SWANS: An Interdisk Wear-Leveling Strategy for RAID-0 Structured SSD Arrays

NAND flash memory–based solid state disks (SSDs) have been widely used in enterprise servers. However, flash memory has limited write endurance, as a block becomes unreliable after a finite number of program/erase cycles. Existing wear-leveling techniques are essentially intradisk data distribution schemes, as they can only even wear out across the flash medium within a single SSD. When multiple SSDs are organized in an array manner in server applications, an interdisk wear-leveling technique, which can ensure a uniform wear-out distribution across SSDs, is much needed. In this article, we propose a novel SSD-array level wear-leveling strategy called SWANS (<u>S</u>moothing <u>W</u>ear <u>A</u>cross <u>N</u> <u>S</u>SDs) for an SSD array structured in a RAID-0 format, which is frequently used in server applications. SWANS dynamically monitors and balances write distributions across SSDs in an intelligent way. Further, to evaluate its effectiveness, we build an SSD array simulator on top of a validated single SSD simulator. Next, SWANS is implemented in its array controller. Comprehensive experiments with real-world traces show that SWANS decreases the standard deviation of writes across SSDs on average by 16.7x. The gap in the total bytes written between the most written SSD and the least written SSD in an 8-SSD array shrinks at least 1.3x.