Netbackup Buffer Settings on RHEL 5

[Chuck]

I was wondering if anyone has tweaked netbackup buffers for LTO2
drives on RHEL 5 64-bit? (primarily the network buffer, but the number
and size of data buffers would also be nice) I have a 4 x 2.0 Ghz Xeon
class backup server running RHEL 5 64-bit and NBU 6.5. Im getting very
poor network backup performance (using both the NBU client and by
mounting filesystems via NFS and backing them up locally) It's almost
10 times as slow as the identical backup done by a 2 x 1.5ghz sparc
III solaris 10 based system. (both systems are at identical points in
the network and latencies to the backup clients are identical)

All the obvious things have been checked like NIC speed/duplex or
memory shortages. There are no lost packets or any other transport
related errors, its just grindingly slower. =p

My current settings are:

/usr/openv/netbackup/NET_BUFFER_SZ = 65535 (gone as high as 256k and
as low as 32k)
/usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS = 262144 (tried
everything from 24k - 256k)
/usr/openv/netbackup/db/config/NUMBER_DATA_BUFFERS = 32 (tried 4, 16,
32, and 64 with no visible change in performance)

I have heard rumors from various vendors that there have been various
issues related to the generic scsi driver included in RHEL 5 (NBU uses
the pass-thru sg driver for various things). I was thinking this might
be the source of my performance problems. (however these issues the
vendor brought to my attention were not performance related) Even the
worst possible buffer settings shouldn't be 10 times slower than a
similar system.

Thanks for any info...
CC