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One other question too:
As I created the compressed tablespaces with the DEFAULT COMPRESS FOR OLTP clause,
in my 10.2.0.5 Grid Server, it shows the compression for the tablespace as
Allocation Type Automatic
Segment Space Management Automatic
Enable logging Yes
Compression Enabled on direct-path INSERT operations only
Block Size (B) 8192
I assume that the Grid Server is just confused because it doesn't know about COMPRESS FOR OLTP but it does make me wonder if the tablespace is really compressed for OLTP or if it's compressed for direct-path INSERT operations only as the Grid Server claims.
I'm not sure if OEM Grid is confused or you are so lets look a little further.
For Advanced Compression you must insert with DIRECT LOAD, for example the APPEND hint.
Secondly when you specify Advanced Compressed Oracle changes PCT_FREE to 0 so you are likely to get 10% reduction even if no compression takes place.
Third Oracle will not do the overhead work of attempting to compress data if it does not believe it can achieve enough compression to have a significant enough impact to make the CPU usage pay for itself.
I'd suggest you run the DBMS_COMPRESSION advisor and figure out what you really have.