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…
Blockdiff – How we built our own file format for VM disk snapshots launched on June 23, 2025 with 11 votes, #2,422 of 7,999 launches that month and more than 75% of that year's launches. The site was still online in September 2026, 1 year on.
In plain words
Blockdiff is a custom file format built for the otterlink VM hypervisor that enables fast disk snapshots for virtual machines. It performs block-level differential snapshots, reducing snapshot times from over 30 minutes to seconds. The format supports VM operations like forking, rollback, and suspension, and is used to power both research workloads and production systems. Blockdiff was developed by a researcher who transitioned into systems engineering to optimize VM performance and user experience.
written from the facts on this page · September 2026
From the sources
How we built blockdiff, an open-source tool for rapid block-level diffs and snapshots of VM disks.
We made it open-source here: https://github.com/CognitionAI/blockdiff 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. Devin writes and runs code in a VM environment. Why VMs instead of Docker? For untrusted user workloads we require full isolation for security purposes . Moreover, many realistic dev environments require using Docker (e.g. to spin up a backend service or database). Good luck running Docker inside Docker, so we…from cognition.ai
In the maker’s words, at launch
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.…
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