Limit the Scope

Does the performance issue originate in the operating system (OS), the instance, or the application SQL? This question is not always easy to answer. 

• For e.g. Poorly performing SQL can cause excessive physical reads and writes, appearing to be an I/O issue.
• Improperly sized memory components (an instance configuration issue) can lead to excessive swapping in the OS. 
• Poor disk configuration can appear to be an instance configuration problem, causing a large redo file waits or commit waits, and other problems.

Does the performance issue originate in the operating system (OS), the instance, or the application SQL? This question is not always easy to answer. Poorly performing SQL can cause excessive physical reads and writes, appearing to be an I/O issue. Improperly sized memory components (an instance configuration issue) can lead to excessive swapping in the OS. Poor disk configuration can appear to be an instance configuration problem, causing a large redo file waits or commit waits, and other problems.

Eliminate possibilities. When the instance appears to have I/O problems, compare the instance file I/O statistics to OS level statistics. The differences can guide you to the actual problem. For example: A higher than normal average wait time on a particular tablespace, could be due to: 

• Hardware: A file is on a slow drive or an improper RAID configuration.
• OS: The OS is busy with other files on the same drive or partition.
• Instance: The tablespace was created with different properties than other tablespaces have, other busy database files are on the same disk or partition (the database I/O is not balanced across all the drives), or the objects being accessed are mostly in the same tablespace, file, or disk. 
• Application: The application is doing excessive I/O due to poor access path choice by the optimizer due to out of date statistics, inefficient indexes, or other reasons.

Determine the scope of the problem to focus your efforts on the solutions that provide the most benefit.