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
variable to omit buffer pool pages from core dumps. The
variable was introduced in MySQL 8.0.14 and is enabled by
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.
Access to the data present in buffer pool pages at the time the mysqld process died may be beneficial in some debugging scenarios. If in doubt whether to include or exclude buffer pool pages, consult MySQL Support.
takes effect only if the
core_file variable is enabled
and the operating system supports the
MADV_DONTDUMP non-POSIX extension to the
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
options to generate core files without buffer pool pages.
shell> mysqld --core-file --innodb-buffer-pool-in-core-file=OFF
core_file variable is read
only and disabled by default. It is enabled by specifying the
--core-file option at startup.
variable is dynamic. It can be specified at startup or
configured at runtime using a
mysql> SET GLOBAL innodb_buffer_pool_in_core_file=OFF;
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.
Table 15.5 Core File Configuration Scenarios
||madvise() MADV_DONTDUMP Support||Outcome|
|OFF (default)||Not relevant to outcome||Not relevant to outcome||Core file is not generated|
|ON||ON (default)||Not relevant to outcome||Core file is generated with buffer pool pages|
|ON||OFF||Yes||Core file is generated without buffer pool pages|
|ON||OFF||No||Core file is not generated,
The reduction in core file size achieved by disabling the
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.
Table 15.6 Core File Size with Buffer Pool Pages Included and Excluded