI storage controller: LSI Logic / Symbios Logic SAS1068
PCI-X Fusion-MPT SAS (rev 01)

The OS is SLES9SP4+updates.
I also tried booting with a LiveCD of openSUSE 11.2 and Mandriva 2010,
the writing speed is the same.

Next step is testing the drive on a different server.

Does anyone have any idea what could be the cause of the slowness?
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