WB-RAIS: White-Box Redundant Array of Independent SSDs

Abstract

Solid-state drives (SSDs) have been widely deployed in many platforms including consumer electronics, desktops and enterprise data centers due to their high performance and low power consumption. However, SSDs suffer from bit errors, and the bit error rate is time dependent since it increases as an SSD wears down. Traditional storage systems mainly use parity-based RAID to provide reliability guarantees by striping redundancy across multiple devices, but the effectiveness of traditional RAID schemes in SSDs remains debatable. Existing solutions are based on the block device SSDs, which used as faster HDD. But the characteristics of NAND flash memory and HDD are very different, Non-write-in place and background Garbage-collection have a negative impact on the real-time performance and reliability of the system. And almost all of the existing solutions are built on Black-Box SSD, which we cannot know the internal detail and data layout. As a result, they cannot take full advantage of flash memory features, reduce the parity updates costs and improve the system reliability. In this work, we proposed a new RAID system built on White-Box SSD, we call it WB-RAIS. For White-Box SSD, the FTL implement on the host side and it could be co-designed with the RAID controller. We exploit the characteristics of flash memory and build the strip mapping based on the physical block address. We also proposed a partial strip mechanism and smooth equipment replacement method. Experimental results show that WB-RAIS improves the write performance by an average of 20% and reduces the flash erase count 17–52% compared to Black-Box SSD RAID, and enhances the system reliability significantly.