Atlas: diagnose and fix disk pressure

TL;DR: Disk fills up and writes start failing. Work out whether it is data growth, indexes, or oplog, then free space or scale deliberately. Check the biggest collections and indexes (Data Explorer storage sizes). Check for unbounded growth: logs, events, or history collections with no TTL or archival.

The error

Disk Usage metric near 100%, or writes failing with out-of-space errors. On the free tier the cap is small and this arrives fast.

The fix

1. Atlas Metrics: Disk Usage trend. Sudden jump suggests a bulk load or runaway job; steady climb suggests organic growth.
2. Check the biggest collections and indexes (Data Explorer storage sizes). Indexes can rival data size on write-heavy collections with many secondary indexes.
3. Check for unbounded growth: logs, events, or history collections with no TTL or archival.
4. Drop unused collections and indexes (the Performance Advisor flags unused indexes).
5. Add TTL indexes or an Online Archive rule for time-series-ish data.
6. Compact only if fragmentation is proven; it needs the right role and takes time.
7. Scale the tier for genuine growth, and set a disk alert at 75% so this never surprises you again.

When to use this

• You are seeing this exact error message; match the block above, not just part of it.
• The failing call matches the scenario in the title: Atlas.
• You want the fastest verified fix before digging through logs.

When not to use this

• Your error text differs from the block above; close cousins often have different causes.
• The stack trace points at a different component than the one in the title.
• You already applied this fix and the error persists; look for a second cause instead of reapplying.

Compatibility

• Not pinned to a specific version; follows current Atlas behavior.

Disk usage monitoring per https://www.mongodb.com/docs/atlas/monitor-cluster-metrics/; alert thresholds per https://www.mongodb.com/docs/atlas/alerts/; Online Archive for data lifecycle per https://www.mongodb.com/docs/atlas/online-archive/manage-online-archive/, verified 2026-09-26.