Enabling Core Dumps

Enabling in an Option File

In order to enable core dumps, you need to set the core_file system variable either on the command-line or in a relevant server option group in an option file. For example:

[mariadb]
...
core_file

You can check your current value by executing:

my_print_defaults --mariadbd

core_file is a system variable (prior to MariaDB 10.3.9 it was just an option). Its value can be checked at runtime by executing the following:

SHOW GLOBAL VARIABLES LIKE 'core_file';

Core Files on Linux

There are some additional details related to using core files on Linux.

Disabling Core File Size Restrictions on Linux

On some systems there is a limit on the sizes of core files that can be dumped. You can check the system's current system-wide limit by executing the following:

ulimit -c

You can check the current limit of the mariadbd process specifically by executing the following:

sudo cat /proc/$(pidof mariadbd)/limits | grep "core file"

If you need to change the core size limit, the method you use depends on how you start mariadbd. See the sections below for more details.

Running mariadbd Using mysqld_safe

If you are starting MariaDB by running mysqld_safe, then configuring the following in the [mysqld_safe] option group in an option file should allow for unlimited sized core files:

[mysqld_safe]
...
core_file_size=unlimited

You can check your current values by executing:

my_print_defaults mysqld_safe

See mysqld_safe: Configuring the Core File Size for more details.

Running mariadbd Manually

If you are starting mariadbd manually or in a custom script, then you can allow for unlimited sized core files by executing the following in the same shell or script in which mariadbd is executed:

ulimit -c unlimited

Running mariadbd Using systemd

If you are starting mariadbd using systemd, then you may need to customize the MariaDB service to allow for unlimited size core files. For example, you could execute the following:

Using sudo systemctl edit mariadb.service add the contents:

[Service]

LimitCORE=infinity

See systemd: Configuring the Core File Size for more details.

Running MariaDB Containers

To get a core dump in a mariadb container requires setting the path on Linux to not include a sysctl kernel.core_pattern beginning with a pipe to an executable that doesn't exist in the container. Setting to a straight core is recommended.

Also see Container with Debug Symbols.

Changing the System-Wide Limit

If you want to change the system-wide limit to allow for unlimited size core files for for the mysql user account, then you can do so by adding the following lines to a file in /etc/security/limits.d/. For example:

sudo tee /etc/security/limits.d/mariadb_core.conf <<EOF
mysql soft core unlimited
mysql hard core unlimited
EOF

The system would have to be restarted for this change to take effect.

See Configuring Linux for MariaDB: Configuring the Core File Size for more details.

Setting the Path on Linux

If you are using Linux, then it can be helpful to change a few settings to alter where the core files is written and what file name is used. This is done by setting the kernel.core_pattern and kernel.core_uses_pid attributes. You can check the current values by executing the following:

sysctl kernel.core_pattern
sysctl kernel.core_uses_pid

If you are using mysql-test-run and want to have the core as part of the test result, the optimal setting is probably the following (store cores in the current directory as core.number-of-process-id):

sudo sysctl kernel.core_pattern=core.%p kernel.core_uses_pid=0

If you are using a production system, you probably want to have the core files in a specific directory, not in the data directory. They place to store cores can be temporarily altered using the sysctl utility, but it is often more common to alter them via the /proc file system. See the following example:

sudo mkdir /tmp/corefiles
sudo chmod 777 /tmp/corefiles
sudo sysctl kernel.core_pattern=/tmp/corefiles/core
sudo sysctl kernel.core_uses_pid=1

The above commands will tell the system to put core files in /tmp/corefiles, and it also tells the system to put the process ID in the file name.

If you want to make these changes permanent, then you can add the following to a file in /etc/sysctl.conf.d/. For example:

sudo tee /etc/sysctl.d/mariadb_core.conf <<EOF
kernel.core_pattern=/tmp/corefiles/core
kernel.core_uses_pid=1
EOF

Note: if you are using containers, the pid is always going to be 1, so this may not be a useful setting. Appending an identifier like %t to the kernel.core_pattern will generate more unique files.

Extracting Linux core dumps with systemd-coredump

Core dump management can be automated using systemd, which then centrally manages all core dump files and provides information about detected core dumps and access to collected core files using the coredumpctl utility.

This is enabled per default on Red Hat Enterprise Linux 8 and CentOS 8, and maybe other contemporary Linux distribution releases by now, too. It can be easily checked for by looking at the kernel.core_pattern setting. If it looks like this systemd-coredump is enabled:

# sysctl kernel.core_pattern
kernel.core_pattern = |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %e

On other distributions like Ubuntu (at least up to 21.10) it is not enabled by default, but can be set up manually.

