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14.18.3 InnoDB INFORMATION_SCHEMA Buffer Pool Tables

The InnoDB INFORMATION_SCHEMA buffer pool tables provide buffer pool status information and metadata about the pages within the InnoDB buffer pool. The tables were introduced in MySQL 5.6.2 and later backported to MySQL 5.5 (in MySQL 5.5.28) and MySQL 5.1 (in MySQL 5.1.66).

The InnoDB INFORMATION_SCHEMA buffer pool tables include those listed below:

mysql> SHOW TABLES FROM INFORMATION_SCHEMA LIKE 'INNODB_BUFFER%';
+-----------------------------------------------+
| Tables_in_INFORMATION_SCHEMA (INNODB_BUFFER%) |
+-----------------------------------------------+
| INNODB_BUFFER_PAGE_LRU                        |
| INNODB_BUFFER_PAGE                            |
| INNODB_BUFFER_POOL_STATS                      |
+-----------------------------------------------+

Table Overview

INNODB_BUFFER_PAGE: Holds information about each page in the InnoDB buffer pool.
INNODB_BUFFER_PAGE_LRU: Holds information about the pages in the InnoDB buffer pool, in particular how they are ordered in the LRU list that determines which pages to evict from the buffer pool when it becomes full. The INNODB_BUFFER_PAGE_LRU table has the same columns as the INNODB_BUFFER_PAGE table, except that the INNODB_BUFFER_PAGE_LRU table has an LRU_POSITION column instead of a BLOCK_ID column.
INNODB_BUFFER_POOL_STATS: Provides buffer pool status information. Much of the same information is provided by SHOW ENGINE INNODB STATUS output, or may be obtained using InnoDB buffer pool server status variables.

Warning

Querying the INNODB_BUFFER_PAGE table or INNODB_BUFFER_PAGE_LRU table can introduce significant performance overhead. Do not query these tables on a production system unless you are aware of the performance impact that your query may have, and have determined it to be acceptable. To avoid impacting performance, reproduce the issue you want to investigate on a test instance and run your queries on the test instance.

Example 14.2 Querying System Data in the INNODB_BUFFER_PAGE Table

This query provides an approximate count of pages that contain system data by excluding pages where the TABLE_NAME value is either NULL or includes a slash / or period . in the table name, which indicates a user-defined table.

SELECT COUNT(*) FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
WHERE TABLE_NAME IS NULL OR (INSTR(TABLE_NAME, '/') = 0 AND INSTR(TABLE_NAME, '.') = 0);
+----------+
| COUNT(*) |
+----------+
|      381 |
+----------+

This query returns the approximate number of pages that contain system data, the total number of buffer pool pages, and an approximate percentage of pages that contain system data.

SELECT
(SELECT COUNT(*) FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
WHERE TABLE_NAME IS NULL OR (INSTR(TABLE_NAME, '/') = 0 AND INSTR(TABLE_NAME, '.') = 0)
) AS system_pages,
(
SELECT COUNT(*)
FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
) AS total_pages,
(
SELECT ROUND((system_pages/total_pages) * 100)
) AS system_page_percentage;
+--------------+-------------+------------------------+
| system_pages | total_pages | system_page_percentage |
+--------------+-------------+------------------------+
|          381 |        8192 |                      5 |
+--------------+-------------+------------------------+

The type of system data in the buffer pool can be determined by querying the PAGE_TYPE value. For example, the following query returns eight distinct PAGE_TYPE values among the pages that contain system data:

mysql> SELECT DISTINCT PAGE_TYPE FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
WHERE TABLE_NAME IS NULL OR (INSTR(TABLE_NAME, '/') = 0 AND INSTR(TABLE_NAME, '.') = 0);
+-------------------+
| PAGE_TYPE         |
+-------------------+
| IBUF_BITMAP       |
| SYSTEM            |
| INDEX             |
| UNDO_LOG          |
| FILE_SPACE_HEADER |
| UNKNOWN           |
| INODE             |
| EXTENT_DESCRIPTOR |
+-------------------+

