Each MySQL version is tested on many platforms before it is released. This doesn't mean that there are no bugs in MySQL, but if there are bugs, they should be very few and can be hard to find. If you have a problem, it always helps if you try to find out exactly what crashes your system, because you have a much better chance of getting the problem fixed quickly.
First, you should try to find out whether the problem is that the mysqld server dies or whether your problem has to do with your client. You can check how long your mysqld server has been up by executing mysqladmin version. If mysqld has died and restarted, you may find the reason by looking in the server's error log. See Section 5.2.2, “The Error Log”.
On some systems, you can find in the error log a stack trace
of where mysqld died that you can resolve
with the resolve_stack_dump program. See
MySQL
Internals: Porting. Note that the variable values
written in the error log may not always be 100% correct.
Many server crashes are caused by corrupted data files or
index files. MySQL updates the files on disk with the
write() system call after every SQL
statement and before the client is notified about the result.
(This is not true if you are running with
--delay-key-write, in which
case data files are written but not index files.) This means
that data file contents are safe even if
mysqld crashes, because the operating
system ensures that the unflushed data is written to disk. You
can force MySQL to flush everything to disk after every SQL
statement by starting mysqld with the
--flush option.
The preceding means that normally you should not get corrupted tables unless one of the following happens:
The MySQL server or the server host was killed in the middle of an update.
You have found a bug in mysqld that caused it to die in the middle of an update.
Some external program is manipulating data files or index files at the same time as mysqld without locking the table properly.
You are running many mysqld servers using the same data directory on a system that doesn't support good file system locks (normally handled by the
lockdlock manager), or you are running multiple servers with external locking disabled.You have a crashed data file or index file that contains very corrupt data that confused mysqld.
You have found a bug in the data storage code. This isn't likely, but it is at least possible. In this case, you can try to change the storage engine to another engine by using
ALTER TABLEon a repaired copy of the table.
Because it is very difficult to know why something is crashing, first try to check whether things that work for others crash for you. Please try the following things:
Stop the mysqld server with mysqladmin shutdown, run myisamchk --silent --force */*.MYI from the data directory to check all
MyISAMtables, and restart mysqld. This ensures that you are running from a clean state. See Chapter 5, MySQL Server Administration.Start mysqld with the general query log enabled (see Section 5.2.3, “The General Query Log”). Then try to determine from the information written to the log whether some specific query kills the server. About 95% of all bugs are related to a particular query. Normally, this is one of the last queries in the log file just before the server restarts. See Section 5.2.3, “The General Query Log”. If you can repeatedly kill MySQL with a specific query, even when you have checked all tables just before issuing it, then you have been able to locate the bug and should submit a bug report for it. See Section 1.7, “How to Report Bugs or Problems”.
Try to make a test case that we can use to repeat the problem. See MySQL Internals: Porting.
Try running the tests in the
mysql-testdirectory and the MySQL benchmarks. See Section 23.1.2, “The MySQL Test Suite”. They should test MySQL rather well. You can also add code to the benchmarks that simulates your application. The benchmarks can be found in thesql-benchdirectory in a source distribution or, for a binary distribution, in thesql-benchdirectory under your MySQL installation directory.Try the
fork_big.plscript. (It is located in thetestsdirectory of source distributions.)If you configure MySQL for debugging, it is much easier to gather information about possible errors if something goes wrong. Reconfigure MySQL with the
-DWITH_DEBUG=1option to CMake and then recompile. See MySQL Internals: Porting.Make sure that you have applied the latest patches for your operating system.
Use the
--skip-external-lockingoption to mysqld. On some systems, thelockdlock manager does not work properly; the--skip-external-lockingoption tells mysqld not to use external locking. (This means that you cannot run two mysqld servers on the same data directory and that you must be careful if you use myisamchk. Nevertheless, it may be instructive to try the option as a test.)Have you tried mysqladmin -u root processlist when mysqld appears to be running but not responding? Sometimes mysqld is not comatose even though you might think so. The problem may be that all connections are in use, or there may be some internal lock problem. mysqladmin -u root processlist usually is able to make a connection even in these cases, and can provide useful information about the current number of connections and their status.
Run the command mysqladmin -i 5 status or mysqladmin -i 5 -r status in a separate window to produce statistics while you run your other queries.
Try the following:
Start mysqld from gdb (or another debugger). See MySQL Internals: Porting.
Run your test scripts.
Print the backtrace and the local variables at the three lowest levels. In gdb, you can do this with the following commands when mysqld has crashed inside gdb:
backtrace info local up info local up info local
With gdb, you can also examine which threads exist with
info threadsand switch to a specific thread withthread, whereNNis the thread ID.
Try to simulate your application with a Perl script to force MySQL to crash or misbehave.
Send a normal bug report. See Section 1.7, “How to Report Bugs or Problems”. Be even more detailed than usual. Because MySQL works for many people, it may be that the crash results from something that exists only on your computer (for example, an error that is related to your particular system libraries).
If you have a problem with tables containing dynamic-length rows and you are using only
VARCHARcolumns (notBLOBorTEXTcolumns), you can try to change allVARCHARtoCHARwithALTER TABLE. This forces MySQL to use fixed-size rows. Fixed-size rows take a little extra space, but are much more tolerant to corruption.The current dynamic row code has been in use for several years with very few problems, but dynamic-length rows are by nature more prone to errors, so it may be a good idea to try this strategy to see whether it helps.
Do not rule out your server hardware when diagnosing problems. Defective hardware can be the cause of data corruption. Particular attention should be paid to your memory and disk subsystems when troubleshooting hardware.