Recently someone asked my if it's possible to find the total free space within InnoDB. I thought this would be very easy as the INFORMATION_SCHEMA.TABLES table has a DATA_FREE column. So we could just use SELECT SUM(DATA_FREE) FROM INFORMATION_SCHEMA.TABLES couldn't we?
So what does the DATA_FREE column tell us? It tells us the free data within InnoDB for that particular table. A table can share a tablespace with multiple other tables.
The tablespace which is used by a table depends on whether the innodb_file_per_table was enabled during table creation and/or at the last time the table was rebuild (e.g. by OPTIMIZE TABLE).
If innodb_file_per_table was always disabled then this query probably reports the correct free space:
SELECT DATA_FREE FROM INFORMATION_SCHEMA.TABLES WHERE ENGINE='InnoDB' LIMIT 1;
This is because all tables will share 1 tablespace.
If innodb_file_per_table was always enabled (new default for 5.6!) then this would report the free space:
SELECT SUM(DATA_FREE) FROM INFORMATION_SCHEMA.TABLES WHERE ENGINE='InnoDB';
This is because eache table will have it's own tablespace.
But how could we combine these two?
To give this a try I create a MySQL 5.6.10 sandbox with 4 tables of which only 2 have their own tablespace.
My first try is to use
my udf_fileexists UDF:
mysql> SELECT TABLE_NAME,DATA_FREE,
-> udf_fileexists(CONCAT(TABLE_SCHEMA,'/',TABLE_NAME,'.ibd')) AS ibd_file
-> FROM information_schema.tables WHERE ENGINE='InnoDB';
+----------------------+-----------+----------+
| TABLE_NAME | DATA_FREE | ibd_file |
+----------------------+-----------+----------+
| t1 | 4194304 | 1 |
| t2 | 4194304 | 1 |
| t3 | 66060288 | 0 |
| t4 | 66060288 | 0 |
| innodb_index_stats | 0 | 1 |
| innodb_table_stats | 0 | 1 |
| slave_master_info | 0 | 1 |
| slave_relay_log_info | 0 | 1 |
| slave_worker_info | 0 | 1 |
+----------------------+-----------+----------+
9 rows in set (0.02 sec)
mysql> SELECT (SELECT DATA_FREE FROM INFORMATION_SCHEMA.TABLES WHERE
-> NOT udf_fileexists(CONCAT(TABLE_SCHEMA,'/',TABLE_NAME,'.ibd'))
-> AND ENGINE='InnoDB' LIMIT 1) + (SELECT SUM(DATA_FREE)
-> FROM INFORMATION_SCHEMA.TABLES WHERE
-> udf_fileexists(CONCAT(TABLE_SCHEMA,'/',TABLE_NAME,'.ibd'))
-> AND ENGINE='InnoDB') AS TOTAL_DATA_FREE;
+-----------------+
| TOTAL_DATA_FREE |
+-----------------+
| 74448896 |
+-----------------+
1 row in set (0.03 sec)
So that works, but it requires loading a UDF. Luckily it's also possible to only use INFORMATION_SCHEMA by using the
INNODB_SYS_TABLES table. This works only for 5.6 and newer.
mysql> SELECT T.TABLE_SCHEMA,T.TABLE_NAME,T.DATA_FREE,ST.SPACE,SD.PATH
-> FROM INFORMATION_SCHEMA.TABLES T LEFT JOIN INFORMATION_SCHEMA.INNODB_SYS_TABLES ST
-> ON ST.NAME=CONCAT(T.TABLE_SCHEMA,'/',T.TABLE_NAME)
-> LEFT JOIN INFORMATION_SCHEMA.INNODB_SYS_DATAFILES SD ON SD.SPACE=ST.SPACE WHERE T.ENGINE='InnoDB';
+---------------------+----------------------+-----------+-------+----------------------------------+
| TABLE_SCHEMA | TABLE_NAME | DATA_FREE | SPACE | PATH |
+---------------------+----------------------+-----------+-------+----------------------------------+
| mysql | innodb_table_stats | 0 | 1 | ./mysql/innodb_table_stats.ibd |
| mysql | innodb_index_stats | 0 | 2 | ./mysql/innodb_index_stats.ibd |
| mysql | slave_relay_log_info | 0 | 3 | ./mysql/slave_relay_log_info.ibd |
| mysql | slave_master_info | 0 | 4 | ./mysql/slave_master_info.ibd |
| mysql | slave_worker_info | 0 | 5 | ./mysql/slave_worker_info.ibd |
| innodbfreespacetest | t1 | 4194304 | 6 | ./innodbfreespacetest/t1.ibd |
| innodbfreespacetest | t2 | 4194304 | 7 | ./innodbfreespacetest/t2.ibd |
| innodbfreespacetest | t3 | 66060288 | 0 | NULL |
| innodbfreespacetest | t4 | 66060288 | 0 | NULL |
+---------------------+----------------------+-----------+-------+----------------------------------+
9 rows in set (0.02 sec)
mysql> SELECT (SELECT SUM(DATA_FREE) FROM INFORMATION_SCHEMA.TABLES T
-> LEFT JOIN INFORMATION_SCHEMA.INNODB_SYS_TABLES ST ON
-> ST.NAME=CONCAT(T.TABLE_SCHEMA,'/',T.TABLE_NAME) WHERE SPACE<>0) +
-> (SELECT DATA_FREE FROM INFORMATION_SCHEMA.TABLES T
-> LEFT JOIN INFORMATION_SCHEMA.INNODB_SYS_TABLES ST
-> ON ST.NAME=CONCAT(T.TABLE_SCHEMA,'/',T.TABLE_NAME)
-> WHERE SPACE=0 LIMIT 1) AS INNODB_FREE_GLOBAL;
+--------------------+
| INNODB_FREE_GLOBAL |
+--------------------+
| 74448896 |
+--------------------+
1 row in set (0.04 sec)
Calculating the free space can be useful if you don't have autoextend enabled on your datafiles (requires innodb_file_per_table to be disabled). It can also be useful to calculate how much space could be freed by running OPTIMIZE TABLE (requires innodb_file_per_table to be enabled).
Conclusion:
If you're on 5.6 you can use INFORMATION_SCHEMA and for 5.5 and earlier you have to use a UDF or have (a script) to peek in the MySQL datadir.
