Databases Reference
In-Depth Information
It's most useful to look at the values of all these variables and metrics over the course
of the last several minutes, as well as over the entire uptime of the server.
The following is an overview—not an exhaustive list—of the different categories of
variables you'll see in
SHOW STATUS
. For full details on a given variable, you should
consult the MySQL manual, which helpfully documents them at
http://dev.mysql.com/
doc/en/mysqld-option-tables.html
.
When we discuss a set of related variables whose
name begins with a common prefix, we refer to the group collectively as “the
<pre
fix>_*
variables.”
Thread and Connection Statistics
These variables track connection attempts, aborted connections, network traffic, and
thread statistics:
•
Connections
,
Max_used_connections
,
Threads_connected
•
Aborted_clients
,
Aborted_connects
•
Bytes_received
,
Bytes_sent
•
Slow_launch_threads
,
Threads_cached
,
Threads_created
,
Threads_running
If
Aborted_connects
isn't zero, it might mean that you have network problems or that
someone is trying to connect and failing (perhaps a user is specifying the wrong pass-
word or an invalid database, or maybe a monitoring system is opening TCP port 3306
to check if the server is alive). If this value gets too high, it can have serious side effects:
it can cause MySQL to block a host.
Aborted_clients
has a similar name but a completely different meaning. If this value
increments, it usually means there's been an application error, such as the programmer
forgetting to close MySQL connections properly before terminating the program. This
is not usually indicative of a big problem.
Binary Logging Status
The
Binlog_cache_use
and
Binlog_cache_disk_use
status variables show how many
transactions have been stored in the binary log cache, and how many transactions were
too large for the binary log cache and so had their statements stored in a temporary file.
MySQL 5.5 also includes
Binlog_stmt_cache_use
and
Binlog_stmt_cache_disk_use
,
which show corresponding metrics for nontransactional statements. The so-called
“binary log cache hit ratio” is not usually useful for configuring the binary log cache
size. See
Chapter 8
for more on this topic.
