Storage Gardening: Using a Virtualization Layer for Efficient Defragmentation in the WAFL File System

Ram Kesavan, Matthew Curtis-Maury, Vinay Devadas, and Kesari Mishra, NetApp

7th USENIX Conference on File and Storage Technologies (FAST)
FEBRUARY 25–28, 2019

As a file system ages, it can experience multiple forms of fragmentation. Fragmentation of the free space in the file system can lower write performance and subsequent read performance. Client operations as well as internal operations, such as deduplication, can fragment the layout of an individual file, which also impacts file read performance. File systems that allow sub-block granular addressing can gather intra-block fragmentation, which leads to wasted free space. This paper describes how the NetApp® WAFL® file system leverages a storage virtualization layer for defragmentation techniques that physically relocate blocks efficiently, including those in read-only snapshots. The paper analyzes the effectiveness of these techniques at reducing fragmentation and improving overall performance across various storage media.