Example 14.3 Querying User Data in the INNODB_BUFFER_PAGE Table

This query provides an approximate count of pages containing user data by counting pages where the TABLE_NAME value is NOT NULL.

mysql> SELECT COUNT(*) FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
WHERE TABLE_NAME IS NOT NULL;
+----------+
| COUNT(*) |
+----------+
|     7811 |
+----------+

This query returns the approximate number of pages that contain user data, the total number of buffer pool pages, and an approximate percentage of pages that contain user data.

mysql> SELECT
(SELECT COUNT(*) FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
WHERE TABLE_NAME IS NOT NULL AND (INSTR(TABLE_NAME, '/') > 0 OR INSTR(TABLE_NAME, '.') > 0)
) AS user_pages,
(
SELECT COUNT(*)
FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
) AS total_pages,
(
SELECT ROUND((user_pages/total_pages) * 100)
) AS user_page_percentage;
+------------+-------------+----------------------+
| user_pages | total_pages | user_page_percentage |
+------------+-------------+----------------------+
|       7811 |        8192 |                   95 |
+------------+-------------+----------------------+

This query identifies user-defined tables with pages in the buffer pool:

mysql> SELECT DISTINCT TABLE_NAME
FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
WHERE TABLE_NAME IS NOT NULL
AND (INSTR(TABLE_NAME, '/') > 0 OR INSTR(TABLE_NAME, '.') > 0);
+---------------------+
| TABLE_NAME          |
+---------------------+
| employees/salaries  |
| employees/employees |
+---------------------+

Example 14.4 Querying Index Data in the INNODB_BUFFER_PAGE Table

For information about index pages, query the INDEX_NAME column using the name of the index. For example, the following query returns the number of pages and total data size of pages for the emp_no index that is defined on the employees.salaries table:

mysql> SELECT INDEX_NAME, COUNT(*) AS Pages,
ROUND(SUM(IF(COMPRESSED_SIZE = 0, 16384, COMPRESSED_SIZE))/1024/1024)
AS 'Total Data (MB)'  FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
WHERE INDEX_NAME='emp_no' AND TABLE_NAME = 'employees/salaries';
+------------+-------+-----------------+
| INDEX_NAME | Pages | Total Data (MB) |
+------------+-------+-----------------+
| emp_no     |  1244 |              19 |
+------------+-------+-----------------+

This query returns the number of pages and total data size of pages for all indexes defined on the employees.salaries table:

mysql> SELECT INDEX_NAME, COUNT(*) AS Pages,
ROUND(SUM(IF(COMPRESSED_SIZE = 0, 16384, COMPRESSED_SIZE))/1024/1024)
AS 'Total Data (MB)'
FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE
WHERE TABLE_NAME = 'employees/salaries'
GROUP BY INDEX_NAME;
+------------+-------+-----------------+
| INDEX_NAME | Pages | Total Data (MB) |
+------------+-------+-----------------+
| emp_no     |  1244 |              19 |
| PRIMARY    |  6086 |              95 |
+------------+-------+-----------------+

Example 14.5 Querying LRU_POSITION Data in the INNODB_BUFFER_PAGE_LRU Table

The INNODB_BUFFER_PAGE_LRU table holds information about the pages in the InnoDB buffer pool, in particular how they are ordered that determines which pages to evict from the buffer pool when it becomes full. The definition for this page is the same as for INNODB_BUFFER_PAGE, except this table has an LRU_POSITION column instead of a BLOCK_ID column.

