24.4.3 The INFORMATION_SCHEMA INNODB_BUFFER_PAGE_LRU Table

The INNODB_BUFFER_PAGE_LRU table provides 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 LRU_POSITION and COMPRESSED columns instead of BLOCK_ID and PAGE_STATE columns.

For related usage information and examples, see Section 14.16.5, “InnoDB INFORMATION_SCHEMA Buffer Pool Tables”.

Warning

Querying the INNODB_BUFFER_PAGE_LRU table can affect performance. Do not query this table on a production system unless you are aware of the performance impact and have determined it to be acceptable. To avoid impacting performance on a production system, reproduce the issue you want to investigate and query buffer pool statistics on a test instance.

The INNODB_BUFFER_PAGE_LRU table has these columns:

• POOL_ID

  The buffer pool ID. This is an identifier to distinguish between multiple buffer pool instances.

• LRU_POSITION

  The position of the page in the LRU list.

• SPACE

  The tablespace ID; the same value as INNODB_SYS_TABLES.SPACE.

• PAGE_NUMBER

  The page number.

• PAGE_TYPE

  The page type. The following table shows the permitted values.

  Table 24.6 INNODB_BUFFER_PAGE_LRU.PAGE_TYPE Values

  Page Type	Description
  ALLOCATED	Freshly allocated page
  BLOB	Uncompressed BLOB page
  COMPRESSED_BLOB2	Subsequent comp BLOB page
  COMPRESSED_BLOB	First compressed BLOB page
  EXTENT_DESCRIPTOR	Extent descriptor page
  FILE_SPACE_HEADER	File space header
  IBUF_BITMAP	Insert buffer bitmap
  IBUF_FREE_LIST	Insert buffer free list
  IBUF_INDEX	Insert buffer index
  INDEX	B-tree node
  INODE	Index node
  RTREE_INDEX	R-tree index
  SYSTEM	System page
  TRX_SYSTEM	Transaction system data
  UNDO_LOG	Undo log page
  UNKNOWN	Unknown

  
  

• FLUSH_TYPE

  The flush type.

• FIX_COUNT

  The number of threads using this block within the buffer pool. When zero, the block is eligible to be evicted.

• IS_HASHED

  Whether a hash index has been built on this page.

• NEWEST_MODIFICATION

  The Log Sequence Number of the youngest modification.

• OLDEST_MODIFICATION

  The Log Sequence Number of the oldest modification.

• ACCESS_TIME

  An abstract number used to judge the first access time of the page.

• TABLE_NAME

  The name of the table the page belongs to. This column is applicable only to pages with a PAGE_TYPE value of INDEX.

• INDEX_NAME

  The name of the index the page belongs to. This can be the name of a clustered index or a secondary index. This column is applicable only to pages with a PAGE_TYPE value of INDEX.

• NUMBER_RECORDS

  The number of records within the page.

• DATA_SIZE

  The sum of the sizes of the records. This column is applicable only to pages with a PAGE_TYPE value of INDEX.

• COMPRESSED_SIZE

  The compressed page size. NULL for pages that are not compressed.

• COMPRESSED

  Whether the page is compressed.

• IO_FIX

  Whether any I/O is pending for this page: IO_NONE = no pending I/O, IO_READ = read pending, IO_WRITE = write pending.

• IS_OLD

  Whether the block is in the sublist of old blocks in the LRU list.

• FREE_PAGE_CLOCK

  The value of the freed_page_clock counter when the block was the last placed at the head of the LRU list. The freed_page_clock counter tracks the number of blocks removed from the end of the LRU list.

Example

mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_BUFFER_PAGE_LRU LIMIT 1\G
*************************** 1. row ***************************
            POOL_ID: 0
       LRU_POSITION: 0
              SPACE: 97
        PAGE_NUMBER: 1984
          PAGE_TYPE: INDEX
         FLUSH_TYPE: 1
          FIX_COUNT: 0
          IS_HASHED: YES
NEWEST_MODIFICATION: 719490396
OLDEST_MODIFICATION: 0
        ACCESS_TIME: 3378383796
         TABLE_NAME: `employees`.`salaries`
         INDEX_NAME: PRIMARY
     NUMBER_RECORDS: 468
          DATA_SIZE: 14976
    COMPRESSED_SIZE: 0
         COMPRESSED: NO
             IO_FIX: IO_NONE
             IS_OLD: YES
    FREE_PAGE_CLOCK: 0

Notes

• This table is useful primarily for expert-level performance monitoring, or when developing performance-related extensions for MySQL.

• You must have the PROCESS privilege to query this table.

• Use the INFORMATION_SCHEMA COLUMNS table or the SHOW COLUMNS statement to view additional information about the columns of this table, including data types and default values.

• Querying this table can require MySQL to allocate a large block of contiguous memory, more than 64 bytes times the number of active pages in the buffer pool. This allocation could potentially cause an out-of-memory error, especially for systems with multi-gigabyte buffer pools.

• Querying this table requires MySQL to lock the data structure representing the buffer pool while traversing the LRU list, which can reduce concurrency, especially for systems with multi-gigabyte buffer pools.

• When tables, table rows, partitions, or indexes are deleted, associated pages remain in the buffer pool until space is required for other data. The INNODB_BUFFER_PAGE_LRU table reports information about these pages until they are evicted from the buffer pool. For more information about how the InnoDB manages buffer pool data, see Section 14.5.1, “Buffer Pool”.