A data storage system stores and retrieves information on behalf of one or more external host computers. A typical data storage system includes a network adapter, storage processing circuitry, and a set of disk drives. The network adapter provides connectivity between the external host computers and the storage processing circuitry. The storage processing circuitry performs a variety of data storage operations (e.g., load operations, store operations, read-modify-write operations, etc.) as well as provides cache memory which enables the data storage system to optimize its operations (e.g., to provide high-speed storage, data pre-fetching, etc.). The set of disk drives provides robust data storage capacity but in a slower and non-volatile manner.
The storage processing circuitry of some data storage systems includes multiple storage processing units for greater availability and/or greater data storage throughput. In such systems, each storage processing unit is individually capable of performing data storage operations.
For example, one conventional data storage system includes two storage processing units which are configured to communicate with each other through a Cache Mirroring Interface (CMI) bus in order to maintain cache coherency as well as to minimize the impact of cache mirroring disk writes. In particular, the CMI bus enables a copy of data to be available on both storage processing units before the disk write operation is complete. In this system, a first storage processing unit has a first CMI interface circuit, a second storage processing unit has a second CMI interface circuit, and the first and second CMI interface circuits connect to each other through the CMI bus.