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