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22.9.5.2 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 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
 SUM_NUMBER_OF_BYTES_READ: 42211
SUM_NUMBER_OF_BYTES_WRITE: 0
...
*************************** 6. row ***************************
                FILE_NAME: /var/mysql/data/binlog.000001
               EVENT_NAME: wait/io/file/sql/binlog
               COUNT_READ: 0
              COUNT_WRITE: 0
 SUM_NUMBER_OF_BYTES_READ: 0
SUM_NUMBER_OF_BYTES_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:

  • COUNT_READ

    The number of read operations in the summarized events.

  • COUNT_WRITE

    The number of write operations in the summarized events.

  • SUM_NUMBER_OF_BYTES_READ

    The number of bytes read in the summarized events.

  • SUM_NUMBER_OF_BYTES_WRITE

    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.


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