📖 ~1 min read
Table of contents
Symptom & Impact
Array enters degraded mode, reducing redundancy and raising risk of data loss on second fault.
Environment & Reproduction
Typically caused by failing media, transient controller issues, or unexpected drive disconnects.
Root Cause Analysis
One member device is marked failed, leaving array with reduced fault tolerance.
Quick Triage
Confirm degraded state and identify failed member while avoiding unnecessary rebuild interruption.
Step-by-Step Diagnosis
Collect mdadm detail, SMART metrics, and controller logs to pinpoint root hardware failure.
Solution – Primary Fix
Replace failed disk, add new member, and monitor RAID rebuild to completion.
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Solution – Alternative Approaches
Migrate workload to replicated storage tier if immediate hardware replacement is delayed.
Verification & Acceptance Criteria
Array returns to clean state with full redundancy and stable read/write performance.
Rollback Plan
Restore from backup if rebuild fails and array reliability cannot be guaranteed.
Prevention & Hardening
Enable proactive disk health alerts and maintain hot spares for critical arrays.
Related Errors & Cross-Refs
Related to ext4 read-only remounts, block timeout events, and controller reset warnings.
Related tutorial: View the step-by-step tutorial for Debian 9.
View all Debian 9 tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
mdadm and Debian RAID operational references for replacement and rebuild best practices.
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