This query counts the number of positions at a specific location in the LRU list occupied by pages of the employees.employees table.

mysql> SELECT COUNT(LRU_POSITION) FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE_LRU
WHERE TABLE_NAME='employees/employees' AND LRU_POSITION < 3072;
+---------------------+
| COUNT(LRU_POSITION) |
+---------------------+
|                 481 |
+---------------------+

Example 14.6 Querying the INNODB_BUFFER_POOL_STATS Table

The INNODB_BUFFER_POOL_STATS table provides information similar to SHOW ENGINE INNODB STATUS and InnoDB buffer pool status variables.

mysql> SELECT * FROM information_schema.INNODB_BUFFER_POOL_STATS \G
*************************** 1. row ***************************
                         POOL_ID: 0
                       POOL_SIZE: 8192
                    FREE_BUFFERS: 1
                  DATABASE_PAGES: 7942
              OLD_DATABASE_PAGES: 2911
         MODIFIED_DATABASE_PAGES: 0
              PENDING_DECOMPRESS: 0
                   PENDING_READS: 0
               PENDING_FLUSH_LRU: 0
              PENDING_FLUSH_LIST: 0
                PAGES_MADE_YOUNG: 8358
            PAGES_NOT_MADE_YOUNG: 0
           PAGES_MADE_YOUNG_RATE: 0
       PAGES_MADE_NOT_YOUNG_RATE: 0
               NUMBER_PAGES_READ: 7045
            NUMBER_PAGES_CREATED: 12382
            NUMBER_PAGES_WRITTEN: 15790
                 PAGES_READ_RATE: 0
               PAGES_CREATE_RATE: 0
              PAGES_WRITTEN_RATE: 0
                NUMBER_PAGES_GET: 28731589
                        HIT_RATE: 0
    YOUNG_MAKE_PER_THOUSAND_GETS: 0
NOT_YOUNG_MAKE_PER_THOUSAND_GETS: 0
         NUMBER_PAGES_READ_AHEAD: 2934
       NUMBER_READ_AHEAD_EVICTED: 23
                 READ_AHEAD_RATE: 0
         READ_AHEAD_EVICTED_RATE: 0
                    LRU_IO_TOTAL: 0
                  LRU_IO_CURRENT: 0
                UNCOMPRESS_TOTAL: 0
              UNCOMPRESS_CURRENT: 0

For comparison, SHOW ENGINE INNODB STATUS output and InnoDB buffer pool status variable output is shown below, based on the same data set.

For more information about SHOW ENGINE INNODB STATUS output, see Section 14.20.3, “InnoDB Standard Monitor and Lock Monitor Output”.

mysql> SHOW ENGINE INNODB STATUS \G
...
----------------------
BUFFER POOL AND MEMORY
----------------------
Total memory allocated 137363456; in additional pool allocated 0
Dictionary memory allocated 71426
Buffer pool size   8192
Free buffers       1
Database pages     7942
Old database pages 2911
Modified db pages  0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 8358, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 7045, created 12382, written 15790
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 7942, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
...

For status variable descriptions, see Section 5.1.7, “Server Status Variables”.

mysql> SHOW STATUS LIKE 'Innodb_buffer%';
+---------------------------------------+-----------+
| Variable_name                         | Value     |
+---------------------------------------+-----------+
| Innodb_buffer_pool_pages_data         | 7942      |
| Innodb_buffer_pool_bytes_data         | 130121728 |
| Innodb_buffer_pool_pages_dirty        | 0         |
| Innodb_buffer_pool_bytes_dirty        | 0         |
| Innodb_buffer_pool_pages_flushed      | 15790     |
| Innodb_buffer_pool_pages_free         | 1         |
| Innodb_buffer_pool_pages_misc         | 249       |
| Innodb_buffer_pool_pages_total        | 8192      |
| Innodb_buffer_pool_read_ahead_rnd     | 0         |
| Innodb_buffer_pool_read_ahead         | 2934      |
| Innodb_buffer_pool_read_ahead_evicted | 23        |
| Innodb_buffer_pool_read_requests      | 28731589  |
| Innodb_buffer_pool_reads              | 4112      |
| Innodb_buffer_pool_wait_free          | 0         |
| Innodb_buffer_pool_write_requests     | 11965146  |
+---------------------------------------+-----------+

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