Method for fast queue restart after redundant i/o path failover

Abstract

A redundant storage control module (also referred to as RDAC or multi-active controller) maintains a queue of pending I/O requests sent for processing via a first asynchronously operating I/O path. In the event of failure of the first asynchronously operating I/O path, the controller restarts the entire queue of pending I/O requests to a second I/O path without waiting for each request to individually fail from the first path. Some prior techniques required the RDAC module to await failure of each I/O request sent to the failed first I/O path before restarting each failed request on the secondary I/O path. Such techniques greatly extend the total time required to restart all operations sent to a failed I/O path, by awaiting the failure of all I/O requests previously sent to the first I/O path. Other known techniques provide non-standard features in the lower level driver modules to permit the higher level RDAC modules to directly manipulate dispatch queues maintained for each I/O path within the low level device drivers. Such prior techniques reduce the wait time for restart of failed I/O requests but at the expense of portability of the RDAC modules. The methods and structure of the present invention reduce the wait time for restart of failed I/O requests but in such a manner as to remain portable over a variety of host systems.