Recursive Updates in Copy-on-write File Systems - Modeling and Analysis

Abstract

Copy-On-Write (COW) is a powerful technique for data protection in file systems. Unfortunately, it introduces a recursively updating problem, which leads to a side effect of write amplification. Studying the behaviors of write amplification is important for designing, choosing and optimizing the next generation file systems. However, there are many difficulties for evaluation due to the complexity of file systems. To solve this problem, we proposed a typical COW file system model based on BTRFS, verified its correctness through carefully designed experiments. By analyzing this model, we found that write amplification is greatly affected by the distributions of files being accessed, which varies from 1.1x to 4.2x. We further found that write amplification is also affected by the number of files being accessed, the number of files contained in a file system, and as well as the space utilization of file system trees.

COWCache: effective flash caching for Copy-on-Write virtual disks

“…COW-based virtual disks provide rich features (e.g., fast snapshot, thin provisioning) that enhance the flexibility of virtualization [19,27,30], but their complex operations also introduce performance overhead to the VM storage. This overhead mainly comes from two sources: (1) metadata management for maintaining the metadata of COWbased virtual disks (e.g., lookup and update); and (2) disk I/O amplification caused by the Copy-on-Write of data [6,22], i.e., the COW penalties. Our study uncovers that the amount of I/O requests issued from the guest VM can be amplified by COW-based virtual disks to 29*139 to the VM's backing storage (Sect.…”