If you frequently delete rows (or update rows with variable-length data types), you can end up with a lot of wasted space in your data file(s), similar to filesystem fragmentation.
If you’re not using the innodb_file_per_table option, the only thing you can do about it is export and import the database, a time-and-disk-intensive procedure.
But if you are using innodb_file_per_table, you can identify and reclaim this space!
Prior to 5.1.21, the free space counter is available from the table_comment column of information_schema.tables. Here is some SQL to identify tables with at least 100M (actually 97.65M) of free space:
SELECT table_schema, table_name, table_comment FROM information_schema.tables WHERE engine LIKE 'InnoDB' AND table_comment RLIKE 'InnoDB free: ([0-9]{6,}).*';
Starting with 5.1.21, this was moved to the data_free column (a much more appropriate place):
SELECT table_schema, table_name, data_free/1024/1024 AS data_free_MB FROM information_schema.tables WHERE engine LIKE 'InnoDB' AND data_free > 100*1024*1024;
You can reclaim the lost space by rebuilding the table. The best way to do this is using ‘alter table’ without actually changing anything:
ALTER TABLE foo ENGINE=InnoDB;
This is what MySQL does behind the scenes if you run ‘optimize table’ on an InnoDB table. It will result in a read lock, but not a full table lock. How long it takes is completely dependent on the amount of data in the table (but not the size of the data file). If you have a table with a high volume of deletes or updates, you may want to run this monthly, or even weekly.