InnoDB supports page-level compression for tables that reside in file-per-table tablespaces. This feature is referred to as Transparent Page Compression. Page compression is enabled by specifying the COMPRESSION attribute with CREATE TABLE or ALTER TABLE. Supported compression algorithms include Zlib and LZ4.
Page compression requires sparse file and hole punching support. Page compression is supported on Windows with NTFS, and on the following subset of MySQL-supported Linux platforms where the kernel level provides hole punching support:
- RHEL 7 and derived distributions that use kernel version 3.10.0-123 or higher
- OEL 5.10 (UEK2) kernel version 2.6.39 or higher
- OEL 6.5 (UEK3) kernel version 3.8.13 or higher
- OEL 7.0 kernel version 3.8.13 or higher
- SLE11 kernel version 3.0-x
- SLE12 kernel version 3.12-x
- OES11 kernel version 3.0-x
- Ubuntu 14.0.4 LTS kernel version 3.13 or higher
- Ubuntu 12.0.4 LTS kernel version 3.2 or higher
- Debian 7 kernel version 3.2 or higher
Note
All of the available file systems for a given Linux distribution may not support hole punching.
When a page is written, it is compressed using the specified compression algorithm. The compressed data is written to disk, where the hole punching mechanism releases empty blocks from the end of the page. If compression fails, data is written out as-is.
On Linux systems, the file system block size is the unit size used for hole punching. Therefore, page compression only works if page data can be compressed to a size that is less than or equal to the InnoDB page size minus the file system block size. For example, if innodb_page_size=16K and the file system block size is 4K, page data must compress to less than or equal to 12K to make hole punching possible.
On Windows systems, the underlying infrastructure for sparse files is based on NTFS compression. Hole punching size is the NTFS compression unit, which is 16 times the NTFS cluster size. Cluster sizes and their compression units are shown in the following table:
Table 14.8 Windows NTFS Cluster Size and Compression Units
| Cluster Size | Compression Unit |
|---|---|
| 512 Bytes | 8 KB |
| 1 KB | 16 KB |
| 2 KB | 32 KB |
| 4 KB | 64 KB |
Page compression on Windows systems only works if page data can be compressed to a size that is less than or equal to the InnoDB page size minus the compression unit size.
The default NTFS cluster size is 4KB, for which the compression unit size is 64KB. This means that page compression has no benefit for an out-of-the box Windows NTFS configuration, as the maximum innodb_page_size is also 64KB.
For page compression to work on Windows, the file system must be created with a cluster size smaller than 4K, and the innodb_page_size must be at least twice the size of the compression unit. For example, for page compression to work on Windows, you could build the file system with a cluster size of 512 Bytes (which has a compression unit of 8KB) and initialize InnoDB with an innodb_page_size value of 16K or greater.
To enable page compression, specify the COMPRESSION attribute in the CREATE TABLE statement. For example:
CREATE TABLE t1 (c1 INT) COMPRESSION="zlib";You can also enable page compression in an ALTER TABLE statement. However, ALTER TABLE ... COMPRESSION only updates the tablespace compression attribute. Writes to the tablespace that occur after setting the new compression algorithm use the new setting, but to apply the new compression algorithm to existing pages, you must rebuild the table using OPTIMIZE TABLE.
ALTER TABLE t1 COMPRESSION="zlib";
OPTIMIZE TABLE t1;To disable page compression, set COMPRESSION=None using ALTER TABLE. Writes to the tablespace that occur after setting COMPRESSION=None no longer use page compression. To uncompress existing pages, you must rebuild the table using OPTIMIZE TABLE after setting COMPRESSION=None.
ALTER TABLE t1 COMPRESSION="None";
OPTIMIZE TABLE t1;Page compression metadata is found in the INFORMATION_SCHEMA.INNODB_SYS_TABLESPACES table, in the following columns:
FS_BLOCK_SIZE: The file system block size, which is the unit size used for hole punching.FILE_SIZE: The apparent size of the file, which represents the maximum size of the file, uncompressed.ALLOCATED_SIZE: The actual size of the file, which is the amount of space allocated on disk.
