Hi,
i upgraded my development DBs to oracle 10g (10.2.0.2)i didnt see any major problem while doing this. We are planning to upgrade the production servers of our clients. I Made an estimation for my development server as follows

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Inspection/Planning/Preparation of system and statistics collection related to OS and proper release of oracle: 1.5 Hr (May take longer if some components are to be downloaded/installed at run time)
If this is a critical production Database, backup (before upgrade process) time will be added which solely depends on the size of DB.
Time to install 10G Software : 30 Min
Patching 10G software : 1.5 Hr (May take a bit longer if some required components are to be downloaded/installed at run time)
Upgrading the Database : 1 Hr
Any required post up gradation tasks : 10 Min
Testing the application after upgrade depends on the size of application. For SEWN, it should not take more then 30 Min.
If database environment involves clustering or other advance implementations, it may take longer to upgrade
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Every thing went fine and also according to the plan.

Can this plan be a successful plan for a production server as well which contains Tera bytes of data? I have the following questions to ask if you could please guide me.

1) What threats could be involved while upgrading a production server. What things could make DB to fail or crash during upgrade and could need recovery.
2) In this plan, upgrade time of the database is 1 hour. I think it could be 1.5 or 2 hours based on the components of oracle server installed on the server and this is also tru for a production server, am i right?
3) When we run "utlu102i.sql" and result is as follows

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--> Oracle Catalog Views [upgrade] VALID
--> Oracle Packages and Types [upgrade] VALID
--> JServer JAVA Virtual Machine [upgrade] VALID
...The 'JServer JAVA Virtual Machine' JAccelerator (NCOMP)
...is required to be installed from the 10g Companion CD.
--> Oracle XDK for Java [upgrade] VALID
--> Oracle Java Packages [upgrade] VALID
--> Oracle XML Database [install]
--> OLAP Analytic Workspace [upgrade] UPGRADED
--> Oracle OLAP API [upgrade] UPGRADED
--> Oracle Ultra Search [upgrade]
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We should expect a successfull upgrade process

Regards