apple/containerization

[Bug]: Support a partial last block group to avoid expanding the EXT4 filesystem size

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#647 opened on Apr 3, 2026

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help wanted

Description

I have done the following

  • I have searched the existing issues
  • If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

Create an EXT4 filesystem with 160 MiB:

container volume create -s 160M size-test
stat -f%z ~/Library/Application\ Support/com.apple.container/volumes/size-test/volume.img

Current behavior

When creating an EXT4 filesystem, the requested size is expanded to the nearest block group boundary. A filesystem whose requested size falls between two block group boundaries ends up larger than requested - 256 MiB:

268435456

Expected behavior

The fix is to support a partial last block group so the filesystem uses exactly the requested size - 160 MiB:

167772160

Environment

N/A

Relevant log output

N/A

Code of Conduct

  • I agree to follow this project's Code of Conduct

Related to https://github.com/apple/containerization/pull/640.

Contributor guide

Tech stack
swift
Domain
backendinfrastructure
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
EXT4 filesystem knowledgeSwift programming
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The issue states that creating an EXT4 filesystem expands the size to the nearest block group boundary. The fix is to support a partial last block group. The related PR at https://github.com/apple/containerization/pull/640 may contain initial work or discussion. Examine the Swift code that creates EXT4 filesystems, likely in a file related to volume creation. Focus on the logic that calculates block group boundaries and ensure the final block group can be partial to match the exact requested size.