feat(ext4): disable journal on rootfs mkfs#2565
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Add ^has_journal to the mkfs feature list. The ext4 journal commits metadata every 5 s (jbd2/commit timer) regardless of guest activity, which dirties journal blocks on a fixed cadence and inflates every snapshot diff. Removing the journal also reclaims ~32 MiB of disk space at the front of the rootfs. Snapshots are taken from a paused guest, so all in-flight writes have already drained — the journal's crash-recovery guarantee across a clean pause/resume isn't needed. A guest kernel panic between snapshots can leave the rootfs inconsistent; resume always loads the previous good snapshot, so the blast radius is bounded to one in-flight customer interaction. mkfs-time only; affects newly-built bases.
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| // between snapshots can leave the rootfs inconsistent; resume always | ||
| // loads the previous good snapshot, so blast radius is bounded to | ||
| // one in-flight customer interaction. | ||
| "^has_journal", |
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Disabling the journal causes resize2fs (used in Resize and Shrink) to fail on filesystems that are not perfectly clean. Since snapshots of running VMs are crash-consistent but not cleanly unmounted, they are marked as dirty. Without a journal for automatic recovery, resize2fs will refuse to run, breaking disk scaling for resumed sandboxes.
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Drop
has_journalfrom the rootfs mkfs feature list. The jbd2 commit timer dirties journal blocks every 5 s regardless of guest activity, inflating every snapshot diff. Also reclaims ~32 MiB at the front of the rootfs.Snapshots come from a paused guest (no in-flight writes), so the journal's crash-recovery role across pause/resume is unused.
Risk: a guest kernel panic between snapshots can leave the rootfs inconsistent — resume always loads the previous good snapshot, so blast radius is bounded to one in-flight customer interaction. Want to discuss before flipping universally — feature flag is an easy follow-up.