Enhancing shared RAID performance through online profiling

Enterprise storage systems are generally shared by multiple servers in a SAN environment. Our experiments as well as industry reports have shown that disk arrays show poor performance when multiple servers share one RAID due to resource contention as well as frequent disk head movements. We have studied IO performance characteristics of several shared storage settings of practical business operations. To avoid the IO contention, we propose a new dynamic data relocation technique on shared RAID storages, referred to as DROP, Dynamic data Relocation to Optimize Performance. DROP allocates/manages a group of cache data areas and relocates/drops the portion of hot data at a predefined sub array that is a physical partition on the top of the entire shared array. By analyzing profiling data to make each cache area owned by one server, we are able to determine optimal data relocation and partition of disks in the RAID to maximize large sequential block accesses on individual disks and at the same time maximize parallel accesses across disks in the array. As a result, DROP minimizes disk head movements in the array at run time giving rise to high IO performance. A prototype DROP has been implemented as a software module at the storage target controller. Extensive experiments have been carried out using real world IO workloads to evaluate the performance of the DROP implementation. Experimental results have shown that DROP improves shared IO performance greatly. The performance improvements in terms of average IO response time range from 20% to a factor 2.5 at no additional hardware cost.