Effects of Recursive Update in Copy-on-Write File Systems: A BTRFS Case Study

The copy-on-write update policy is a powerful technique for data protection. Unfortunately, it introduces a recursive update problem, which causes several side effects to a storage system, such as WRITE amplification and performance degradation. This paper elaborates on how these effects are introduced by recursive update and how serious they are. In order to evaluate these effects, an extended BTRFS (the Linux B-tree Filesystem) prototype was developed to implement the update-in-place update policy for comparison. This paper reports that recursive update can lead to 29.5x WRITE amplification and 71% performance degradation in a single WRITE operation, as well as 18.3x WRITE amplification and 33% performance degradation in an e-mail server workload. These results indicate that taking recursive update into consideration is important in developing high performance and reliable file and storage systems.