HybRAID: A High-Performance Hybrid RAID Storage Architecture for Write-Intensive Applications in All-Flash Storage Systems

With the ever-increasing demand for higher I/O performance and reliability in data-intensive applications, solid-state drives (SSDs) typically configured as redundant array of independent disks (RAID) are broadly used in enterprise all-flash storage systems. While a mirrored RAID offers higher performance in random access workloads, parity-based RAIDs (e.g., RAID5) provide higher performance in sequential accesses with less cost overhead. Previous studies try to address the poor performance of parity-based RAIDs in small writes (i.e., writes into a single disk) by offering various schemes, including caching or logging small writes. However, such techniques impose a significant performance and/or reliability overheads and are seldom used in the industry. In addition, our empirical analysis shows that partial stripe writes, i.e., writing into a fraction of a full array in parity-based RAIDs, can significantly degrade the I/O performance, which has not been addressed in the previous work. In this paper, we first offer an empirical study which reveals partial stripe writes reduce the performance of parity-based RAIDs by up to 6.85x compared to full stripe writes (i.e., writes into entire disks). Then, we propose a high-performance hybrid RAID storage architecture, called HybRAID, which is optimized for write-intensive applications. HybRAID exploits the advantages of mirror- and parity-based RAIDs to improve the write performance. HybRAID directs a) aligned full stripe writes to parity-based RAID tier and b) small/partial stripe writes to the RAID1 tier. We propose an online migration scheme, which aims to move small/partial writes from parity-based RAID to RAID1, based on access frequency of updates. As a complement, we further offer offline migration, whose aim is to make room in the fast tier for future references. Experimental results over enterprise SSDs show that HybRAID improves the performance of write-intensive applications by 3.3x and 2.6x, as well as enhancing performance per cost by 3.1x and 3.0x compared to parity-based RAID and RAID10, respectively, at equivalent costs.