We are using db2 version 9.5 on windows nt server.
So far only one instance (default) DB2 exists.
Planning on starting another instance that will run in parallel with the current. The same java applications will use both the instances at the same time. The motivation behind this idea is to enable circular logging in one Database, and retain logs in the other. I know that this can be achieved by having two databases as well (instead of two instances). But there may be some performance tuning that we can apply differently in these two instances. So we've to decide whether to go with two instances or with two databases.

How does running multiple instances on the same server affect performance etc? Any info on pros and cons or known problems of running multiple db2 instances would help me.

Thanks.

I prefer 1:1 relationship between instances & databases. So each database has its dedicated instance. It is more easy to manage & to monitor. You know exactly which database goes down when you issue a db2stop and which database is monitored with www.db2mon.com - DB2 Monitor. With v9 stmm can manage various databases in 1 server in more instances. No problem.

just do it

Each intance requires memory for the DB2 code and some misc memory heaps, but if you have enough memory it should be no problem.