# Cognition — Blockdiff: How we built our own file format for VM disk snapshots

- Company: Cognition (cognition.com)
- Announced: 2025-06-23
- Category: not stated
- Coverage: not counted
- Announcement: no
- Group: routine
- Source: https://cognition.com/blog/blockdiff
- Record: https://forck.live/items/7699-blockdiff-how-we-built-our-own-file-format-for-vm-disk-snapshots
- Subject: Devin / SWE

Cognition open-sourced Blockdiff, a file format for instant block-level diffs of VM disks, built for their VM hypervisor Otterlink. It enables fast snapshot creation (a couple of seconds) compared to EC2's 30+ minutes.

## Evidence

Verbatim from https://cognition.com/blog/blockdiff:

> We built our own file format blockdiff for instant block-level diffs of VM disks. ... we’re open-sourcing our implementation today.

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Record: https://forck.live/items/7699-blockdiff-how-we-built-our-own-file-format-for-vm-disk-snapshots
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