fix: increase ext4 filesystem overhead from 20% to 50% #55
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
The 20% overhead was insufficient for small filesystems with many files (like images with timezone data). ext4 requires significant space for superblock, group descriptors, inode tables, and directory entries. This caused mkfs.ext4 to fail with "Could not allocate block in ext2 filesystem" when converting images to disk format. Tested with Alpine + tzdata image which previously failed.
![tembo[bot]](https://avatars.githubusercontent.com/in/1223475?s=60&v=4){kind=small}
There was a problem hiding this comment.
Makes sense to bump overhead here given mkfs.ext4 failures on small roots with lots of files. One thought: if this ever needs tuning again, having a named constant (or config knob) for the overhead ratio would make it easier to adjust without re-auditing the comment/math everywhere.
| // Add 50% overhead for filesystem metadata, minimum 10MB | ||
| // ext4 needs significant overhead for superblock, group descriptors, inode tables, etc. | ||
| // 20% was insufficient for small filesystems with many files (like tzdata) | ||
| diskSizeBytes := sizeBytes + (sizeBytes / 2) |
There was a problem hiding this comment.
Nit: to keep the comment and math from drifting, consider pulling the ratio into a named constant.
Suggested change
| diskSizeBytes := sizeBytes + (sizeBytes / 2) | |
| const ext4MetadataOverheadPercent int64 = 50 | |
| diskSizeBytes := sizeBytes + (sizeBytes*ext4MetadataOverheadPercent)/100 |
rgarcia
approved these changes
Jan 11, 2026
Contributor
rgarcia
left a comment
rgarcia
left a comment
There was a problem hiding this comment.
lgtm - makes sense for small filesystems with many files
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Summary
mkfs.ext4: Could not allocate block in ext2 filesystemerror for images with many small filesProblem
When building and running images with many small files (like Alpine + tzdata), the image conversion to ext4 was failing:
The 20% overhead was insufficient for ext4 metadata (superblock, group descriptors, inode tables, directory entries), especially for small filesystems with many files.
Solution
Increased the overhead from 20% (
sizeBytes / 5) to 50% (sizeBytes / 2).Test plan
hypeman buildNote
Adjusts ext4 image sizing to avoid mkfs failures on rootfs with many small files.
convertToExt4, increase overhead fromsizeBytes/5tosizeBytes/2(retain 10MB minimum) when computingdiskSizeBytesWritten by Cursor Bugbot for commit 5493085. This will update automatically on new commits. Configure here.