MySQL 5.7 Release Notes
The DDL log, or metadata log, records metadata operations
generated by data definition statements affecting table
partitioning, such as
ALTER
TABLE t3 DROP PARTITION p2, where we must make certain
that the partition is completely dropped and that its definition
is removed from the list of partitions for table
t3. MySQL uses this log to recover from a crash
occurring in the middle of a partitioning metadata operation.
A record of partitioning metadata operations is written to the
file ddl_log.log, in the MySQL data
directory. This is a binary file; it is not intended to be
human-readable, and you should not attempt to modify its contents
in any way.
ddl_log.log is not created until it is
actually needed for recording metadata statements, and is removed
following a successful start of mysqld. Thus,
it is possible for this file not to be present on a MySQL server
that is functioning in a completely normal manner.
ddl_log.log can hold up to 1048573 entries,
equivalent to 4 GB in size. Once this limit is exceeded, you must
rename or remove the file before it is possible to execute any
additional DDL statements. This is a known issue (Bug #83708).
There are no user-configurable server options or variables associated with this file.