We have an Apple XServe RAID with 14 x 750GB SATA drives connected via fibre-channel to a Qlogic 5602Q switch. The XServe RAID is configured as two 4.1 TB volumes. Also connected to the switch is a Mac running 10.4. This Mac has a 2 port 4Gbps fibre-channel HBA with port 1 connected to the switch and port 0 directly connected to a Tandberg 224 LTO4 library. The Mac runs Retrospect 6.1 workgroup edition to backup the two 4.1 TB volumes on the XServe RAID to tape.
This works, but it seems like the backups are taking longer than they should. When a job runs, it indicates a throughput of approx 3200 MB/min. Wikipedia lists the max throughput of LTO4 as 120 MB/s (7200 MB/min), which is over double the speed I'm seeing. I normally expect real world numbers to be 15-20% lower than maximum values, but this is 50+ percent slower than the spec.
For comparison, I looked at the logs for another server which is running Backup Exec with an LTO3 drive and it backs up at 4200+ MB/min. I assume there's a problem somewhere and these are not expected numbers?