I've been using Amanda 3.4.3 & LTO6 drive w/ Fibre channel for a few months. Since write speed of LTO6 drive (=160 MB/sec) is not high enough for us, Iím investigating into RAIT. Iím expecting that total 160 x 2 = 320MB/sec of write speed would be achievable when using #3 LTO6 drives.
Iíve completed RAIT configuration with these #3 drives and found that simultaneous recording was successfully achieved. However, the problem is that write performance was worse than expected.
I tried measuring write speed of each tape drive with ďiostat-scsi.stpĒ and it showed 84MB/sec a drive in average. This means that effective write speed was: 84 x 2 = 168 MB/sec - worse than expected 320 MB/sec.
Since I thought that there was a bottleneck at HBA or holding disk, I tried the same test with using #2 drives and RAIT configuration. In this test, I found that each write speed was almost 160 MB/sec. This means that the system has a bandwidth of 320MB/sec from holding disk to tape, at least. According to this result, effective write speed in #3-drive configuration should have been: 320 x 2 / 3 = 213 MB/sec, at least.
My configurations are as follows. I really appreciate any information or suggestion on this.

---Amanda.conf (excerpt)---
inparallel 10 # maximum dumpers that will run in parallel
dumporder "BTBTBTBTBTBT" # specify the priority order of each dumper
taperalgo first # The algorithm used to choose which dump image to send
runtapes 3 # number of tapes to be used in a single run of amdump
tapedev "rait:{/dev/st0,/dev/st1,/dev/st2}"

holdingdisk hd1 {
comment "main holding disk"
directory "/dumps/amanda" # where the holding disk is
use -100 Mb # how much space can we use on it
chunksize 3Gb # size of chunk if you want big dump to be

define tapetype LTO6x3 {
comment "Created by amtapetype; compression enabled"
length 7328456544 kbytes
filemark 0 kbytes
speed 510000 kps

define dumptype comp-user-tar {
compress client fast
priority high
program "GNUTAR"
comment "root partitions dumped with tar"
compress none
# exclude list "/etc/amanda/exclude.gtar"

--- HW configuration---
#2 HBAs: One HBA has one port, the other has 2 ports, 8Gbps
Holding Disk: RAM disk, DDR2. Mounted as tempfs
#3 LTO6 Drives