Techniques for performing compression operations on persistently-stored data blocks during read/write commands. A method embodiment performs in-line data compression operations over data blocks referenced by a caller. The in-line data compression operations are performed during execution of a storage input-output (I/O) command, between the event of receipt of the storage I/O command and the event of returning status of the storage I/O command. The storage I/O operation is associated with at least one data group comprising one or more data blocks that are identified by the caller. Upon receipt of the storage I/O command, one or more compression rules are applied to the data blocks to determine one or more compression parameters, which compression parameters are used to form specific compression operations that are performed over at least a portion of the data group. The status pertaining to the execution of the storage I/O operation is returned to the caller.