15.12.3 Online DDL Space Requirements

Space for temporary log files

A temporary log file records concurrent DML when an online DDL operation creates an index or alters a table. The temporary log file is extended as required by the value of innodb_sort_buffer_size up to a maximum specified by innodb_online_alter_log_max_size. If a temporary log file exceeds the size limit, the online DDL operation fails, and uncommitted concurrent DML operations are rolled back. A large innodb_online_alter_log_max_size setting permits more DML during an online DDL operation, but it also extends the period of time at the end of the DDL operation when the table is locked to apply logged DML.

If the operation takes a long time and concurrent DML modifies the table so much that the size of the temporary log file exceeds the value of innodb_online_alter_log_max_size, the online DDL operation fails with a DB_ONLINE_LOG_TOO_BIG error.

Space for temporary sort files

Online DDL operations that rebuild the table write temporary sort files to the MySQL temporary directory ($TMPDIR on Unix, %TEMP% on Windows, or the directory specified by --tmpdir) during index creation. Temporary sort files are not created in the directory that contains the original table. Each temporary sort file is large enough to hold all secondary index columns plus the primary key columns of the clustered index. Temporary sort files are removed as soon as their contents are merged into the final table or index. Temporary sort files may require space equal to the amount of data in the table plus indexes. An online DDL operation that rebuilds the table reports an error if it uses all of the available disk space on the file system where the data directory resides.

If the MySQL temporary directory is not large enough to hold the sort files, set tmpdir to a different directory. Alternatively, define a separate temporary directory for online DDL operations using innodb_tmpdir. This option was introduced to help avoid temporary directory overflows that could occur as a result of large temporary sort files.

Space for an intermediate table file

Some online DDL operations that rebuild the table create a temporary intermediate table file in the same directory as the original table. An intermediate table file may require space equal to the size of the original table. Intermediate table file names begin with #sql-ib prefix and only appear briefly during the online DDL operation.

The innodb_tmpdir option is not applicable to intermediate table files.