Storage Alternatives for Mobile Computers

Abstract

A B S T R A C TMobile c o m p u t e r s such as notebooks, subnotebooks, a n d p a l m t o p s require low weight, low power consumption, a n d good interactive performance. These requirements impose m a n y challenges on architectures a n d operating systems. This chapter investigates t h r e e alternative storage devices for mobile c o m p u t e r s : magnetic h a r d disks, flash m e m o r y disk emulators, a n d flash m e m o r y cards.We have used h a r d w a r e m e a s u r e m e n t s a n d trace-driven simulation t o evaluate each of t h e alternative storage devices a n d their related design strategies. H a r d w a r e measurem e n t s on an H P O m n i B o o k 300 highlight differences in t h e performance of t h e t h r e e devices as used on t h e O m n i b o o k , especially t h e poor performance of version 2.00 of t h e Microsoft Flash File System [12] when accessing large flies. T h e traces used Permission has been granted by the USENIX Association to reprint the above paper. An earlier version of this appeared in the Proceedings of the First Symposium on Operating Systems Design and Implementation, USENIX Association, November, 1994, Copyright ©USENIX Association, 1994. Unlike that version, this chapter considers an SRAM buffer cache in addition to flash memory. It also uses updated parameters for one of the devices, and measures response time differently. A detailed description of the changes appears at the end of the chapter on page 503. This work was performed in part at the Matsushita Information Technology Laboratory of Panasonic Technologies, Inc. 474CHAPTER 18 in our study came from different environments, including mobile computers (Macintosh PowerBooks) and desktop computers (running Windows or HP-UX), as well as synthetic workloads. Our simulation study shows that flash memory can reduce energy consumption and mean read response time by up to two orders of magnitude, compared to magnetic disk, while providing acceptable write performance. These energy savings can translate into a 25-115% extension of battery life. We also find that the amount of unused memory in a flash memory card has a substantial impact on energy consumption, performance, and endurance: compared to low storage utilizations (40% full), running flash memory near its capacity (95% full) can increase energy consumption by 50-165%, degrade write response time by 24%, and decrease the lifetime of the memory card by up to 70%. For flash disks, asynchronous erasure can improve write response time by a factor of 2.5.

Block Cleaning Process in Flash Memory

“…The effectiveness of the cleaning process is heavily dependent on the efficient cleaning algorithm as well as the data allocation scheme employed by the flash memory system. Moreover, the cleaning process and the block utilization level are key to the cleaning process performance and have substantial impact on the access performance, energy consumption and block endurance [1,10,14]. Block utilization is the ratio between valid cells and total cells and it is represented in percentage.…”

“…The victim block selection procedure refers to the method used to select the block to be erased and the straight forward approach is selecting a victim block that contains the largest amount of garbage. Other parameters include cost to erase, block lifespan, erasure count, and age of data [1,10,21,22]. Again, the victim block amount issue in the semi-automatic cleaning enables single or multiple victim blocks to be erased simultaneously.…”

Mobile computing with the Rover toolkit

“…The use of an alternative to a file system, a different type of file system (e.g., a log-based file system), a flash RAM card, or a small battery-backed static RAM card could offer substantial performance benefits over using an ordinary file system with a disk [21].…”

Online maintenance of very large random samples on flash storage

“…VLDB '08, August 24-30, 2008, Auckland, New Zealand Copyright 2008 VLDB Endowment, ACM 000-0-00000-000-0/00/00. tion, less noise, and faster read performance than disk drives [3,6,15]. While flash has been the primary storage media for embedded devices from the very beginning, many market experts expect that it will soon dominate the market of personal computers too.…”