17.11.2 File Space Management

The data files that you define in the configuration file using the innodb_data_file_path configuration option form the InnoDB system tablespace. The files are logically concatenated to form the system tablespace. There is no striping in use. You cannot define where within the system tablespace your tables are allocated. In a newly created system tablespace, InnoDB allocates space starting from the first data file.

To avoid the issues that come with storing all tables and indexes inside the system tablespace, you can enable the innodb_file_per_table configuration option (the default), which stores each newly created table in a separate tablespace file (with extension .ibd). For tables stored this way, there is less fragmentation within the disk file, and when the table is truncated, the space is returned to the operating system rather than still being reserved by InnoDB within the system tablespace. For more information, see Section 17.6.3.2, “File-Per-Table Tablespaces”.

You can also store tables in general tablespaces. General tablespaces are shared tablespaces created using CREATE TABLESPACE syntax. They can be created outside of the MySQL data directory, are capable of holding multiple tables, and support tables of all row formats. For more information, see Section 17.6.3.3, “General Tablespaces”.

Pages, Extents, Segments, and Tablespaces

Each tablespace consists of database pages. Every tablespace in a MySQL instance has the same page size. By default, all tablespaces have a page size of 16KB; you can reduce the page size to 8KB or 4KB by specifying the innodb_page_size option when you create the MySQL instance. You can also increase the page size to 32KB or 64KB. For more information, refer to the innodb_page_size documentation.

The pages are grouped into extents of size 1MB for pages up to 16KB in size (64 consecutive 16KB pages, or 128 8KB pages, or 256 4KB pages). For a page size of 32KB, extent size is 2MB. For page size of 64KB, extent size is 4MB. The “files” inside a tablespace are called segments in InnoDB. (These segments are different from the rollback segment, which actually contains many tablespace segments.)

When a segment grows inside the tablespace, InnoDB allocates the first 32 pages to it one at a time. After that, InnoDB starts to allocate whole extents to the segment. InnoDB can add up to 4 extents at a time to a large segment to ensure good sequentiality of data.

Two segments are allocated for each index in InnoDB. One is for nonleaf nodes of the B-tree, the other is for the leaf nodes. Keeping the leaf nodes contiguous on disk enables better sequential I/O operations, because these leaf nodes contain the actual table data.

Some pages in the tablespace contain bitmaps of other pages, and therefore a few extents in an InnoDB tablespace cannot be allocated to segments as a whole, but only as individual pages.

When you ask for available free space in the tablespace by issuing a SHOW TABLE STATUS statement, InnoDB reports the extents that are definitely free in the tablespace. InnoDB always reserves some extents for cleanup and other internal purposes; these reserved extents are not included in the free space.

When you delete data from a table, InnoDB contracts the corresponding B-tree indexes. Whether the freed space becomes available for other users depends on whether the pattern of deletes frees individual pages or extents to the tablespace. Dropping a table or deleting all rows from it is guaranteed to release the space to other users, but remember that deleted rows are physically removed only by the purge operation, which happens automatically some time after they are no longer needed for transaction rollbacks or consistent reads. (See Section 17.3, “InnoDB Multi-Versioning”.)

Configuring the Percentage of Reserved File Segment Pages

The innodb_segment_reserve_factor variable, introduced in MySQL 8.0.26, is an advanced feature that permits defining the percentage of tablespace file segment pages reserved as empty pages. A percentage of pages are reserved for future growth so that pages in the B-tree can be allocated contiguously. The ability to modify the percentage of reserved pages permits fine-tuning InnoDB to address issues of data fragmentation or inefficient use of storage space.

The setting is applicable to file-per-table and general tablespaces. The innodb_segment_reserve_factor default setting is 12.5 percent, which is the same percentage of pages reserved in previous MySQL releases.

The innodb_segment_reserve_factor variable is dynamic and can be configured using a SET statement. For example:

mysql> SET GLOBAL innodb_segment_reserve_factor=10;

How Pages Relate to Table Rows

For for 4KB, 8KB, 16KB, and 32KB innodb_page_size settings, the maximum row length is slightly less than half a database page size. For example, the maximum row length is slightly less than 8KB for the default 16KB InnoDB page size. For a 64KB innodb_page_size setting, the maximum row length is slightly less than 16KB.

If a row does not exceed the maximum row length, all of it is stored locally within the page. If a row exceeds the maximum row length, variable-length columns are chosen for external off-page storage until the row fits within the maximum row length limit. External off-page storage for variable-length columns differs by row format:

• COMPACT and REDUNDANT Row Formats

  When a variable-length column is chosen for external off-page storage, InnoDB stores the first 768 bytes locally in the row, and the rest externally into overflow pages. Each such column has its own list of overflow pages. The 768-byte prefix is accompanied by a 20-byte value that stores the true length of the column and points into the overflow list where the rest of the value is stored. See Section 17.10, “InnoDB Row Formats”.

• DYNAMIC and COMPRESSED Row Formats

  When a variable-length column is chosen for external off-page storage, InnoDB stores a 20-byte pointer locally in the row, and the rest externally into overflow pages. See Section 17.10, “InnoDB Row Formats”.

LONGBLOB and LONGTEXT columns must be less than 4GB, and the total row length, including BLOB and TEXT columns, must be less than 4GB.