Mysql innodb caches

This page documents the various caches used by MySQLs innodb engine.

Documentation

Buffer Pool	https://dev.mysql.com/doc/refman/5.7/en/innodb-buffer-pool.html
MemcacheD Plugin	https://dev.mysql.com/doc/refman/5.7/en/innodb-memcached.html
Data Dictionary	https://dev.mysql.com/doc/refman/5.7/en/innodb-data-dictionary.html

General

Basics

Stores adaptive-hash-table (in-memory db caching), row data, write buffers, locks etc.

• Every modification records an entry in the transaction log (can be used in replication)
• Reads/Writes are abstracted by the buffer-pool, which serves as a cache.
• Writes are recorded twice (one after the other) for atomicity.

                    SQL REQUEST
                         |
                        \ /                            ---+
                         `                                |
 +----------------------------------------------------+   |
 |                   buffer pool                      |   |
 +----------------------------------------------------+   |
         |                 |                .             |
         |                 |               / \            | Memory
         |                 |                |             |
         |                 |                |             |
         |                 |                |             |
  +------------+           |                |             |
  | log buffer |           |                |             |
  +------+-----+           |                |             |
         |                 |                |             |
         |                 |                |          ---+
         |                 |                |          ---+
         |          +------+------+         |             |
         |          |double-write |         |             |
         |          |   buffer    |         |             |
        \ /         +-------------+         |             | Disk
         `                 |                |             |
  +------+------+   +-------------+         |             |
  | transaction |   |  datastore  |         |             |
  |    log      |   | (filesystem)|         |             |
  +-------------+   +-------------+         |             |
                           |                |             |
                           +----------------+          ---+

Disabling

# bytes per chunk
show variables like 'innodb_buffer_pool_chunk_size';

# number of pools
show variables like 'innodb_buffer_pool_instances';

# bytes allowed for buffer pool
# (must be a multiple of (chunk_size * pool_instances) OR VALUE WILL BE IGNORED)
# (dynamic as of mysql-5.7.5)
show variables like 'innodb_buffer_pool_size';

Introspection

select count(*) from information_schema.innodb_buffer_page;   # pages in buffer pool (memory) (16KB/page ?)
SHOW INNODB_BUFFER_POOL_STATS;                                # general info

Components

Buffer Pool

Adaptive Hash Index

The AdaptiveHashIndex, when enabled is designed to make innodb behave like an in-memory database, crossed with an LRU cache. The mysql docs specifically call out disabling it interactively for benchmarking, and document that the table is emptied immediately when this feature is disabled.

If you're looking to evaluate query cost without caching, this is likely what you want to disable.

# interactive
SHOW VARIABLES LIKE 'innodb_adaptive_hash_index';
SET GLOBAL innodb_adaptive_hash_index=OFF;

# CLI
mysqld --skip-innodb-adaptive-hash-index

You could further scale back buffering by configuring mysql with a small buffer_pool_size (untested)

# my.cnf
[mysqld]
innodb_buffer_pool_size=1M
key_buffer_size=8
query_cache_type=0

Tablespace

The tablespace is a virtual filesystem that manages the database-file reads/writes.

Misc items stored in the tablespace:

- transactions: |
  open transactions keep state in the tablespace (I suspect related to MVCC)

- per-table data-dictionary: |
  a ~4KB cache of table statistics

Double-Write Buffer

When the buffer-pool (memory) is flushed,
it is written to the double-write buffer,
and only afterwards written to the datastore.

This is used to ensure write atomicity.
Generally, the double write buffer keeps a rolling buffer of the most recent 100 written pages worth of data.

• If a write to the double-write buffer is interrupted (ex. crash)
  when the database is brought back online, the (old) copy from the datastore is used.
• If a write to the double-write buffer succeeds, but it's write the datastore is interrupted,
  when the database is brought back online, the buffer's successful write is copied into the datastore.

Data Dictionary

Caches table metadata (ex. indexes, tables, columns)

https://dev.mysql.com/doc/refman/5.7/en/innodb-data-dictionary.html

Transaction Log

InnoDB attempts to reduce the performance hit of random I/O by appending writes to a log
and periodically flushing the log (grouping sequential writes together).
This happens in two stages:

• writes to the logfile are buffered in memory
• the logfile itself is periodically flushed and written to the datastore

Tuning

# size of in-memory buffer (before writing to log files)
innodb_log_buffer_size =

# size of log-file (before writing to datastore)
innodb_log_file_size =

Monitoring

SHOW INNODB STATUS

SHOW VARIABLES LIKE innodb_os_log_written

Plugins/Extension Components

Memcached Plugin

If this is enabled, you can use very fast get/set key-value operations instead of SQL queries (which require parsing, optimization).
The memcached instance is embedded in mysql, no separate process required.

This is not used to improve the BufferPool's AdaptiveHashTable for regular SQL queries,
it exposes an entirely different interface.