The headline needs to capture the key facts: ClickHouse, 11 million rows, every 30 seconds, idle, and the fix/outcome.

Short summary

A developer discovered their self-hosted ClickHouse instance was merging 11M rows every 30 seconds while idle, caused by unbounded system logs (trace_log, text_log, etc.) with no default TTL. Disabling heavy logs and tuning background pools, mark cache, and collection intervals freed 2.7 GB and cut merges by 99.9%. The post also corrects a common docker stats misreading: CPU percentages are per-core, not per-machine.

• •ClickHouse system logs (trace_log, text_log) have no TTL by default and can bloat disk 1000:1 over real app data
• •Disabling heavy logs and tuning background_pool_size, mark_cache, and collection intervals cut merges from 11M to 5.7K rows per 30s
• •docker stats reports CPU as percentage of one core, not the whole machine — cross-check with host ps

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