While adopting cache in software RAID brings performance benefit, it can cause data loss at power failure which results in the broken filesystem consistency. Though I/O Barrier can be used to remove the consistency issue, it sacrifices the write performance of software RAID. To mitigate this tradeoff, we suggest R-Barrier for software RAID. The basic idea of R-Barrier is to avoid the synchronization of the entire cache space. Instead, based on given hints from the filesystem layer, R-Barrier makes software RAID cache select one of the following methods when the cache writes a certain write request into the disk: (1)Strictly ordered destaging for the write requests affecting the filesystem consistency, (2)Reordered destaging for the rest of write requests. We show that R-Barrier guarantees same filesystem consistency with I/O Barrier while the performance degradation of R-Barrier is less than that of I/O Barrier.

Traditional studies on disk arrays have focused on parallelism or load balance of disks. However, this paper reveals that the independency of disks is more important than parallelism for concurrent reads of large numbers of processes in striped disk arrays, whereas parallelism is significant only for concurrent reads of small numbers of processes. We investigate and evaluate how much aligning sequential prefetch in strip or stripe boundaries affects the I/O performance by comparing with the proposed two types of sequential prefetching schemes, both of which are implemented in Linux kernel 2.6.18. In the experiments in this study, the combination of our schemes outperforms the original sequential prefetching of Linux by 3.2 times for 128 clients and 2.4 times for a single sequential read.

This paper presents a hybrid flash file system (HFFS) based on both NOR flash and NAND flash memory. In a conventional NAND flash-based flash file system, there is a trade-off between life span and durability in the frequent writing of small amounts of data. Because NAND flash supports only a page-level I/O, at least one page is wasted in the synchronous writing of small amounts of data. The wasting of pages reduces the utilization and life span of the NAND flash. To alleviate the utilization problem, some NAND flash-based flash file systems write small amounts of data asynchronously with RAM buffers, though buffering in RAM decreases the durability of the system. Our HFFS eliminates the trade-off between life span and durability. It synchronously stores data as a log in the NOR flash, whenever we append small amounts of data to a file. The merged logs are then flushed to the NAND flash in a page-aligned fashion. The implementation of our HFFS is based on our previous NAND flash-based file system, called CFFS, The experimental results reveal that our HFFS provides a longer life span than a conventional NAND flash-based synchronous flash file system with a similar level of durability.

Given that contiguous reads and writes between a cache and a disk outperform fragmented reads and writes, fragmented reads and writes are forcefully transformed into contiguous reads and writes via a proposed matrix-stripe-cache-based contiguity transform (MSC-CT) method which employs a rule of consistency for data integrity at the block level and a rule of performance that ensures no performance degradation. MSC-CT performs for reads and writes, both of which are produced by write requests from a host as a write request from a host employs reads for parity update and writes to disks in a redundant array of independent disks (RAID)-5. MSC-CT is compatible with existing disk technologies. The proposed implementation in a Linux kernel delivers a peak throughput that is 3.2 times higher than a case without MSC-CT on representative workloads. The results demonstrate that MSC-CT is extremely simple to implement, has low overhead, and is ideally suited for RAID controllers not only for random writes but also for sequential writes in various realistic scenarios.

Redundant arrays of inexpensive disks (RAID) offer fault tolerance against disk failures. However a storage system having more disks suffers from less reliability and performance. A RAID architecture tolerating multiple disk failures shows severe performance degradation in comparison to the RAID Level 5 due to the complexity of implementation. We present a new RAID architecture that tolerates at least three disk failures and offers similar throughout to the RAID Level 5. We call it the hierarchical RAID, which is hierarchically composed of RAID Levels. Furthermore, we formally introduce the mean-time-to-data-loss (MTTDL) of traditional RAID and the hierarchical RAID using Markov process for detailed comparison.