[Veritas-bu] Very slow write speed to the LTO tape drive (NBU 5.0 MP3)

We have a Sun StorEdge L25 tape libarary hooked to a v880 running
Solaris 8. The L25 has two LTO tape drives in it. When backing up a
*local* filesystem, NBU can get 20MB/sec write speed on one of the tape
drives and only get 3MB/sec write speed on another tape drive. The
backup test was done on two tape drives seperately with the same local
filesystem.  Has anybody had such experience before, or does anybody
know what would have caused this behavior?

During the backup test, no network data got involved.

I also did a backup test using ufsdump on two tape drives seperately
(backing up local filesystem). Surprising ufsdump only got 2MB/sec write
speed on both tape drives:

tape drive 1:

# ufsdump 0cfu /dev/rmt/0 /dev/md/rdsk/d6
  DUMP: Writing 63 Kilobyte records
  DUMP: Date of this level 0 dump: Mon Nov 15 21:34:51 2004
  DUMP: Date of last level 0 dump: the epoch
  DUMP: Dumping /dev/md/rdsk/d6 (server:/log) to /dev/rmt/0.
  DUMP: Mapping (Pass I) [regular files]
  DUMP: Mapping (Pass II) [directories]
  DUMP: Estimated 8143000 blocks (3976.07MB).
  DUMP: Dumping (Pass III) [directories]
  DUMP: Dumping (Pass IV) [regular files]
  DUMP: 30.40% done, finished in 0:22
  DUMP: 59.52% done, finished in 0:13
  DUMP: 89.11% done, finished in 0:03
  DUMP: Tape rewinding
  DUMP: 8142874 blocks (3976.01MB) on 1 volume at 2007 KB/sec
  DUMP: DUMP IS DONE
  DUMP: Level 0 dump on Mon Nov 15 21:34:51 2004

tape drive 2:

# ufsdump 0cfu /dev/rmt/1 /dev/md/rdsk/d6
  DUMP: Writing 63 Kilobyte records
  DUMP: Date of this level 0 dump: Mon Nov 15 22:12:34 2004
  DUMP: Date of last level 0 dump: the epoch
  DUMP: Dumping /dev/md/rdsk/d6 (server:/log) to /dev/rmt/1.
  DUMP: Mapping (Pass I) [regular files]
  DUMP: Mapping (Pass II) [directories]
  DUMP: Estimated 8143000 blocks (3976.07MB).
  DUMP: Dumping (Pass III) [directories]
  DUMP: Dumping (Pass IV) [regular files]
  DUMP: 28.12% done, finished in 0:25
  DUMP: 57.03% done, finished in 0:15
  DUMP: 85.65% done, finished in 0:05
  DUMP: Tape rewinding
  DUMP: 8142874 blocks (3976.01MB) on 1 volume at 1957 KB/sec
  DUMP: DUMP IS DONE
  DUMP: Level 0 dump on Mon Nov 15 22:12:34 2004

System information:

# cat /usr/openv/netbackup/db/config/NUMBER_DATA_BUFFERS
512

# cat /usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS
262144

# cat /usr/openv/netbackup/NET_BUFFER_SZ 
262144

# uname -a
SunOS tychousa7 5.8 Generic_117350-07 sun4u sparc SUNW,Sun-Fire-880

NBU version: Netbackup Enterprise Server 5.0 MP3.

Thanks,

Rongsheng Fang