db2 performance issue - large number of IOPS how to determine problem?
The message you include from your db2diag.log shows that there is a potential space problem (i.e. the file system holding your data is filling up).  The monitoring data here shows that you are ranging between 89% and 94% over a 90 minute period with increases and decreases – this is probably associated with system temporary tables in the database.  It may be worthwhile to increase the size of the filesystem(s) holding the data since you're getting very close to running out of space.

This space utilization, however, has nothing to do with how much I/O activity (IOPS) the database is generating.

Generally when you have an I/O problem where you see a much different usage pattern than normal.  You mention that normally there the system is generating 300 IOPS, but lately it's been generating 5000 IOPS.  

Assuming that the I/O activity can be traced to the file system(s) holding DB2 data, then you need to monitor the database during these periods of high I/O to find out what queries are active and are causing lots of physical I/O, and then do some investigation to figure out why.  You can use the db2top utility as well as many other monitoring tools to figure this out.

At that point it's a tuning exercise.  Are your bufferpools too small?  Was an index dropped?  Or is the cause a new query that needs to be reviewed / tuned?