Note
On Unix-like systems, ls -l *tablespace_name*.ibd shows the apparent file size (equivalent to FILE_SIZE) in bytes. To view the actual amount of space allocated on disk (equivalent to ALLOCATED_SIZE), use du --block-size=1 *tablespace_name*.ibd. The --block-size=1 option prints the allocated space in bytes instead of blocks, so that it can be compared to ls -l output.
Use SHOW CREATE TABLE to view the current page compression setting (Zlib, Lz4, or None). A table may contain a mix of pages with different compression settings.
In the following example, page compression metadata for the employees table is retrieved from the INFORMATION_SCHEMA.INNODB_SYS_TABLESPACES table.
# Create the employees table with Zlib page compression
CREATE TABLE employees (
emp_no INT NOT NULL,
birth_date DATE NOT NULL,
first_name VARCHAR(14) NOT NULL,
last_name VARCHAR(16) NOT NULL,
gender ENUM ('M','F') NOT NULL,
hire_date DATE NOT NULL,
PRIMARY KEY (emp_no)
) COMPRESSION="zlib";
# Insert data (not shown)
# Query page compression metadata in INFORMATION_SCHEMA.INNODB_SYS_TABLESPACES
mysql> SELECT SPACE, NAME, FS_BLOCK_SIZE, FILE_SIZE, ALLOCATED_SIZE FROM
INFORMATION_SCHEMA.INNODB_SYS_TABLESPACES WHERE NAME='employees/employees'\G
*************************** 1. row ***************************
SPACE: 45
NAME: employees/employees
FS_BLOCK_SIZE: 4096
FILE_SIZE: 23068672
ALLOCATED_SIZE: 19415040Page compression metadata for the employees table shows that the apparent file size is 23068672 bytes while the actual file size (with page compression) is 19415040 bytes. The file system block size is 4096 bytes, which is the block size used for hole punching.
To identify tables for which page compression is enabled, you can query the INFORMATION_SCHEMA.TABLES CREATE_OPTIONS column for tables defined with the COMPRESSION attribute:
mysql> SELECT TABLE_NAME, TABLE_SCHEMA, CREATE_OPTIONS FROM INFORMATION_SCHEMA.TABLES
WHERE CREATE_OPTIONS LIKE '%COMPRESSION=%';
+------------+--------------+--------------------+
| TABLE_NAME | TABLE_SCHEMA | CREATE_OPTIONS |
+------------+--------------+--------------------+
| employees | test | COMPRESSION="zlib" |
+------------+--------------+--------------------+SHOW CREATE TABLE also shows the COMPRESSION attribute, if used.
- Page compression is disabled if the file system block size (or compression unit size on Windows) * 2 >
innodb_page_size. - Page compression is not supported for tables that reside in shared tablespaces, which include the system tablespace, the temporary tablespace, and general tablespaces.
- Page compression is not supported for undo log tablespaces.
- Page compression is not supported for redo log pages.
- R-tree pages, which are used for spatial indexes, are not compressed.
- Pages that belong to compressed tables (
ROW_FORMAT=COMPRESSED) are left as-is. - During recovery, updated pages are written out in an uncompressed form.
- Loading a page-compressed tablespace on a server that does not support the compression algorithm that was used causes an I/O error.
- Before downgrading to an earlier version of MySQL that does not support page compression, uncompress the tables that use the page compression feature. To uncompress a table, run
ALTER TABLE ... COMPRESSION=NoneandOPTIMIZE TABLE. - Page-compressed tablespaces can be copied between Linux and Windows servers if the compression algorithm that was used is available on both servers.
- Preserving page compression when moving a page-compressed tablespace file from one host to another requires a utility that preserves sparse files.
- Better page compression may be achieved on Fusion-io hardware with NVMFS than on other platforms, as NVMFS is designed to take advantage of punch hole functionality.
- Using the page compression feature with a large
InnoDBpage size and relatively small file system block size could result in write amplification. For example, a maximumInnoDBpage size of 64KB with a 4KB file system block size may improve compression but may also increase demand on the buffer pool, leading to increased I/O and potential write amplification.