Mysql – What does ‘innodb_buffer_pool_reads’ and ‘innodb_buffer_pool_read_requests’ actually mean

innodbMySQLperformance

Mysql version: 5.7.18 (Innodb)

I have a table with the following specification

Structure:

CREATE TABLE `dummybigint10` (
  `pk` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT 'A wonderful serenity has taken possession of my entire soul, like these sweet mornings of spring which I enjoy with my whole heart. I am alone, and feel the charm of existence in this spot, which was created for the bliss of souls like mine. I am so.',
  `num1` bigint(20) DEFAULT NULL,
  `num2` bigint(20) DEFAULT NULL,
  `num3` bigint(20) DEFAULT NULL,
  `num4` bigint(20) DEFAULT NULL,
  `num5` bigint(20) DEFAULT NULL,
  `num6` bigint(20) DEFAULT NULL,
  `num7` bigint(20) DEFAULT NULL,
  `num8` bigint(20) DEFAULT NULL,
  `num9` bigint(20) DEFAULT NULL,
  `num10` bigint(20) DEFAULT NULL,
  PRIMARY KEY (`pk`)
) ENGINE=InnoDB AUTO_INCREMENT=2163184 DEFAULT CHARSET=utf8

Row count: 1049088

The B+ tree of this table has 19104 leaf nodes (out of which 19075 used) and 20 internal nodes (out of which 20 is used). [Thanks to innodb_ruby project]

I fired a query to cause a full table scan as follows:

select count(*) from dummybigint10 where num9 is not null;

I observed for the values in 'innodb_buffer_pool_reads from performance_schema.

Observed values are:

innodb_buffer_pool_reads: 98

Some resource available online say that, 'innodb_buffer_pool_reads' is the number of pages which are loaded by Innodb from disk, which in my case , a full table scan on the given table has to cause at least 19104 but the actual value reported by Innodb is 98. I think that resource is misleading the interpretation of innodb_buffer_pool_reads. Can any one explain what does innodb_buffer_pool_reads actually mean?

Best Answer

YOUR QUERY

SELECT post.postid, post.attach FROM newbb_innopost AS post WHERE post.threadid = 51506;

At first glance, that query should only touches 1.1597% (62510 out of 5390146) of the table. It should be fast given the key distribution of threadid 51506.

REALITY CHECK

No matter which version of MySQL (Oracle, Percona, MariaDB) you use, none of them can fight to one enemy they all have in common : The InnoDB Architecture.

InnoDB Architecture

CLUSTERED INDEX

Please keep in mind that the each threadid entry has a primary key attached. This means that when you read from the index, it must do a primary key lookup within the ClusteredIndex (internally named gen_clust_index). In the ClusteredIndex, each InnoDB page contains both data and PRIMARY KEY index info. See my post Best of MyISAM and InnoDB for more info.

REDUNDANT INDEXES

You have a lot of clutter in the table because some indexes have the same leading columns. MySQL and InnoDB has to navigate through the index clutter to get to needed BTREE nodes. You should reduced that clutter by running the following:

ALTER TABLE newbb_innopost
    DROP INDEX threadid,
    DROP INDEX threadid_2,
    DROP INDEX threadid_visible_dateline,
    ADD INDEX threadid_visible_dateline_index (`threadid`,`visible`,`dateline`,`userid`)
;

Why strip down these indexes ?

The first three indexes start with threadid
threadid_2 and threadid_visible_dateline start with the same three columns
threadid_visible_dateline does not need postid since it's the PRIMARY KEY and it's embedded

BUFFER CACHING

The InnoDB Buffer Pool caches data and index pages. MyISAM only caches index pages.

Just in this area alone, MyISAM does not waste time caching data. That's because it's not designed to cache data. InnoDB caches every data page and index page (and its grandmother) it touches. If your InnoDB Buffer Pool is too small, you could be caching pages, invalidating pages, and removing pages all in one query.

TABLE LAYOUT

You could shave of some space from the row by considering importthreadid and importpostid. You have them as BIGINTs. They take up 16 bytes in the ClusteredIndex per row.

You should run this

SELECT importthreadid,importpostid FROM newbb_innopost PROCEDURE ANALYSE();

This will recommend what data types these columns should be for the given dataset.

CONCLUSION

MyISAM has a lot less to contend with than InnoDB, especially in the area of caching.

While you revealed the amount of RAM (32GB) and the version of MySQL (Server version: 10.0.12-MariaDB-1~trusty-wsrep-log mariadb.org binary distribution, wsrep_25.10.r4002), there are still other pieces to this puzzle you have not revealed

The InnoDB settings
The Number of Cores
Other settings from my.cnf

If you can add these things to the question, I can further elaborate.

UPDATE 2014-08-28 11:27 EDT

You should increase threading

innodb_read_io_threads = 64
innodb_write_io_threads = 16
innodb_log_buffer_size = 256M

I would consider disabling the query cache (See my recent post Why query_cache_type is disabled by default start from MySQL 5.6?)

query_cache_size = 0

I would preserve the Buffer Pool

innodb_buffer_pool_dump_at_shutdown=1
innodb_buffer_pool_load_at_startup=1

Increase purge threads (if you do DML on multiple tables)

innodb_purge_threads = 4

Best Answer

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