OATM conversion - should I use unform or auto allocated extents

Question

Hi, as part of an 11i to R12 E-Biz upgrade, I am migrating to the OATM model of tablespaces. One issue I am not sure about is whether to create the new OATM tablespaces so that they use Uniform Extents or whether I should set them to use Auto Allocate. I appreciate that some folk believe Auto Allocate can in the long run lead to freespace fragmentation. However I have not seen any recommendation from Oracle in any of the upgrade documentation as to which method of extent allocation to use.
Q1. Any ideas on which is the recommended / better / wiser extent allocation method to use ?
Q2. Are the concerns around free space fragmentation with the Auto Allocate method really likely ?
Also when I ran the Sizing Report in the OATM migration utility against my existing non OATM 11i tablespaces to predict the space I required for the migration, I got very different results / predictions !
For Uniform Extent Allocation I got the following prediction
Required 20% buffer Current
======= ======= =====
Total(in KB): 83,552,256 100,261,687 31,653,400
For Auto Allocate I got the following prediction
Required 20% buffer Current
======= ======= =====
Total(in KB): 29,842,048 35,809,437 31,653,400
Q3. Any idea why there is such a large variation between the predicted figures for both methods ?
any insight or shared thoughts greatly appreciated,
Jim

Answer 1

Thanks Hussein,
The notes indicate that the recommendation for OATM is Uniform Extnts of 128 Kb. However it caveats that with the statement "if that suits your database". How are you meant to pre-determine what extent size suits your current data / database ?
Also when I ran the pre-sizing report in the OATM migration utility I got vastly different space predictions ( see below ) when proposing to use Uniform Extent Allocation ( of 128 Kb ) and Auto Allocate. Do you know why there should be such a large difference in the figures ( especially as the recommended Unform Extent allocation is so much larger ! )
For Uniform Extent Allocation
==================
Required 20% buffer Current
Total(in KB): 83,552,256 100,261,687 31,653,400
For Auto Allocation
============
Required 20% buffer Current
Total(in KB): 29,842,048 35,809,437 31,663,600