If the underlying system has a very high disk utilization, it can adversely impact server performance. It could delay the ability to read or write database files or write log files. It could also raise concerns for server stability if excessive disk I/O inhibits the ability of the cleaner threads to keep the database size under control.
The iostat tool can be used to obtain information about the disk activity on the system.
iostat -x 5
A number of different types of information will be displayed, but to obtain an initial feel for how busy the underlying disks are, look at the "%util" column on Linux. This field shows the percentage of the time that the underlying disks are actively servicing I/O requests. A system with a high disk utilization likely exhibits poor server performance.
If the high disk utilization is on one or more disks that are used to provide swap space for the system, the system might not have enough free memory to process requests. As a result, it might have started swapping blocks of memory that have not been used recently to disk. This can cause very poor server performance. It is important to ensure that the server is configured appropriately to avoid this condition. If this problem occurs on a regular basis, then the server is likely configured to use too much memory. If swapping is not normally a problem but it does arise, then check to see if there are any other processes running, which are consuming a significant amount of memory, and check for other potential causes of significant memory consumption (for example, large files in a tmpfs file system).