On-callMediumoc-g183

Subject Inode exhaustionLevel Mid–Senior~30 minCommon in Reliability & on-call interviewsIndustries Software development

Question

A PHP application server starts throwing 'No space left on device' on file creation at 10:00, yet `df -h` shows / at only 55% used — there's plenty of free space in bytes. New session creation fails so users can't log in. The app stores PHP session files as one tiny file per session under /var/lib/php/sessions, and the session garbage-collector probability was effectively set to 0 in a config change last month. A marketing push tripled new (anonymous) sessions today. Walk through triage and remediation.

What a strong answer looks like

Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.

Learn the concepts

Diagram & narrate the incident

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Run or narrate your approach, then ask the coach.