9.17 Monitoring the MySQL Server
9.17.1 Problem
You
want to find out how the server was configured or monitor its state.
9.17.2 Solution
SHOW VARIABLES and
SHOW STATUS are useful for
this.
9.17.3 Discussion
The SHOW VARIABLES and
SHOW STATUS statements provide
server configuration and performance information:
mysql> SHOW VARIABLES;
+---------------------------------+-------------------+
| Variable_name | Value |
+---------------------------------+-------------------+
| back_log | 50 |
| basedir | /usr/local/mysql/ |
| bdb_cache_size | 8388600 |
| bdb_log_buffer_size | 0 |
| bdb_home | |
...
mysql> SHOW STATUS;
+--------------------------+----------+
| Variable_name | Value |
+--------------------------+----------+
| Aborted_clients | 319 |
| Aborted_connects | 22 |
| Bytes_received | 32085033 |
| Bytes_sent | 26379272 |
| Connections | 65684 |
...
This information can be useful for writing administrative
applications. For example, you might write a long-running program
that probes the server periodically to monitor its activity. A simple
application of this type might ask the server to report the number of
connections it's received and its uptime, to
determine a running display of average connection activity. The
queries to obtain this information are:
SHOW STATUS LIKE 'Connections';
SHOW STATUS LIKE 'Uptime';
If you want to avoid having to reconnect each time you issue the
queries, you can ask the server for its client timeout period and
probe it at intervals shorter than that value. You can get the
timeout value (in seconds) with this query:
SHOW VARIABLES LIKE 'wait_timeout';
The default value is 28800 (8 hours), but it may be different on your
system.
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Heisenberg's
uncertainty principle for measurement of quantum phenomena has a
MySQL analog. If you monitor MySQL's status to see
how it changes over time, you may notice a curious effect for some of
the indicators: Each time you take a measurement, you change the
value you're measuring! For example, you can
determine the number of queries the server has received by using the
following statement:
SHOW STATUS LIKE 'Questions'
However, that statement is itself a query, so each time you issue it,
you cause the Questions value to change. In
effect, your performance assessment instrument contaminates its own
measurements, something you may want to take into account.
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