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

Hacker News

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

Visit product View on Hacker NewsJun 23, 2025

Share card

Actual performance

11points

1comments

Made the leaderboard

Launch Intel predictions

Analyze your own launch →

Product HuntOn track for Day 1 leaderboard · Strong signals: agents, agent, user · Missing: mac, macos, cursor

81%81% predicted probability of success on Product Hunt, based on ML models trained on real launch data.

best fitHighest predicted score across all platforms for this description.

Indie HackersFits the IH revenue-focused audience · Missing: supports, reddit linkedin, podcasting

74%74% predicted probability of success on Indie Hackers, based on ML models trained on real launch data.

Hacker NewsStrong engagement from HN community · Strong signals: exist, existing, io · Missing: https docs, excited, just released

69%69% predicted probability of success on Hacker News, based on ML models trained on real launch data.

nativeThis product was originally launched on this platform.

AppSumoMay struggle as an AppSumo deal · Strong signals: users · Missing: plus, platform, intuitive

33%33% predicted probability of success on AppSumo, based on ML models trained on real launch data.

TrustMRRLess likely to generate early MRR · Strong signals: users · Missing: mobile apps, ios, personal

29%29% predicted probability of success on TrustMRR, based on ML models trained on real launch data.

Acquire.comPre-revenue stage for this audience · Missing: arr, mrr, revenue

14%14% predicted probability of success on Acquire.com, based on ML models trained on real launch data.

BetaListMay not resonate with beta-testers · Missing: web3, chat, crypto

0%0% predicted probability of success on BetaList, based on ML models trained on real launch data.

Correct prediction on native model

Similar products

Labfile, a declarative file format for orchestrating experiments46%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

Labfile, a declarative file format for orchestrating experiments

Hacker News3

Hessellate – Hyperbolic Tessellations on the Poincaré Disk60%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

Hessellate – Hyperbolic Tessellations on the Poincaré Disk

Hacker News2

Budding new file format44%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

Budding new file format

Hacker News8

I made Confetti: a configuration language file format49%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

I made Confetti: a configuration language file format

Hacker News67

Backup/restore tool for CouchDb with rsnapshot suited disk format33%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

Backup/restore tool for CouchDb with rsnapshot suited disk format

Hacker News2

SAN syntax for Vim – a Sane and Nice configuration format59%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

SAN syntax for Vim – a Sane and Nice configuration format

Hacker News1

The CFG Configuration Format51%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

The CFG Configuration Format

Hacker News1

Clang_format_all – Configure clang-format with YAML49%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

Clang_format_all – Configure clang-format with YAML

Hacker News1

Ragzip format specification (and Java implementation)53%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

Ragzip format specification (and Java implementation)

Hacker News4

DiskBOX48%Launch Intel prediction score: how likely this product is to succeed on its source platform, based on its name, tagline, and description.

See what’s filling your disk — and take control

Indie Hackerscommitment-side-project

Browse all products Analyze your launch