Cognition details blockdiff, an open-source file format for instant VM disk snapshots

Cognition built and open-sourced blockdiff, a file format that creates block-level diffs of VM disks using only filesystem metadata.

The company reports its otterlink hypervisor cut snapshot times from 30 to 60 minutes on EC2 to about 5 to 10 seconds for a 128 GB disk with a 5 GB diff, roughly a 200x speedup.

The post also covers the sparse file and copy-on-write concepts behind the approach and why OverlayFS, ZFS, and qcow2 were not chosen.