Mitigating Sync Amplification for Copy-on-write Virtual Disk

Copy-on-write virtual disks (e.g., qcow2 images) provide many useful features like snapshot, de-duplication, and full-disk encryption. However, our study uncovers that they introduce additional metadata for block organization and notably more disk sync operations (e.g., more than 3X for qcow2 and 4X for VMDK images). To mitigate such sync amplification, we propose three optimizations, namely per virtual disk internal journaling, dual-mode journaling, and adaptive-preallocation, which eliminate the extra sync operations while preserving those features in a consistent way. Our evaluation shows that the three optimizations result in up to 110% performance speedup for varmail and 50% for TPCC.