Difference Between RMAN Incremental and Full Backups - Driving me Nuts!

Well, nuttier anyway. Oracle 9.2.0.7 on AIX5.3L. IBM DS4800 SAN with CIO mounted RAID5 Oracle Datafile filesystems. init.ora filesystems_io parameter can be "ASYNCH" or "SETALL" - it doesn't make a difference. TSM5.5 for tape backup. So here's what happens -

RMAN parameters - maxopenfiles=1, maxsetsize=250G, device type sbt_tape parallelism=2 (there are 5 drives).

Full backup: System runs 'normally' (whatever that means), people can work, jobs complete within expected time limits, etc. Statspack snapshot covering period when backup is running shows top 5 timed events as follows:

Top 5 Timed Events

~~~~~~~~~~~~~~~~~~                                                     %
Total
Event                                               Waits    Time (s) Ela

sbtwrite2                                      20,616,329      46,382
62.39
CPU time                                                        9,302
12.51
db file sequential read                         1,086,174       9,296
12.51
async disk IO                                  10,541,867       7,678
10.33
db file scattered read                            145,084       1,282
1.72
          -------------------------------------------------------------

Incremental backup (Level 1): System slows to a crawl. People complain. Jobs take hours longer than expected (it's like they don't even start until the backup completes). Statspack snapshot covering period when backup is running shows top 5 timed events as follows:

Top 5 Timed Events

~~~~~~~~~~~~~~~~~~                                                     %
Total
Event                                               Waits    Time (s) Ela

async disk IO                                   9,805,203      45,977
59.34
db file sequential read                         2,401,975      23,764
30.67
db file scattered read                            130,897       3,458
4.46
CPU time                                                        2,790
3.60
sbtwrite2                                         314,812         895
1.15
          -------------------------------------------------------------

Didn't have this problem when we were using plain old jfs2 filesystems mounted without cio.

What is the difference between disk access for the two types of backups?

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