Introduction of metadata-request queue with immediate response for I/O path optimizations on iSCSI-based storage subsystem

Software delays have become the bottleneck to the overall storage system. Particularly, in iSCSI-based storage system, more disks in the target-end system are configured to make up a target node, but requests initiated by the initiator-end system will go through the traditional single I/O path before they are re-requested by the target-end system, which cannot exploit the serviceability of the target multi-disks adequately. This paper introduces the metadata-request queue to handle metadata I/O requests and scheduling differs from the request queue that exists in the initiator-end system of iSCSI-based storage system, given the different characteristics of metadata and data I/O requests. Instead of dispatching all requests exclusively to the single queue and unified scheduling, a new metadata-request queue is introduced for every logical device and is implemented with immediate response mode to eliminate waiting delays, considering the characteristics of small size, high frequency and scattered accesses that require for real-time response. Data requests in the data-request queue are delivered to the scheduling queue in an I/O waiting state, which is known as plugged-unplugged mechanism. Evaluation of our system using Postmarks and IOMeter shows 9%–19% I/O performance improvement.