create a cache group and create a sql table using that cache group, then insert considerable rows of records to the table, monitor the disk space usage, now stop inserting records and delete records from the table using
DELETE FROM table; (not a drop table operation)
once this is done the select count(*) from table; shows the count 0 but after this when disk space is monitored it will be in the same usage level as it is in the level before rows deletion.
When I start inserting records again, the new records re-used the space occupied by the deleted records but still, it is not a good idea to unnecessarily hold disk resource.
The same can be reproduced by configuring cache expiry policy (CreatedExpiryPolicy) where after records expires it will not be visible in sql query (select * from table) but still it will hold disk resource.