Separating the File System Journal to Reduce Write Amplification of Garbage Collection on ZNS SSDs

Abstract

Solid-State Drives (SSDs), despite their outstanding I/O performance, suffer from the garbage collection overhead incurred by out-place update scheme that addresses the erase-before-write constraint of NAND flash memory. The Zoned Namespace SSD (ZNS SSD) is a state-of-the-art storage technology that divides NAND flash memory space within an SSD into zones and allows the host to directly assign certain zone for a write request. Therefore, the ZNS SSD can minimize the garbage collection overhead by allocating data with similar update patterns to the same zone. However, legacy journaling file systems still cannot actively exploit the advantages of ZNS SSDs, and journal data and file data with obviously different update patterns are written together in the same zone. In this paper, we propose new file system journal placement scheme that separates journal data into a dedicated zone to minimize write amplification due to garbage collection, noting the characteristics of journal data in the Ext4 file system. The proposed scheme is implemented in Linux kernel and carefully evaluated. The evaluation results show that our scheme removes the unnecessary copying of journal data during garbage collection of ZNS SSD and reduces the valid page copies caused by garbage collection by up to 26.8%.