Show HN: Blockdiff – How we built our own file format for VM disk snapshots

Usually, I’m a researcher working on areas like RL for coding agents – but one day I became annoyed by our slow VM startup times. So I took the plunge into systems engineering and built the first version of our VM hypervisor called otterlink. It now powers both our research & all of Devin production workloads.

Compared to EC2, otterlink was able to bring down VM startup times by about 10x. The real pain point, however, were EC2’s long snapshot times. We want a lot of flexibility (e.g. forking, rollback and suspending VMs) that all require taking disk snapshots. On EC2 taking disk snapshots usually took a whopping 30+ minutes, which would be a terrible experience for our users. With otterlink we were able to bring this down to just a couple of seconds – a 200x speed-up.

To achieve this, we built our own file format blockdiff for instant block-level diffs of VM disks. Creating block-level diffs of two files is a much broader problem that goes beyond VM disks. We assumed there must be an existing open-source solution. To our surprise we couldn’t find such a tool, so we’re open-sourcing our implementation today: https://github.com/CognitionAI/blockdiff