Abstract: A method, apparatus and computer program product for storing data in a disk storage system is presented. A high-performance dictionary data structure is defined. The dictionary data structure is stored on a disk storage system. Key-value pairs can be inserted and deleted into the dictionary data structure. Updates run faster than one insertion per disk-head movement. The structure can also be stored on any system with two or more levels of memory. The dictionary is high performance and supports with full transactional semantics, concurrent access from multiple transactions, and logging and recovery. Keys can be looked up with only a logarithmic number of transfers, even for keys that have been recently inserted or deleted. Queries can be performed on ranges of key-value pairs, including recently inserted or deleted pairs, at a constant fraction of the bandwidth of the disk.