Offloading raid update operations to disk controllers

An improved technique involves performing computations for partial stripe updates in a RAID at individual disk controllers rather than at the RAID controller. When a RAID controller receives a request to update old payload data at a block in a particular disk with update data, it sends the update data to the controller of that particular disk. The disk controller reads internally old data from the block, computes the difference between new and old data, replaces the old data on disk with the new data, and returns the difference to the RAID controller. The RAID controller computes difference values of the parity data from the difference values of the payload data received from the disk controllers. It then sends these difference values to the controllers of disks storing parity data. A controller of a disk storing parity data reads internally the corresponding data block, adds to it the difference value, and writes the result back to disk.