Is this scenario possible with db2 udb version 8?

I have two instances (inst1 and inst2) on the same server.

1. upgrade to fix pack 15
2. run db2iupdt on inst1
3. test inst1 for some time period
4. continue to use inst2
5. eventually run db2iupdt on inst2

Is this possible or do I have to run db2iupdt on inst2 also?

Thanks

Depends on whether you have ESE version and installed an alternate fixpack, which allows 2 different fixpack levels (on different instances) to run at the same time.

If you don't have ESE, and/or you did not install an alternate fixpack, then I believe that you need to upgrade all instances on the server at the same time.

Marcus is right. Each DB2 instance has some local file and some shared files/executables. Installing a FixPak (w/o db2iupdt) updates the shared files and, thus implicitly affects all instances referring to this installation. You need different install paths to run different code levels.