reFresh SSDs: Enabling High Endurance, Low Cost Flash in Datacenters

Storage performance and power are critical issues in modern datacenters. Solid State Drives (SSDs) offer both performance and power advantages over hard disk drives. With the advent of MLC flash, the cost-per-Gigabyte of Flash has dropped significantly enough to make it attractive for use in largescale storage in datacenters. However, flash suffers from limited endurance and wears away after a certain number of writes and erases, thereby entailing higher replacement and service costs and hence an increase in the overall server infrastructure costs. In this paper, we develop a physically-accurate model of flash memory reliability and show that there exists a tradeoff between endurance and retention time. We then quantify this tradeoff and show how to exploit it without causing data integrity problems by refreshing the data in the flash memory cells. We propose and evaluate an Earliest-Deadline First (EDF) based refresh policy that can increase the endurance limit by 6-56% for an MLC-based SSD.