MySQL Overview

This dashboard provides basic information about MySQL hosts.

MySQL Uptime

The amount of time since the MySQL server process was started.

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Current QPS

Based on the queries reported by MySQL’s SHOW STATUS command, this metric shows the number of queries executed by the server during the last second.n This metric includes statements executed within stored programs.

This variable does not include the following commands:

  • COM_PING
  • COM_STATISTICS

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InnoDB Buffer Pool Size

Absolute value of the InnoDB buffer pool used for caching data and indexes in memory.

The goal is to keep the working set in memory. In most cases, this should be between 60%-90% of available memory on a dedicated database host, but depends on many factors.

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Buffer Pool Size % of Total RAM

The ratio between InnoDB buffer pool size and total memory. In most cases, the InnoDB buffer pool should be between 60% and 90% of available memory on a dedicated database host, but it depends on many factors.

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MySQL Connections

Max Connections
The maximum permitted number of simultaneous client connections. This is the value of the max_connections variable.
Max Used Connections
The maximum number of connections that have been in use simultaneously since the server was started.
Connections
The number of connection attempts (successful or not) to the MySQL server.

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MySQL Active Threads

Threads Connected
The number of open connections.
Threads Running
The number of threads not sleeping.

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MySQL Questions

The number of queries sent to the server by clients, excluding those executed within stored programs.

This variable does not count the following commands:

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  • COM_PING
  • COM_STATISTICS
  • COM_STMT_PREPARE
  • COM_STMT_CLOSE
  • COM_STMT_RESET

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MySQL Thread Cache

The thread_cache_size metric informs how many threads the server should cache to reuse. When a client disconnects, the client’s threads are put in the cache if the cache is not full. It is autosized in MySQL 5.6.8 and above (capped to 100).

Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created.

  • Threads_created: The number of threads created to handle connections.
  • Threads_cached: The number of threads in the thread cache.

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MySQL Select Types

As with most relational databases, selecting based on indexes is more efficient than scanning the data of an entire table. Here, we see the counters for selects not done with indexes.

  • Select Scan is how many queries caused full table scans, in which all the data in the table had to be read and either discarded or returned.
  • Select Range is how many queries used a range scan, which means MySQL scanned all rows in a given range.
  • Select Full Join is the number of joins that are not joined on an index, this is usually a huge performance hit.

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MySQL Sorts

Due to a query’s structure, order, or other requirements, MySQL sorts the rows before returning them. For example, if a table is ordered 1 to 10 but you want the results reversed, MySQL then has to sort the rows to return 10 to 1.

This graph also shows when sorts had to scan a whole table or a given range of a table in order to return the results and which could not have been sorted via an index.

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MySQL Slow Queries

Slow queries are defined as queries being slower than the long_query_time setting. For example, if you have long_query_time set to 3, all queries that take longer than 3 seconds to complete will show on this graph.

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Aborted Connections

When a given host connects to MySQL and the connection is interrupted in the middle (for example due to bad credentials), MySQL keeps that info in a system table (since 5.6 this table is exposed in performance_schema).

If the amount of failed requests without a successful connection reaches the value of max_connect_errors, mysqld assumes that something is wrong and blocks the host from further connections.

To allow connections from that host again, you need to issue the FLUSH HOSTS statement.

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Table Locks

MySQL takes a number of different locks for varying reasons. In this graph we see how many Table level locks MySQL has requested from the storage engine. In the case of InnoDB, many times the locks could actually be row locks as it only takes table level locks in a few specific cases.

It is most useful to compare Locks Immediate and Locks Waited. If Locks Waited is rising, it means you have lock contention. Otherwise, Locks Immediate rising and falling is normal activity.

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MySQL Network Traffic

This metric shows how much network traffic is generated by MySQL. Outbound is network traffic sent from MySQL and Inbound is the network traffic that MySQL has received.

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MySQL Network Usage Hourly

This metric shows how much network traffic is generated by MySQL per hour. You can use the bar graph to compare data sent by MySQL and data received by MySQL.

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MySQL Internal Memory Overview

This metric shows the various uses of memory within MySQL.

