Storage Alternatives for Mobile Computers

Mobile computers such as notebooks, subnotebooks, and palmtops require low weight, low power consumption, and good interactive performance. These requirements impose many challenges on architectures and operating systems. This chapter investigates three alternative storage devices for mobile computers: magnetic hard disks, flash memory disk emulators, and flash memory cards.We have used hardware measurements and trace-driven simulation to evaluate each of the alternative storage devices and their related design strategies. Hardware measurements on an HP OmniBook 300 highlight differences in the performance of the three devices as used on the Omnibook, especially the poor performance of version 2.00 of the Microsoft Flash File System [12] when accessing large files. The traces used 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.