Dong-young Seo

Recently-evicted-first buffer replacement policy for flash storage devices

Flash memory has been widely used as a storage device for consumer devices. Recently, applications using flash memory are becoming complex and diverse. One of obstacles to its wide use is the slow write performance of flash memory caused by its erase-before-write characteristic. To enhance the write performance, FTL (Flash Translation Layer) generally uses the flash log-buffer, where data is written by the out-of-place scheme. However, current log buffer-based FTL schemes show poor performance for random write requests due to the block thrashing problem and high block associativity. Recently, flash-aware buffer cache management schemes are proposed to solve the block thrashing problem. However, they cannot also eliminate the problem completely. In this paper, we propose a novel flashaware buffer cache replacement policy. The technique selects the victim page to be evicted from buffer cache considering the recent victim page sent to the flash log buffer. Our experimental results show that the proposed technique reduces the flash I/O execution time by up to 30%.

WAM: Wear wear-out-aware memory management for SCRAM-based low power mobile systems

Emerging storage class RAM (SCRAM) devices are power-efficient, byte-addressable, and non-volatile. However, each SCRAM memory cell has a limited lifetime. To use SCRAM to replace power-hungry DRAM in mobile consumer devices, wear-leveling among cells will be a critical issue. Especially, due to the locality of memory access pattern, current operating systems. SCRAM-unaware memory allocator may intensify the wear of certain SCRAM blocks. Wear-leveling requires the redesign of operating systems to distribute memory accesses evenly throughout the entire memory space. This paper suggests two operating-system-level wear-leveling techniques for SCRAM devices. First, this paper proposes a novel memory allocator, called W-Buddy, that considers the wear of memory chunks as it selects a free memory chunk to be allocated. Second, this paper proposes a variable metadata formatting technique for file systems. The experiment results show that the proposed W-Buddy allocator and metadata reformatting technique improve the lifetime of SCRAM devices compared with the techniques of conventional operating systems.

W-Buddy: Wear-out-aware memory allocator for SCRAM-based low power mobile systems

Since recently emerging storage class RAM (SCRAM) devices are power-efficient, byte-addressable, and non-volatile, they are expected to replace the power-hungry DRAM in mobile consumer devices. However, SCRAM has a limited number of program and erase cycling, requiring a wear-leveling technique. Since the locality in memory access pattern may intensify the discriminated wearing of memory blocks, it is required to change the operating systems such that memory accesses are distributed evenly over all memory space for wear-leveling. We propose a novel memory allocator, called W-Buddy, which selects a free memory chunk to be allocated considering the wear-outs of memory chunks. Our experimental results show that the proposed W-Buddy achieves fourteen time longer lifetime compared with the conventional buddy memory allocator.