July 23, 2014

How much memory Innodb Dictionary can take ?

The amount of memory Innodb will require for its data dictionary depends on amount of tables you have as well as number of fields and indexes. Innodb allocates this memory once table is accessed and keeps until server is shut down. In XtraDB we have an option to restrict that limit.

So how much memory can it really take ? Here is some production stats from real system:

In this case The memory stats from SHOW INNODB STATUS look like:

So considering about 700M were allocated for dictionary cache in this case we have about 15KB per Table or 1.5KB per Index. These numbers will change a lot depending on your table structure – This given schema had many tables with 50+ columns but I believe it can be helpful as ballpark figure.

Also note Dictionary Cache is not included in “Total Memory Allocated” in SHOW INNODB STATUS any more, as that allocations were moved to use malloc() and so Innodb does not track them any more.

About Peter Zaitsev

Peter managed the High Performance Group within MySQL until 2006, when he founded Percona. Peter has a Master's Degree in Computer Science and is an expert in database kernels, computer hardware, and application scaling.

Comments

  1. Vadim says:

    Peter,

    did you open all
    48246 tables ?

    Dictionary cache shows only opened in memory tables.

  2. peter says:

    Vadim,

    Yes. I’ve done mysqldump -d -A > /dev/null which should have opened all the tables.

  3. Vivek says:

    MySQL Documentation says:

    “Do not convert MySQL system tables in the mysql database from MyISAM to InnoDB tables! This is an unsupported operation.”
    In my db when I run sql get below error:

    mysql> SELECT count(*) FROM INNODB_SYS_TABLES;
    ERROR 1146 (42S02): Table ‘test.INNODB_SYS_TABLES’ doesn’t exist

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