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If you are using some functionality that is very new in MySQL,
you can try to run mysqld with the --skip-new (which will disable all
new, potentially unsafe functionality) or with --safe-mode which
disables a lot of optimisation that may cause problems.
See section What To Do If MySQL Keeps Crashing.
If mysqld doesn't want to start, you should check that you don't have
any `my.cnf' files that interfere with your setup!
You can check your `my.cnf' arguments with mysqld --print-defaults
and avoid using them by starting with mysqld --no-defaults ....
If mysqld starts to eat up CPU or memory or if it "hangs", you
can use mysqladmin processlist status to find out if someone is
executing a query that takes a long time. It may be a good idea to
run mysqladmin -i10 processlist status in some window if you are
experiencing performance problems or problems when new clients can't connect.
The command mysqladmin debug will dump some information about
locks in use, used memory and query usage to the mysql log file. This
may help solve some problems. This command also provides some useful
information even if you haven't compiled MySQL for debugging!
If the problem is that some tables are getting slower and slower you
should try to optimise the table with OPTIMIZE TABLE or
myisamchk. See section Database Administration. You should also
check the slow queries with EXPLAIN.
You should also read the OS-specific section in this manual for problems that may be unique to your environment. See section Operating System Specific Notes.
If you have some very specific problem, you can always try to debug
MySQL. To do this you must configure MySQL with the
--with-debug or the --with-debug=full option. You can check
whether MySQL was compiled with debugging by doing:
mysqld --help. If the --debug flag is listed with the
options then you have debugging enabled. mysqladmin ver also
lists the mysqld version as mysql ... --debug in this case.
If you are using gcc or egcs, the recommended configure line is:
CC=gcc CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors \ -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql \ --with-debug --with-extra-charsets=complex |
This will avoid problems with the libstdc++ library and with C++
exceptions (many compilers have problems with C++ exceptions in threaded
code) and compile a MySQL version with support for all character sets.
If you suspect a memory overrun error, you can configure MySQL
with --with-debug=full, which will install a memory allocation
(SAFEMALLOC) checker. Running with SAFEMALLOC is however
quite slow, so if you get performance problems you should start
mysqld with the --skip-safemalloc option. This will
disable the memory overrun checks for each call to malloc() and
free().
If mysqld stops crashing when you compile it with
--with-debug, you have probably found a compiler bug or a timing
bug within MySQL. In this case you can try to add -g to
the CFLAGS and CXXFLAGS variables above and not use
--with-debug. If mysqld now dies, you can at least attach
to it with gdb or use gdb on the core file to find out
what happened.
When you configure MySQL for debugging you automatically enable a
lot of extra safety check functions that monitor the health of mysqld.
If they find something "unexpected," an entry will be written to
stderr, which safe_mysqld directs to the error log! This also
means that if you are having some unexpected problems with MySQL and
are using a source distribution, the first thing you should do is to
configure MySQL for debugging! (The second thing, of course, is to
send mail to a MySQL mailing list and ask for help.
See section The MySQL Mailing Lists.
Please use the
mysqlbug script for all bug reports or questions regarding the
MySQL version you are using!
In the Windows MySQL distribution, mysqld.exe is by
default compiled with support for trace files.
If the mysqld server doesn't start or if you can cause the
mysqld server to crash quickly, you can try to create a trace
file to find the problem.
To do this you have to have a mysqld that is compiled for debugging.
You can check this by executing mysqld -V. If the version number
ends with -debug, it's compiled with support for trace files.
Start the mysqld server with a trace log in `/tmp/mysqld.trace'
(or `C:\mysqld.trace' on Windows):
mysqld --debug
On Windows you should also use the --standalone flag to not start
mysqld as a service:
In a console window do:
mysqld --debug --standalone |
After this you can use the mysql.exe command-line tool in a
second console window to reproduce the problem. You can take down the above
mysqld server with mysqladmin shutdown.
Note that the trace file will get very big! If you want to have a smaller trace file, you can use something like:
mysqld --debug=d,info,error,query,general,where:O,/tmp/mysqld.trace
which only prints information with the most interesting tags in `/tmp/mysqld.trace'.
If you make a bug report about this, please only send the lines from the trace file to the appropriate mailing list where something seems to go wrong! If you can't locate the wrong place, you can ftp the trace file, together with a full bug report, to ftp://support.mysql.com/pub/mysql/secret/ so that a MySQL developer can take a look a this.
The trace file is made with the DBUG package by Fred Fish. See section The DBUG Package.
On most systems you can also start mysqld from gdb to get
more information if mysqld crashes.
With some older gdb versions on Linux you must use run
--one-thread if you want to be able to debug mysqld threads. In
this case you can only have one thread active at a time. We recommend you
to upgrade to gdb 5.1 ASAP as thread debugging works much better with this
version!
When running mysqld under gdb, you should disable the stack trace
with --skip-stack-trace to be able to catch segfaults within gdb.
In MySQL 4.0.14 and above you should use the --gdb option to
mysqld. This will install an interrupt handler for SIGINT (needed
to stop mysqld with ^C to set breakpoints) and disable stack
tracing and core file handling.
It's very hard to debug MySQL under gdb if you do a lot of
new connections the whole time as gdb doesn't free the memory for
old threads. You can avoid this problem by starting mysqld with
-O thread_cache_size= 'max_connections +1'. In most cases just
using -O thread_cache_size=5' will help a lot!
If you want to get a core dump on Linux if mysqld dies with a
SIGSEGV signal, you can start mysqld with the --core-file option.
This core file can be used to make a backtrace that may help you
find out why mysqld died:
shell> gdb mysqld core gdb> backtrace full gdb> exit |
See section What To Do If MySQL Keeps Crashing.
