Yimo Du

CD-RAIS: Constrained dynamic striping in redundant array of independent SSDs

Solid state drives (SSDs) are increasingly deployed to construct storage arrays (RAIDs) in enterprise environments. The design decisions in RAID are traditionally devised for HDD RAIDs, which cannot fully exploit the characteristics of SSDs. In particular, SSD lacks the ability to update pages in-place. Random writes in traditional parity-based RAIS (SSD RAID) systems that has static striping result in significantly more writes, degraded performance, and shortened SSD lifetime. By dynamically forming full stripes, log-based design originally proposed in HDD RAID can mitigate the write-hole problem caused by random writes. However, it needs a directory to record locations for all data blocks, resulting in large space overhead and consequently sacrificing addressing efficiency. In this paper, we propose CD-RAIS, a compromise between static striping and dynamic striping. CD-RAIS groups requests that are from different SSD drives and places their corresponding unnecessarily consecutive logical blocks in one stripe. It mitigates the write-hole problem, meanwhile remains the same addressing efficiency as static striping. To enable dynamic data striping fit SSDs, CD-RAIS performs lazy data invalidation and consolidates updates to parity blocks. CD-RAIS greatly alleviates the write request increase due to parity block update in RAIS. Our experimental results show that, for random write dominated workloads, CD-RAIS achieves 65% response time improvement and 31% longer SSD lifespan over traditional RAIS schemes.

R-Dedup: Content Aware Redundancy Management for SSD-Based RAID Systems

While high density SSDs are increasingly adopted in enterprise computing environment, it remains a challenge to meet the high performance and reliability demands of server applications as well as the demands for longer system lifetime and high space utilization in such environment. Existing schemes often address these issues separately. In particular, deduplication schemes improve write performance and SSD lifetime while SSDbased RAID designs improve reliability and read performance. Naively integration of deduplication and RAID results in suboptimal designs. In this paper, we propose R-Dedup, a content-aware redundancy management scheme for SSD-based RAID storage. By combining deduplication with replication, R-Dedup evaluates system performance, reliability, endurance and space utilization, and dynamically manages replicas to achieve better trade off. Our experimental results show that R-Dedup achieves 18% and 20% improvements on read and write performance, respectively, and extends SSD lifetime by 20% with no reliability compromise.

PBFTL: The Page to Block Mapping FTL with Low Response Time

NAND flash has some inherent peculiarities which increase the access delay seriously. We propose the Page to Block mapping Flash Translation Layer (PBFTL). Solid State Drives (SSDs) adopting PBFTL have lower response time. To achieve low response time for read requests, PBFTL adopts hybrid-level mapping scheme. But, hybrid-level FTL behaves awkwardly for write due to the high overhead of garbage collection. PBFTL takes two measures to optimize garbage collection. The first is to direct hot and cold data to separate blocks, which mitigates write amplification significantly. The second is to reduce the latency of reclaiming a block, which enables PBFTL to spend less time on garbage collection. Users requests are unlikely to be congested for a long time. Trace-driven simulations show that, PBFTL achieves low response for both read- and write-intensive workloads.

RAID-Aware SSD: Improving the Write Performance and Lifespan of SSD in SSD-Based RAID-5 System

Flash memory-based SSD RAID has an excellent I/O performance with high stability, which making it get more and more attention from companies and manufacturers, especially in I/O-intensive environments. However, frequently updating parity also makes the SSD have a higher overhead in the process of garbage collection. To this end, we propose RAID-Aware SSD (RA-SSD) that could distinguish user data from parity by detecting the different access patterns from the upper RAID layer, and store them separately at different flash blocks. RA-SSD could effectively reduce the overhead of garbage collection. Simulation results show that, being deployed in a RAID-5 System, RA-SSD could reduce the number of pages copied in the process of garbage collection by up to 10%. As the overhead of garbage collection decreases, the write performance and lifespan will be improved. The extra space consumed by RA-SSD is very small, it is only about 1/10000 of the capacity of the device. Moreover, the processing logic of RA-SSD is so simple that it has very little impact on read performance.

