Absolutely the first thing to do is clean the tape drive. The drive should have come with a cleaning cartridge.
After that, the most common problem I've observed when trying to maximize tape throughput is that the machine being backed up cannot pump data fast enough. A 600MB/s disk subsystem is good, obviously, but if there is overhead involved then the throughput will drop. Off the top of my head, here are some things that induce overhead:
- Fragmentation of the files.
- Insufficient read caching on the RAID controllers.
- Host/server-based encryption.
- Compression.
- Lots of small files v. fewer but larger.
- Crappy backup software.
- Various combinations of the above.
Basically, a bottleneck occurs somewhere. That causes the tape to not only have to pause, but sometimes to have to rewind and reposition before writing can resume. This "see-saw" effect can take a few seconds and, over the course of an entire backup, can add significant time to the backup. LTO4 minimizes the see-saw effect by varying the write speed, but even then it is slowing down.
I would suggest defragmenting the file system if you can. Second, I would monitor memory & CPU consumption during the backup to see if the machine is hitting an internal bottleneck. On the CPU front, assuming a multi-core or multi-CPU environment, check to see if a single core/CPU is being hit hard while the others are idle; that would show a CPU bottleneck and non-multithreaded backup software. Also monitor the disk subsystem - how busy the disks are, how busy the RAID card controller is - to see what's going on there. Disk busy other 40% is bad & indicates thrashing.
Third, listen very closely to the tape drive during a backup so see if you can hear changes in the noise it makes. This will be tough as LTO4 is pretty quiet. If you hear variances, the tape drive is having to speed up/down/pause as it waits for data. That indicates a problem outside of the drive. Unless the media you have is of poor quality & you're getting read/write errors. Try a brand new tape. Also, make sure you're buying quality tapes. Some drives are finicky and like certain brands of tape better than others. Imation (3M), IBM, and Fuji seem to work well for us.
Oh, and verify you are using LTO4 media. LTO3 media will work but not at LTO4 speeds.
I would suggest disabling compression and encryption for your tests. If either of those solves your problem, and you're not using the LTO4's internal encryption capability, then you've found your bottleneck and it's most likely the server's CPU.
Another thought .. is the disk internal or is it actually a SAN or NAS?
And another .. is read-after-write verification on? That'll add significant time.
BTW, LTO4 should ideally be cabled via fibre channel & not SCSI. U320 in theory has adequate bandwidth for LTO4, but in practice there's not that much head room.
