A core file records the status and memory image of a running process. Because the buffer pool resides in main memory, and the memory image of a running process is dumped to the core file, systems with large buffer pools can produce large core files when the mysqld process dies.

Large core files can be problematic for a number of reasons including the time it takes to write them, the amount of disk space they consume, and the challenges associated with transferring large files.

To reduce core file size, you can disable the innodb_buffer_pool_in_core_file variable to omit buffer pool pages from core dumps. The innodb_buffer_pool_in_core_file variable was introduced in MySQL 8.0.14 and is enabled by default.

Excluding buffer pool pages may also be desirable from a security perspective if you have concerns about dumping database pages to core files that may be shared inside or outside of your organization for debugging purposes.

Disabling innodb_buffer_pool_in_core_file takes effect only if the core_file variable is enabled and the operating system supports the MADV_DONTDUMP non-POSIX extension to the madvise() system call, which is supported in Linux 3.4 and later. The MADV_DONTDUMP extension causes pages in a specified range to be excluded from core dumps.

Assuming the operating system supports the MADV_DONTDUMP extension, start the server with the --core-file and --innodb-buffer-pool-in-core-file=OFF options to generate core files without buffer pool pages.

shell> mysqld --core-file --innodb-buffer-pool-in-core-file=OFF

The core_file variable is read only and disabled by default. It is enabled by specifying the --core-file option at startup. The innodb_buffer_pool_in_core_file variable is dynamic. It can be specified at startup or configured at runtime using a SET statement.

mysql> SET GLOBAL innodb_buffer_pool_in_core_file=OFF;

If the innodb_buffer_pool_in_core_file variable is disabled but MADV_DONTDUMP is not supported by the operating system, or an madvise() failure occurs, a warning is written to the MySQL server error log and the core_file variable is disabled to prevent writing core files that unintentionally include buffer pool pages. If the read-only core_file variable becomes disabled, the server must be restarted to enable it again.

The following table shows configuration and MADV_DONTDUMP support scenarios that determine whether core files are generated and whether they include buffer pool pages.

The reduction in core file size achieved by disabling the innodb_buffer_pool_in_core_file variable depends on the size of the buffer pool, but it is also affected by the InnoDB page size. A smaller page size means more pages are required for the same amount of data, and more pages means more page metadata. The following table provides size reduction examples that you might see for a 1GB buffer pool with different pages sizes.