Snapshot Action Storage Saturation

Snapshot creation, deletion, purge, rebuild, and backup can temporarily consume more space than operators expect. When a disk is already near full, the safe behavior is to block the action with a clear reason before volume integrity depends on luck.

Runbook: Block Before The Action Amplifies Space

1. Classify each snapshot action separately. Create, delete, purge, rebuild, backup, and restore have different temporary space profiles.
2. Use absolute bytes, percentage free, largest replica size, and action-specific reserve. A percent-only threshold misses large modern volumes.
3. Block user-initiated actions that need more temporary space than the disk can safely provide.
4. For system-initiated rebuild or upgrade snapshots, add automatic cleanup and alert if cleanup cannot complete.
5. Show the operator a clear reason: current free space, required free space, action name, affected volume, and next safe action.
6. Alert when snapshot counts or missing recurring cleanup jobs make a future purge unsafe.
7. Manual test plans should include a nearly-full disk, largest-replica threshold, and cleanup failure path.

I would make this an action-specific admission gate rather than a single generic disk threshold.

Acceptance checks I would want:
- Snapshot create, delete, purge, rebuild, backup, and restore each declare their temporary free-space requirement.
- The gate uses absolute free bytes, free percent, largest affected replica size, and an emergency reserve.
- User-initiated actions are blocked before they can push the disk into ENOSPC.
- The error includes required free space, current free space, action name, affected volume, and the safest next step.
- System snapshots created during rebuild/upgrade have a cleanup guarantee and a failed-cleanup alert.
- The UI or alerting warns when snapshot count or missing recurring cleanup makes the next purge unsafe.
- Manual tests cover a nearly-full disk, a largest-replica threshold breach, and a cleanup job that cannot reclaim space.

Do Not Treat As Ordinary Cleanup

• Snapshot deletion that coalesces data before freeing space.
• Rebuild or upgrade flows that create system snapshots without guaranteed cleanup.
• Backup or restore staging that competes with live replica headroom.
• UI actions that look harmless while the disk is below the largest-replica threshold.