Backup your database!!!
By Mike Dietrich-Oracle on Aug 19, 2009
Yes, I appologize for writing this sentence ... "Backup your database!!!" ... it sounds a bit frumpy, such as something from the past. But still nowadays with those high-class super storage solutions a valid backup can become of vital importance. Especially before you start the upgrade process.
RMAN (Oracle Recovery Manager) does a great job in backing up your database completely online. Have I said online? Yes!! Even though some Metalink notes still recommend to do an offline backup you shouldn't stop your 58 TB data warehouse database for 2 days to do a complete offline backup. Use RMAN for your backups - and since Oracle Database 10g RMAN is really simple to use.
For your upgrades a complete online backup of your database will be your life insurance. So this should be always the first task. But: Don't forget to do a test restore. In my 6 years doing mission critical support in Oracle I've seen too often empty backup tapes, broken tape robots etc.
And after the test restore has finished you should definitely do a full recovery and measure the time it'll take to have your database up and running. These timings are important for your service level agreements. Make also sure you'll be able to access your backups when you'll really need them e.g. in case of an emergency. In todays data centers backups got written often to so called Virtual Tape Drives, storage drives. But what if your restore job's priority is too low? A German insurance (yes, an insurance!!) discovered in the test phase (and yes, they did test a lot!!!) that it'll take 3 days to access their backups for restoration just because of the low priority of their jobs. Very clever if you've found this out before you started your upgrades.
Make sure you have a valid and complete online backup - and do at least once a test restore - and don't forget the recovery!!!