fix: increase ext4 filesystem overhead from 20% to 50% #55
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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.
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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) |
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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
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Jan 11, 2026
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lgtm - makes sense for small filesystems with many files
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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.