This topic describes I/O concepts and the causes of high I/Os.

I/O concepts

Term Description
I/O The read and write operations on disks.
I/O latency The latency of an I/O operation.
Logical I/O The I/O that is sent by an application to a file system.
Physical I/O The I/O that is sent by the file system to disks.
Sequential I/O The sequential read and write operations on disks.
Random I/O The random access to disks for read and write operations.
Synchronous write The results can be returned only after all the data is written to disks.
Asynchronous write The results can be returned before data is written to disks. This releases CPU resources.

Causes of high I/Os

  • If a large number of SQL statements are executed to scan many rows, shared_buffer cannot cache all the data. Therefore, a large number of physical I/Os are required. In this case, you must locate and fix SQL problems. For more information, see Performance insight.
  • The VACUUM operation triggers high I/Os because the parameter settings related to the VACUUM operation are improper. You must adjust these parameters, such as vacuum_cost_delay and vacuum_cost_limit. For more information about how to specify these parameters, see Specify parameters.