CSWL: Cross-SSD Wear-Leveling Method in SSD-Based RAID Systems for System Endurance and Performance

Flash memory has limited erasure/program cycles. Hence, to meet their advertised capacity all the time, flash-based solid state drives (SSDs) must prolong their life span through a wear-leveling mechanism. As a very important part of flash translation layer (FTL), wear leveling is usually implemented in SSD controllers, which is called internal wear leveling. However, there is no wear leveling among SSDs in SSD-based redundant array of independent disks (RAIDs) systems, making some SSDs wear out faster than others. Once an SSD fails, reconstruction must be triggered immediately, but the cost of this process is so high that both system reliability and availability are affected seriously. We therefore propose cross-SSD wear leveling (CSWL) to enhance the endurance of entire SSD-based RAID systems. Under the workload of random access pattern, parity stripes suffer from much more updates because updating to a data stripe will cause the modification of other all related parity stripes. Based on this principle, we introduce an age-driven parity distribution scheme to guarantee wear leveling among flash SSDs and thereby prolong the endurance of RAID systems. Furthermore, age-driven parity distribution benefits performance by maintaining better load balance. With insignificant overhead, CSWL can significantly improve both the life span and performance of SSD-based RAID.

2F: A Special Cache for Mapping Table of Page-Level Flash Translation Layer

The development of flash memory drives flash based SSDs to enter into enterprise-scale storage systems. As the kernel of SSD, flash translation layer (FTL) attracts many attentions. Generally, there are two types of FTLs according to the granularity of address mapping: block-level and page-level mapping FTLs. We focus on the latter one. Typically, page-level mapping scheme must employ a cache to alleviate the memory pressure introduced by the big mapping table. We argue that classic cache replacement policies aren’t competent for the page table cache of FTLs. The major contribution of this work is to design a dedicated cache replacement policy called Two Filters (abbreviated as 2F) for page-level mapping FTLs. 2F aims at two goals. The first is higher hit ratio as all the replacement policies pursue. As 2F not only protects frequently accessed pages, but also protects sequentially accessed pages at little cost, it does achieve a higher hit ratio. The second goal is to distinguish hot pages from the cold. This goal is special for page table of FTLs. If hot and cold pages are directed to separate blocks, garbage collection will be more efficient. In order to achieve this goal, 2F employs two filters. One is used for containing sequentially accessed pages. Another is used for selecting hot pages. Trace driven simulations present that 2F outperforms classic replacement policies in both hit ratio and data classification.

Hot Data-Aware FTL Based on Page-Level Address Mapping

The development of flash memory drives flash based SSD to enter into large-scale storage systems. The performance of SSD is highly dependent on the design of FTL. For the last few years, several FTL schemes have been proposed. Such as FAST, BAST, SAST etc. we design a novel FTL based on page-level mapping scheme. Since one of the major troubles of page-level mapping FTL is the unendurable memory consuming of the fine-grained mapping table. We propose a dedicated cache replacement policy called SRC to mitigate the memory pressure. Our FTL based on SRC is able to distinguish hot data from the cold. This capability highlights the garbage collection efficiency of page-level mapping schemes. As a result, the hot data-aware FTL reduces extra read/write operations by 10 times or more compared with FAST and BAST. As our FTL erases less blocks, the lifetime of SSD is extended by more than 30%. Further experiment shows that the hot data-aware FTL outperforms hybrid-level FTLs on workloads with varied read/write ratios.

Dual-Page Mode: Exploring Parallelism in MLC Flash SSDs

MLC flash memory SSDs have gained much popularity in storage subsystems from mobile devices to data centers because of their high performance and low power consumption. The naturally rich internal parallelism of SSDs is one of the most important factors that contribute to the high performance. Many studies make use of the parallelism from various perspectives such as scheduler, SSD organization and data layout etc. To promote the performance. However, internal parallelism of SSDs has been limited to plane level so far. This paper extends the parallelism beyond plain level, exploring the parallelism among pages within a block. We propose dual-page mode that can access two pages simultaneously. Experimental results demonstrate that SSDs with dual-page mode can reduce the average response time by up to 38% compared with contemporary SSDs.

Erasure codes in big data era

Data continues to explode exponentially in big data era, making the 3-replication scheme unaffordably expensive and space-consuming for a storage system. To this end, erasure code schemes prevail by offering the same or higher reliability at a relatively lower spatial cost. In this paper, an overview of up-to-date research work of erasure codes in big data era is presented. Additionally, we identify some problems of recent work and attempt to share our insights about possible future work of erasure codes.