LOB pctversion vs retention definition

Question

I am trying to understand the difference between pctversion and retention definitions of LOB column.
I am using outline LOB (5.5Kb size) that is stored in ASSM tablespace with UNIFORM size .
If i create a LOB without any settings then default is RETENTION that is derived from UNDO_RETENTION of the instance.
In addition i can see in user_lobs PCTVERSION is set to 10 which is a default value.
The question is what definition is taken precedence RETENTION or PCTVERSION ?
What is the exact meaning of RETENTION inside the LOB definition , does it mean that previous LOB pages are not overwritten for the UNDO_RETENTION period of time ?
My application is running massive update commands against LOB column with very few select statemnets .
I see that my LOB Segment is growing and application is waiting for "enq HW contention" wait.
Thank you
Michael

Answer 1

Yes, of course i read the docs but got confused by what is defined in the docs and especially what was not written there.
Oracle documentation does not mention that in ASSM tablespaces LOBS default definition is retention and not pctversion. This information i found in metalink.
My question was when i see both definitions in user_lobs what definition is really taken place.
I have played a little with PCTVERSION by setting it to 100 or to 0.
When i set it to 100 i see that each time i do an update LOB segment allocates new extent and does not overwrite any of already allocated extents in a future.
Test case:
Create table with LOB
In a LOOP update LOB column one by one (all table rows) doing a commit after each update.
The result of this test case with PCTVERSION 0 is that LOB segment is increased by initial value of extents
If at the begining it was 100 extents , after 1 full update it was extended to 200 extents and so on.
The question is for how long time old versions will be kept (please notice that all these old versions are from committed changes) ?
Another question about RETENTION, does it mean that for X sec (defined by UNDO_RETENTION) Oracle is keeping old versions of the LOB without doing overwrite ?
If this is true then this may very quickly lead to significant LOB segment growth.
For example application that performs massive updates on LOBS and UNDO_RETENTION is defined for 12 hours.
The problem here that i can not tune RETENTION period on a LOB level .
Michael