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  Oracle Tips by Burleson

Preparing to Migrate

Invariably, other “experts” (using the term loosely) and I will be asked hundreds of questions about installation and migration to Oracle that could have been answered if the people asking the questions had read the documentation and tested new features before implementation. The point is, you have to take the time to study the new features sections of the manuals, read the readme.doc, and review utility scripts for “hidden” changes.       

Back when the Oracle8 migration was the rage, I mentioned the compatible parameter in one of my columns. A couple of weeks later, Oracle announced a spate of bugs that related to the parameter. Were the two related? Probably. It demonstrated that people who thought they had “migrated” to a new version of Oraclehadn’t. Why? Because of the setting of the parameter, they hadn’t even tested the new features that depended on a new redo log format! Reading the documentation before they migrated would have prevented this.      

How about the person who buys one of those great new sport utility vehicles only to discover it won’t fit in their garage? Obviously, they didn’t check available resources before “migrating” to a new vehicle. Oracle will take up to 150 percent more space for initial load than Oracle8 and up to three times the space of Oracle 7.3. Do you have enough free space? You may want to run both an Oracle and an Oracle8i (or earlier release) system in parallel. Do you have enough memory (9i may require twice to three times as much memory as previous releases (512 megabytes on Linux, for example) or enough CPU? You will require at least 50 meg of free space in an 8i SYSTEM tablespace to upgrade to 9i, or the upgrade will fail. You may also want to take note that the default installation of an Oracle database contains a SYSTEM tablespace that is near 350 megabytes in size with only 40 Kilobytes free, you should plan your sizing accordingly.

Getting back to our geese versus lemmings migration analogy: Which chose the better migration path (overlooking hunters in this case…)? Don’t begin your migration without a clear plan of how to get from your Oracle8i or earlier instance to your Oracle instance. Decide on your path, preferably one that doesn’t lead you over a cliff! Review the possible methods to migrate your system; choose the best for your situation, and then plan, plan, plan! In Oracle migration, you have the choice between export and import, the Data Migration GUI tool, and the migration command-line utility. Of course, there is also the CTAS method, but that is only applicable to real S-and-M fanatics. In a 6.x version database, you must first migrate to Oracle7.3.x and then migrate the 7.3.x database into 9i. In a 7.3.x to 9i, all of the methods are applicable; however, in an 8.1.x to 9i, only the Data Migration Assistant, export and import, unload, and SQLLOADER or CTAS are available. Figures 1.22 through 1.25 show these migration paths.

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