- 21.5.1 Debugging a MySQL Server
- 21.5.1.1 Compiling MySQL for Debugging
- 21.5.1.2 Creating Trace Files
- 21.5.1.3 Using pdb to create a Windows crashdump
- 21.5.1.4 Debugging mysqld under gdb
- 21.5.1.5 Using a Stack Trace
- 21.5.1.6 Using Server Logs to Find Causes of Errors in mysqld
- 21.5.1.7 Making a Test Case If You Experience Table Corruption
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. It is best to
upgrade to gdb 5.1 because thread debugging
works much better with this version!
NPTL threads (the new thread library on Linux) may cause problems while running mysqld under gdb. Some symptoms are:
In this case, you should set the following environment variable in the shell before starting gdb:
LD_ASSUME_KERNEL=2.4.1 export LD_ASSUME_KERNEL
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 installs an interrupt handler
for SIGINT (needed to stop
mysqld with ^C to set
breakpoints) and disable stack tracing and core file handling.
It is 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
thread_cache_size set to a
value equal to
max_connections + 1. In most
cases just using
--thread_cache_size=5' helps 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> quit
See Section B.1.4.2, “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 bug report, which you can file using the instructions in Section 1.6, “How to Report Bugs or Problems”.
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.
User Comments
On Solaris you can use instead of strace system
utility called truss. For example, "truss
-t\!ioctl,\!lseek,\!stat,\!fstat,\!fstat64 -e
-vall -xall -a -f ./mysqld"
To debug under a newly started process, chdir() to
the source tree, compile mysql using -g3 CFLAGS
option at the best and run:
# gdb ./sql/mysqld
(gdb) set args --datadir=/data/mysql2
--basedir=/usr/local/mysql-BK20021104
--pid-file=/usr/local/mysql/var/kulan.pidd
--port=3307 --socket=/tmp/mysql2.sock
(gdb) break mysqld.cc:2039
(gdb) break open() <- stop on every open() call
(gdb) run <- run until we reach our breakpoint
(gdb) where <- where are we now in the sources?
(gdb) l <- list source code
(gdb) s <- step
(gdb) n <- next
(gdb) p varname <- print variable contents
(gdb) quit
To debug an already running mysqld process and see
what happens to it if you issue a specific SQL
command from the interactive mysql session, you
have to figure out first to which PID you have to
connect. Issue from you client commands line
"SELECT 3+3" and figure out the PID. (Sinisa says
that you can in principle connect to any mysqld
process and switch context to access another
thread, but NOT on Linux these days yet. Use `info
threads' command at the gdb prompt in such case)
chdir() to unpacked source tree. At the best,
compile next time mysql using CFLAGS="-g3".
# gdb
(gdb) attach 1234
(gdb) detach
(gdb) break sql_parse.cc:981
(gdb) b pthread_mutex_unlock
(gdb) info break
(gdb) attach 12345
(gdb) c
(gdb) s
(gdb) n
(gdb) where
(gdb) l
(gdb) detach
(gdb) quit
Where to set breakpoints as a general start place?
In client in real_connect(). On the server in handle_connections(), but in version above 4.0.8 it got renamed, so use handle_one_connection() for example.
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