The error log contains information indicating when mysqld was started and stopped and also any critical errors that occur while the server is running. If mysqld notices a table that needs to be automatically checked or repaired, it writes a message to the error log.
On some operating systems, the error log contains a stack trace if mysqld dies. The trace can be used to determine where mysqld died. See MySQL Internals: Porting to Other Systems.
In the following discussion, “console” means
stderr, the standard error output; this is your
terminal or console window unless the standard error output has
been redirected. (For example, if invoked with the
--syslog option,
mysqld_safe arranges for the server's
stderr to be sent to the
syslog facility, as described later.)
On Windows, the --log-error and
--console options both affect error
logging:
Without --log-error,
mysqld writes error messages to
host_name.err
With
--log-error[=,
mysqld writes error messages to an error
log file. The server uses the named file if present, creating
it in in the data directory unless an absolute path name is
given to specify a different directory. If no file is named,
the default name is
file_name]host_name.err
If --console is given,
mysqld writes error messages to the
console, unless --log-error is
also given. If both options are present, the last one takes
precedence.
In addition, on Windows, events and error messages are written to
the Windows Event Log within the Application log. Entries marked
as Warning and Note are
written to the Event Log, but not informational messages such as
information statements from individual storage engines. These log
entries have a source of MySQL. You cannot
disable writing information to the Windows Event Log.
On Unix and Unix-like systems, mysqld writes error log messages as follows:
Without --log-error,
mysqld writes error messages to the
console.
With
--log-error[=,
mysqld writes error messages to an error
log file. The server uses the named file if present, creating
it in the data directory unless an absolute path name is given
to specify a different directory. If no file is named, the
default name is
file_name]host_name.err
At runtime, log_error system
variable indicates the the error log file name if error output is
written to a file.
If you flush the logs using
FLUSH LOGS or
mysqladmin flush-logs and
mysqld is writing the error log to a file (for
example, if it was started with the
--log-error option), the effect is
version dependent:
As of MySQL 5.1.51, the server closes and reopens the log file. To rename the file, do so manually before flushing. Then flushing the logs reopens a new file with the original file name. For example, to rename the file and create a new one, use the following commands:
shell>mvshell>host_name.errhost_name.err-oldmysqladmin flush-logsshell>mvhost_name.err-oldbackup-directory
On Windows, use rename rather than mv.
Prior to MySQL 5.1.51, the server renames the current log file
with the suffix -old, then creates a new
empty log file. Be aware that a second log-flushing operation
thus causes the original error log file to be lost unless you
save it under a different name. On Windows, you cannot rename
the error log while the server has it open before MySQL
5.1.51. To avoid a restart, flush the logs first to cause the
server to rename the original file and create a new one, then
save the renamed file. That also works on Unix, or you can use
the commands shown earlier.
No error log renaming occurs when the logs are flushed in any case if the server is not writing to a named file.
If you use mysqld_safe to start
mysqld, mysqld_safe arranges
for mysqld to write error messages to a log
file or (as of MySQL 5.1.20) to syslog:
Before 5.1.20, mysqld_safe behavior is to
log to a file, using the default error log file if the
--log-error option is not given
to mysqld_safe. Otherwise,
mysqld_safe uses the file name specified
using
--log-error=.
file_name
From 5.1.20 on, mysqld_safe has two
additional error-logging options,
--syslog and
--skip-syslog.
In 5.1.21 and up, the default with no logging options is
--skip-syslog,
which is compatible with the default behavior of writing an
error log file for releases prior to 5.1.20. To explicitly
specify use of an error log file, specify
--log-error=
to mysqld_safe, and
mysqld_safe will arrange for
mysqld to write messages to a log file. To
use file_namesyslog instead, specify the
--syslog option.
In 5.1.20 only, the
following conditions apply: 1) The default is to
use syslog, which is not compatible with
releases prior to 5.1.20. 2) Logging to
syslog may fail to operate correctly in
some cases; if so, use
--skip-syslog
or --log-error.
For logging to syslog, messages from
mysqld_safe and mysqld
are written with a tag of mysqld_safe and
mysqld, respectively. As of MySQL 5.1.21,
to specify a suffix for the tag, use
--syslog-tag=,
which modifies the tags to be
tagmysqld_safe-
and tagmysqld-.
tag
If you specify --log-error in an
option file in a [mysqld],
[server], or [mysqld_safe]
section, mysqld_safe will find and use the
option.
If mysqld_safe is used to start
mysqld and mysqld dies
unexpectedly, mysqld_safe notices that it needs
to restart mysqld and writes a
restarted mysqld message to the error log.
The --log-warnings option or
log_warnings system variable can
be used to control warning logging to the error log. The default
value is enabled (1). Warning logging can be disabled using a
value of 0. If the value is greater than 1, aborted connections
are written to the error log. See
Section C.5.2.11, “Communication Errors and Aborted Connections”.