Apparatus and method for analyzing and modifying data transfer reguests in a raid system

DETAILED DESCRIPTION The invention provides a method and apparatus for increasing the efficiency of data transfer between a host computer and a disk array in a RAID system.
The invention operates by splitting up large I/O requests from the computer into smaller, more manageable pieces and processing the pieces as though they were individual I/O requests.
In one embodiment, the invention keeps only a limited number of these smaller individual I/O requests "active" at any particular time so that a single large I/O request cannot preclude other I/O requests from making progress in the controller.
Both the size of the smaller I/O request pieces and the limited number of these pieces which will be "active" at any one time may be tunable parameters.
The invention improves the efficiency of data transfer between the host computer and the array of disk drives by allowing for increased overlap of activity in the controller.
In other words, controller operations such as: (1) data transfers between the host and the buffer, (2) buffer setup in the controller, and (3) data transfers between the buffer and the drives can take place, to an increased extent, at the same time.
This increased overlap of activity results in increased controller throughput.
In one aspect of the present invention, a data storage system is provided comprising: (a) an array of disk drives; (b) a host computer capable of issuing I/O requests for read and write operations to be performed using the array of disk drives, the read and write operations requiring a transfer of data between the host computer and the array of disk drives; (c) means for coordinating the transfer of data between the host computer and the array of disk drives in response to the I/O requests from the host computer; and (d) means for determining an amount of data required to be transferred between the host computer and the array of disk drives for each I/O request issued by the host computer, comparing the amount of data to a predetermined amount, dividing each I/O request into a plurality of separate block requests whenever the amount of data exceeds the predetermined amount, each block request requiring a transfer of no more than the predetermined amount of data, and issuing each of the plurality of separate block requests to the means for coordinating as an individual I/O request