/proc/mdstat

The /proc/mdstat file in Linux provides real-time information about the status of software RAID (Redundant Array of Independent Disks) devices and arrays managed by the mdadm utility. Here’s an overview of its purpose and usage:

1. Purpose:

   • Status Reporting: /proc/mdstat is a pseudo-file system located at /proc/mdstat.

   • Real-time Monitoring: It provides real-time status and information about software RAID devices and arrays configured on the system.

2. Contents:

   • RAID Arrays: Displays information about each RAID array configured on the system.

   • Device States: Shows the state of each device within the RAID arrays (active, inactive, degraded, resyncing, recovering, etc.).

   • Progress Indicators: If a RAID array is in the process of rebuilding (resyncing) or recovering from a failure (recovering), progress indicators are shown.

   • Errors and Warnings: Logs errors or warnings related to RAID devices or arrays.

3. Example Output:

   Personalities : [raid1]
   md0 : active raid1 sda1[0] sdb1[1]
         1953517568 blocks super 1.2 [2/2] [UU]
         bitmap: 0/15 pages [0KB], 65536KB chunk
   
   md1 : active raid5 sdc1[0] sdd1[1] sde1[2]
         7814049280 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]
         bitmap: 3/30 pages [12KB], 65536KB chunk
   
   unused devices: <none>

   • md0 is a RAID 1 array (raid1) consisting of sda1 and sdb1.

   • md1 is a RAID 5 array (raid5) with devices sdc1, sdd1, and sde1.

4. Usage:

   • Monitoring: Administrators use /proc/mdstat to monitor the health and status of RAID arrays.

   • Troubleshooting: Helps diagnose issues like device failures, array degradation, or resynchronization processes (resyncing).

   • Automation: Can be parsed by monitoring tools or scripts for automated alerting or reporting on RAID status changes.

5. Maintenance:

   • Resynchronization: Monitor ongoing resynchronization (resyncing) processes to ensure completion.

   • Device Replacement: Identify failed devices ([U_] indicates a missing device) and replace them using mdadm.

6. Access:

   • Read access to /proc/mdstat is typically restricted to privileged users (root) due to its system status reporting nature.

Conclusion

Understanding /proc/mdstat is crucial for administrators managing software RAID configurations in Linux. It provides vital real-time information about RAID arrays, helping maintain data redundancy and availability. Regularly monitor /proc/mdstat to promptly address any issues with RAID devices or arrays, ensuring optimal system performance and data integrity.