Oracle performance tuning doesn’t like mystery. Performance slowness would start in your database server and slowly it would be impacting all active sessions. Performance bottleneck doesn’t occur immediately and reflect drastically. But it started from very beginning and increasing rapidly sometimes. If you are well experienced Oracle DBA then you can judge for future impacts of some incidents. These symptoms and characteristics can be judged using constant monitoring of databases. If you are constantly monitoring your databases, then you can able to mark some incidents like buffer cache hit ratio suddenly dropped, temp segment usage suddenly improving, physical read is increasing, etc.
If you are keen in system administration then you would get these all characteristics of performance bottleneck very quickly and easily. Because in Linux and Unix environment so many utilities available to monitor Oracle server accurately about usage and load. As per my own opinion, I always use system administration tools for server monitoring like "top" command or "vmstat" command. Those are best monitoring tools.
Oracle installs on operating system. We need to concentrate on server performance and system load. If server has too much load then obviously Oracle database performance would affect. There is no question about it. So many performance issues are starting from Oracle server consumption only. It is just like health of people. If your stomach is not healthy then you would get so many health problems at any part of body.
I got one very good article on performance tuning of Oracle server from a resource. Oracle Performance Tuning Troubleshooting is art and techniques. To get expertise in tuning, you need to get various kind of knowledge like system administration, network administration, hardware, and applications. Without that knowledge, it is very difficult to understand main reason behind server slowness. In another word we can say it "without knowledge of application, system, network, hardware, Oracle server tuning becomes mystery for you".
