Yang Ou

A Scalable Multi-channel Parallel NAND Flash Memory Controller Architecture

With more data-intensive applications appearing in the present social life, the NAND flash memory acting as a replacement candidate of hard disk drives is popularly used in some data centers due to its lower power consumption, faster random access, and higher shock resistance. But the traditional solid state disk exposes the limitation of bandwidth. To this end, we deliver the scalable multi-channel flash memory controller architecture in this paper to exploit the parallelism of multiple chips. It supports all the flash operations, and boosts the performance by evenly distributing multiple accesses among different chips when using the error correction code to improve its reliability. The functions of the multi-channel parallel controller are validated according to a wide spread of workloads. And the evaluation results show that our proposed eight-channel controller outperforms the traditional controller by more than three times and can be well scaled to be 128 channels without extra critical timing path.

HIFFS: A Hybrid Index for Flash File System

Flash memory, especially NAND flash memory, has become a popular alternative for the design of storage system. Index schemes of conventional file systems do not take the flash memory characteristics into account and will cause poor performance. The current flash file systems work well only in the case of small capacity. To address the problem, we propose a new hybrid indexing scheme both in directory structure and file data index for NAND flash file system in this paper, which is called HIFFS (a Hybrid Index for Flash File System). HIFFS contains two components: hash tree directory and adaptive file data index. The hash tree directory uses a hash-based index to get better update performance and search efficiency. The adaptive file data index uses different file index strategy according to file size. The experiment results show that HIFFS outperforms the state-of-the-art file systems both in throughput.

A theoretical analysis of lifespan impact on flash memory imposed by erasure code

Each cell of flash memory only survives a nominally given number of write/erasure cycles. Beyond the nominal lifespan, flash memory can still record digital information but the bit error rate increases rapidly with the increment of write/erase cycles. Erasure code is a conventional method used to recover corrupted data, but its redundant data produce a large number of additional writes, making the erasure code seem to be unsuitable for the write-sensitive flash memory. We argue that, erasure code influences the lifespan of flash memory in two conflicting directions: its inherent error correction capability enables the flash memory to survive beyond the nominal lifespan, while its redundant data wear out the lifespan of flash memory by increasing the write/erase cycles. This paper builds a theoretical model to analyze both the two aspects and demonstrates that the erasure code is able to extend the nominal lifespan of flash memory by as many as 30×.

NIS: A New Index Scheme for Flash File System

NAND flash memory has become a popular alternative to replace disks in a storage system because of its non-volatility, high I/O performance, shock resistance, and low power consumption. However, NAND flash memory has characteristics of erase-before-write and limited cycles of write and erase operations, which introduce new challenges into the design of file system. Index schemes of conventional file systems do not consider flash memory characteristics, which results in poor performance. We propose a new index scheme both in directory structure and file data for NAND flash file system in this paper, which is called NIS. NIS has two features: hybrid extendable hash-based directory structure, HyEx Hash and adaptive file data index. The HyEx Hash integrates the static hashing that has the fixed number of hash items and dynamic hashing that can create overflow bucket at run-time to achieve better update performance and search efficiency. The adaptive file data index also uses hybrid scheme that includes traditional indirect index for large files and direct index for small files to reduce locating cost in terms of file size. To further reduce cost of locating small files, we separate direct index from indirect index and store it together with files themselves. The experimental results show that NIS reduces the index cost by 25% to 55%, and outperforms the state-of-the-art file systems both in throughput and in read/write bandwidth.

RAID-6Plus: A Fast and Reliable Coding Scheme Aided by Multi-failure Degradation

Existing triple-failure-tolerant codes assume that failures are independent and instantaneous. Such assumptions overlook the underlying mechanism of multi-failure occurrences and ignored the effect of reconstruction window. These codes are not adapted to the occurrence pattern of failure in real-world applications. As a result, the third parity drive is almost idle as it set to handle the triple-failure scenario only with lower-level failure situations unattended. Furthermore, the problem of single failure rebuild deteriorates with the increasing disk capacity, and the systems reliability will decrease with user experience impaired. Aiming at these problems, a fast reconstructable coding scheme extended from RAID-6 has been developed in this study. RAID-6Plus maintains a smaller reconstruction window by recoding the third parity drive. Existing codes provide absolute reliability for triple failures via full combinations. As a contrast, RAID-6Plus employs short combinations which are able to greatly reuse overlapped elements during reconstruction to remake the third parity drive. The short combinations shorten the reconstruction window of single failure, which avoids multi-failure overlapping in the reconstruction window. The capability of multi-failure degradation provides RAID-6Plus with 1 a better system performance comparing to RTP and STAR and 2 an enhanced reliability comparing to RAID-6.