To see all recent core dumps on the system you can then simply run

# coredumpctl list

Or you can check for MariaDB Server core dumps specifically with:

# coredumpctl list /usr/sbin/mariadbd 

If an actual core file got stored you'll see present in the COREFILE column of the output, you can then extract the core file with:

# coredumpctl dump -o mariadbd.core ...PID...

using the process id number from the PID column, or when you just want to retrieve the latest MariaDB Server related entry:

# coredumpctl dump -o mariadb.core /usr/sbin/mariadbd

Starting with systemd 248 it is also possible to invoke the gdb debugger directly using the new --debugger-arguments=... option, e.g. making the extraction of all thread backtraces from the most recent MariaDB server crash a one liner without even having to extract the core dump file first (requires gdb to be installed):

# coredumpctl debug --debugger-arguments="-batch -ex 'thread apply all bt full'" /usr/sbin/mariadbd

So far none of the long term support Linux distribution releases have a new enough systemd version for this, the (as of this writing) still upcoming Ubuntu 22.04 "Jammy Jellyfish" will probably the first to support this.

Core Dumps and setuid on Linux

Since mariadbd executes setuid, you may have to set fs.suid_dumpable=2 to allow core dumps on Linux. You can check the current fs.suid_dumpable value by using the sysctl utility. For example:

sysctl fs.suid_dumpable

You can temporarily set it to 2 by using the sysctl utility. For example:

sudo sysctl -w fs.suid_dumpable=2

Or you can temporarily set it to 2 by writing to the /proc file system. For example:

sudo echo 2 > /proc/sys/fs/suid_dumpable

If you want to permanently set it to 2 then you can add the following to a file in /etc/sysctl.conf.d/:

sudo tee /etc/sysctl.d/mariadb_fs_suid_dumpable.conf <<EOF
fs.suid_dumpable=2
EOF

Forcing a Core File on Linux

You can get a core dump from a running server with:

sudo gcore -o filename $(pidof mariadbd)

This will store a core dump in filename.pid where pid is the process ID of mariadbd. mariadbd will continue to be running and will not be affected by gcore.

Another method is to force a core file for mariadbd by sending the process the sigabrt signal, which has the signal code 6. This is very useful to get the state of the unresponsive mariadbd process. However, this will cause mariadbd to crash, and crash recovery will be run on restart.

You can send the signal with the kill command. For example:

sudo kill -6 $(pidof mariadbd)

As an alternative to $(pidof mariadbd), you can find the process ID either by using the ps utility or by checking the file defined by the pid_file system variable.

Core Files on Windows

In MariaDB 10.4 and later, core dumps are enabled by default on Windows. See MDEV-18439 for more information.

There are some additional details related to using core files on Windows.

Core Files on Kubernetes

See the IBM Core Dump Handler project.

Minidump Files on Windows

On Windows, the core file is created as a minidump file.

For details on how to configure and read the minidump file, see How to read the small memory dump file that is created by Windows if a crash occurs.

Core Files and Address Sanitizer (ASAN)

If your mariadbd binary is built with Address Sanitizer (ASAN) then it will not be able to generate a core file.

What's Included in Core Files

Core files usually contain a dump of all memory in the process's full address space. This means that if a server has some large buffers configured (such as a large InnoDB buffer pool), then the server's core files can get very large.

However, in MariaDB 10.3.7 and later, some large buffers have been excluded from core files on some systems as a way to reduce the size.

The following buffers are excluded:

• InnoDB buffer pool
• InnoDB log buffer
• InnoDB Redo log buffer (fixed 2M)
• Query cache

The buffers are only excluded on Linux when using kernel version 3.4 and above and when using a non-debug build of mariadbd. Some Linux kernel versions have a bug which would cause the following warning to be printed to the log:

Sep 25 10:41:19 srv1 mysqld: 2018-09-25 10:41:19 0 [Warning] InnoDB: Failed to set memory to DODUMP: Invalid argument ptr 0x2aaac3400000 size 33554432

In those cases, the core dump may exclude some additional data. If that is not a concern, then the warning can be ignored. The problem can be fixed by upgrading to a Linux kernel version in which the bug is fixed.

See Also

• How to Produce a Full Stack Trace for mariadbd
• HowTo: Debug Crashed Linux Application Core Files Like A Pro
• A Nice Feature in MariaDB 10.3: no InnoDB Buffer Pool in Core Dumps
• Getting MySQL Core file on Linux