To rebuild a RAID array after a disk has failed, you will need to follow the steps below:
Identify the failed disk: The first step is to identify which disk has failed. You can use the cat /proc/mdstat command to view the status of your RAID array and see if any disks are listed as failed. Alternatively, you can use the mdadm command with the --detail option to view the status of your RAID array in more detail.
Replace the failed disk: Once you have identified the failed disk, you will need to replace it with a new disk. Be sure to use a disk of the same size or larger as the failed disk.
Rebuild the RAID array: After replacing the failed disk, you will need to rebuild the RAID array using the mdadm command. To do this, use the mdadm command with the --add option and specify the name of the RAID array and the path to the new disk. For example: mdadm --add /dev/md0 /dev/sdb. This will add the new disk to the RAID array and begin the rebuild process.
Monitor the rebuild process: You can use the cat /proc/mdstat command to monitor the progress of the rebuild process. Alternatively, you can use the mdadm command with the --detail option to view more detailed information about the rebuild process.
Verify the rebuild process: Once the rebuild process is complete, you can use the mdadm command with the --misc option to verify the integrity of the rebuilt RAID array. For example: mdadm --misc --test /dev/md0. This will check the RAID array for any errors or inconsistencies.
Reboot the system: After rebuilding the RAID array, it is recommended to reboot the system to ensure that the changes take effect.
It's important to note that rebuilding a RAID array can take a significant amount of time, depending on the size of the array and the speed of the disks. Be sure to plan accordingly and make sure you have enough downtime to complete the rebuild process.