We are seeing very similar performance issues from our F840Cs. We have opened a case with NetApps, but have received very little information back from them.

In our case, a 3 drive volume backing up gzip'ed files maxes out our SDLT drives at nearly 11 MB/sec. We have a 6 drive volume of highly compressible information that will stream to tape at over 20 MB/sec. However, a 7 drive volume that only contains a few dozen very large Oracle datafiles, now at times struggles to write 4 MB/sec!

The only correlation that I can make is that when disk utilization goes above 40%, the backup speed plummets below 10 MB/sec. Have you tried running sysstat or statit during the backup?

My biggest concern is that this may be the first symptom of some major problem. If 7 drives can't read faster than 4MB/sec, then wouldn't you be a little worried?