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MySQL 5.5 Reference Manual
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Excerpts from this Manual File I/O Summary Tables

The Performance Schema maintains file I/O summary tables that aggregate information about I/O operations.

Example file I/O event summary information:

mysql> SELECT * FROM performance_schema.file_summary_by_instance\G
*************************** 2. row ***************************
                FILE_NAME: /var/mysql/share/english/errmsg.sys
               EVENT_NAME: wait/io/file/sql/ERRMSG
               COUNT_READ: 3
              COUNT_WRITE: 0
*************************** 6. row ***************************
                FILE_NAME: /var/mysql/data/binlog.000001
               EVENT_NAME: wait/io/file/sql/binlog
               COUNT_READ: 0
              COUNT_WRITE: 0

Each file I/O summary table has one or more grouping columns to indicate how the table aggregates events. Event names refer to names of event instruments in the setup_instruments table:

Each file I/O summary table has these summary columns containing aggregated values:


    The number of read operations in the summarized events.


    The number of write operations in the summarized events.


    The number of bytes read in the summarized events.


    The number of bytes written in the summarized events.

TRUNCATE TABLE is permitted for file I/O summary tables. It resets the summary columns to zero rather than removing rows.

The MySQL server uses several techniques to avoid I/O operations by caching information read from files, so it is possible that statements you might expect to result in I/O events will not. You may be able to ensure that I/O does occur by flushing caches or restarting the server to reset its state.