If you are using gdb 4.17.x or above on Linux, you should install a `.gdb' file, with the following information, in your current directory:
set print sevenbit off handle SIGUSR1 nostop noprint handle SIGUSR2 nostop noprint handle SIGWAITING nostop noprint handle SIGLWP nostop noprint handle SIGPIPE nostop handle SIGALRM nostop handle SIGHUP nostop handle SIGTERM nostop noprint |
If you have problems debugging threads with gdb, you should download gdb 5.x and try this instead. The new gdb version has very improved thread handling!
Here is an example how to debug mysqld:
shell> gdb /usr/local/libexec/mysqld gdb> run ... backtrace full # Do this when mysqld crashes |
Include the above output in a mail generated with mysqlbug and
mail this to the general MySQL mailing list.
See section The MySQL Mailing Lists.
If mysqld hangs you can try to use some system tools like
strace or /usr/proc/bin/pstack to examine where
mysqld has hung.
strace /tmp/log libexec/mysqld |
If you are using the Perl DBI interface, you can turn on
debugging information by using the trace method or by
setting the DBI_TRACE environment variable.
See section Perl DBI Class.
On some operating systems, the error log will contain a stack trace if
mysqld dies unexpectedly. You can use this to find out where (and
maybe why) mysqld died. See section The Error Log. To get a stack trace,
you must not compile mysqld with the -fomit-frame-pointer
option to gcc. See section Compiling MYSQL for Debugging.
If the error file contains something like the following:
mysqld got signal 11; The manual section 'Debugging a MySQL server' tells you how to use a stack trace and/or the core file to produce a readable backtrace that may help in finding out why mysqld died Attemping backtrace. You can use the following information to find out where mysqld died. If you see no messages after this, something went terribly wrong stack range sanity check, ok, backtrace follows 0x40077552 0x81281a0 0x8128f47 0x8127be0 0x8127995 0x8104947 0x80ff28f 0x810131b 0x80ee4bc 0x80c3c91 0x80c6b43 0x80c1fd9 0x80c1686 |
you can find where mysqld died by doing the following:
mysqld server:
nm -n libexec/mysqld > /tmp/mysqld.sym |
Note that most MySQL binary distributions (except for the "debug" packages,
where this information is included inside of the binaries themselves)
already ship with the above file, named mysqld.sym.gz.
In this case you can simply unpack it by doing:
gunzip < bin/mysqld.sym.gz > /tmp/mysqld.sym |
resolve_stack_dump -s /tmp/mysqld.sym -n mysqld.stack.
This will print out where mysqld died. If this doesn't help you
find out why mysqld died, you should make a bug report and include
the output from the above command with the bug report.
Note however that in most cases it will not help us to just have a stack
trace to find the reason for the problem. To be able to locate the bug
or provide a workaround, we would in most cases need to know the query
that killed mysqld and preferable a test case so that we can
repeat the problem! See section How to Report Bugs or Problems.
Note that before starting mysqld with --log you should
check all your tables with myisamchk.
See section Database Administration.
If mysqld dies or hangs, you should start mysqld with
--log. When mysqld dies again, you can examine the end of
the log file for the query that killed mysqld.
If you are using --log without a file name, the log is stored in
the database directory as 'hostname'.log In most cases it's the last
query in the log file that killed mysqld, but if possible you
should verify this by restarting mysqld and executing the found
query from the mysql command-line tools. If this works, you
should also test all complicated queries that didn't complete.
You can also try the command EXPLAIN on all SELECT
statements that takes a long time to ensure that mysqld is using
indexes properly. See section EXPLAIN.
You can find the queries that take a long time to execute by starting
mysqld with --log-slow-queries. See section The Slow Query Log.
If you find the text mysqld restarted in the error log file
(normally named `hostname.err') you have probably found a query
that causes mysqld to fail. If this happens you should check all
your tables with myisamchk (see section Database Administration),
and test the queries in the MySQL log files to see if one doesn't
work. If you find such a query, try first upgrading to the newest
MySQL version. If this doesn't help and you can't find anything
in the mysql mail archive, you should report the bug to a MySQL
mailing list.
The mailing lists are described at
http://lists.mysql.com/, which also has links to online
list archives.
If you have started mysqld with myisam-recover,
MySQL will automatically check and try to repair MyISAM
tables if they are marked as 'not closed properly' or 'crashed'. If
this happens, MySQL will write an entry in the
hostname.err file 'Warning: Checking table ...' which is
followed by Warning: Repairing table if the table needs to be
repaired. If you get a lot of these errors, without mysqld having
died unexpectedly just before, then something is wrong and needs to
be investigated further. See section mysqld Command-line Options.
It's of course not a good sign if mysqld did died unexpectedly,
but in this case one shouldn't investigate the Checking table...
messages but instead try to find out why mysqld died.
If you get corrupted tables or if mysqld always fails after some
update commands, you can test if this bug is reproducible by doing the
following:
mysqladmin shutdown).
myisamchk -s database/*.MYI. Repair any
wrong tables with myisamchk -r database/table.MYI.
mysqld with --log-bin. See section The Binary Log.
If you want to find a query that crashes mysqld, you should use
--log --log-bin.
mysqld server.
mysqld server without --log-bin
mysqlbinlog update-log-file | mysql.
The update log is saved in the MySQL database directory with
the name hostname-bin.#.
mysqld to die with the
above command, you have found reproducible bug that should be easy to
fix! FTP the tables and the binary log to
ftp://support.mysql.com/pub/mysql/secret/ and enter it into
our bugs system at http://bugs.mysql.com/.
If you are a support customer), you can also support@mysql.com to
alert the MySQL team about the problem and have it fixed as soon as possible.
You can also use the script mysql_find_rows to just execute some of the
update statements if you want to narrow down the problem.
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Generated: 2007-01-26 17:58:44