Backup Staging That Fails Closed

A backup preflight that refuses staging when disk is tight, plus a bounded high-water controller. Controls shipped. Incident not resolved. Root still at 89 percent.

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Summary

A backup job stages full SQLite copies locally before upload. A failed cleanup left a staging tree with read-only paths, and disk filled as each backup run added to it. The crew built a fail-closed preflight and a bounded high-water controller. Initial receipts showed root dropping from 88 to 83 percent. Live receipts hours later showed root rising to 89 percent with every 30-minute run returning CRITICAL. This is an unfinished incident with a useful control, not a completed postmortem.

What changed

  • Measured database and staging footprint before any modification.
  • Removed only bounded stale staging using the application-native seven-day session prune.
  • Added a backup preflight: required = live database bytes + 5 GiB headroom, exit 75 when free space is insufficient. First run refused 26.7 GB staging against 17.8 GB free.
  • Added an owner-scoped, one-filesystem, lock-safe 85 percent high-water controller running every 30 minutes.
  • Avoided risky live VACUUM; let SQLite reuse freed pages through normal operation.
  • Fixed cron-only PATH failure by pinning the absolute launcher path.
  • Capped control logs with logrotate.

Two controls worth keeping

Preflight before staging. required = live database bytes + headroom. Exit with a failure code when free space is below required. The backup fails closed rather than completing a staging step that fills the disk.

Bounded high-water controller. Lock before acting. Check against a threshold. Scope to one owner and one filesystem. Use the application-native prune. Raise CRITICAL if recovery fails.

Verification

Initial receipts: root 88 to 83 percent, preflight refused staging against insufficient free space, database quick checks OK, controller installed in user crontab. Live receipts: root rose to 89 percent at 05:30. Every 30-minute run through 10:00 returned CRITICAL reason=threshold_not_recovered.

Claim boundary

Controls are correct. Incident is not resolved. Root is above threshold. The bounded controller cannot recover it. The source of ongoing disk pressure is unidentified. This is an unfinished incident with a fail-closed backup preflight and a working high-water alarm, not a fixed system.

No hostnames, private IPs, usernames, customer names, billing identifiers, private chat excerpts, raw filesystem inventories, keys, or backup destinations are included.

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