System Memory

Total Memory for the system.

InnoDB Buffer Pool Data

InnoDB maintains a storage area called the buffer pool for caching data and indexes in memory. Knowing how the InnoDB buffer pool works, and taking advantage of it to keep frequently accessed data in memory, is an important aspect of MySQL tuning.

TokuDB Cache Size

Similar in function to the InnoDB Buffer Pool, TokuDB will allocate 50% of the installed RAM for its own cache. While this is optimal in most situations, there are cases where it may lead to memory over allocation. If the system tries to allocate more memory than is available, the machine will begin swapping and run much slower than normal.

Key Buffer Size

Index blocks for MyISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.

Adaptive Hash Index Size

The InnoDB storage engine has a special feature called adaptive hash indexes. When InnoDB notices that some index values are being accessed very frequently, it builds a hash index for them in memory on top of B-Tree indexes. This allows for very fast hashed lookups.

Query Cache Size

The query cache stores the text of a SELECT statement together with the corresponding result that was sent to the client. The query cache has huge scalability problems in that only one thread can do an operation in the query cache at the same time. This serialization is true for SELECT and also for INSERT, UPDATE, and DELETE. This also means that the larger the query_cache_size is set to, the slower those operations become.

InnoDB Dictionary Size

The data dictionary is InnoDB internal catalog of tables. InnoDB stores the data dictionary on disk, and loads entries into memory while the server is running. This is somewhat analogous to table cache of MySQL, but instead of operating at the server level, it is internal to the InnoDB storage engine.

InnoDB Log Buffer Size

The MySQL InnoDB log buffer allows transactions to run without having to write the log to disk before the transactions commit. The size of this buffer is configured with the innodb_log_buffer_size variable.

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MySQL Handlers

Handler statistics are internal statistics on how MySQL is selecting, updating, inserting, and modifying rows, tables, and indexes.

This is in fact the layer between the Storage Engine and MySQL.

  • read_rnd_next is incremented when the server performs a full table scan and this is a counter you don’t really want to see with a high value.
  • read_key is incremented when a read is done with an index.
  • read_next is incremented when the storage engine is asked to ‘read the next index entry’. A high value means a lot of index scans are being done.

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MySQL Query Cache Memory and MySQL Query Cache Activity

The query cache has huge scalability problems in that only one thread can do an operation in the query cache at the same time. This serialization is true not only for SELECT, but also for INSERT, UPDATE, and DELETE.

This also means that the larger the query_cache_size is set to, the slower those operations become. In concurrent environments, the MySQL Query Cache quickly becomes a contention point, decreasing performance. MariaDB and Amazon Aurora have done work to try and eliminate the query cache contention in their flavors of MySQL, while MySQL 8.0 has eliminated the query cache feature.

The recommended settings for most environments is to set:

query_cache_type=0
query_cache_size=0

Note

While you can dynamically change these values, to completely remove the contention point you have to restart the database.

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MySQL Open Tables, MySQL Table Open Cache Status, and MySQL Table Definition Cache

The recommendation is to set the table_open_cache_instances to a loose correlation to virtual CPUs, keeping in mind that more instances means the cache is split more times. If you have a cache set to 500 but it has 10 instances, each cache will only have 50 cached.

The table_definition_cache and table_open_cache can be left as default as they are autosized MySQL 5.6 and above (do not set them to any value).

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See also

Configuring MySQL for PMM
pmm.conf-mysql.settings.dashboard
MySQL Documentation: InnoDB buffer pool
https://dev.mysql.com/doc/refman/5.7/en/innodb-buffer-pool.html
Percona Server Documentation: Running TokuDB in Production
https://www.percona.com/doc/percona-server/LATEST/tokudb/tokudb_quickstart.html#considerations-to-run-tokudb-in-production
Blog post: Adaptive Hash Index in InnoDB
https://www.percona.com/blog/2016/04/12/is-adaptive-hash-index-in-innodb-right-for-my-workload/
MySQL Server System Variables: key_buffer_size
https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_key_buffer_size
MySQL Server System Variables: table_open_cache
http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_table_open_cache

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