EGS: An Effective Global I/O Scheduler to Improve the Load Balancing of SSD-Based RAID-5 Arrays

摘要

With the expansion in data volumes, disk arrays require high speed and large volumes to adapt the cloud computing environment. Solid State Drives (SSDs) now have the trend to substitute the traditional external storage because of their better performance than Hard Disk Drives (HDDs), so they are proper to be utilized in disk arrays. However, traditional I/O scheduler in RAID systems use uniform I/O distribution to balance the workload, which is not suitable for SSD-based RAID. It is because that typical SSD has asymmetric performance on read/write operations, which can result in unbalanced I/O accesses in an SSD array. Furthermore, an even I/O distribution can have an uneven write distribution, which leads to unbalanced lifetime of various SSDs and affects the lifetime of SSD arrays significantly. Obviously, this problem cannot be well addressed through internal wear leveling mechanisms in SSDs. To address the above problems, in this paper, we propose an effective global I/O scheduler called “EGS”, which can optimize the I/O performance and prolong the lifetime of SSD-based RAID-5 arrays. According to different access time of read/write operations, we separate the read and write operations to give different weights, and make each disk has a same amount of window (consists of several read/write I/Os) simultaneously. On the other hand, to avoid unbalanced write distribution, we design an additional policy to prolong SSDs' lifetime. To demonstrate the effectiveness of EGS, we conduct several simulations via Disksim. The results show that, compared to traditional I/O scheduling algorithms, EGS saves average response time by up to 40%, and increases the lifetime of SSD array by approximately 45%.