If you have a software RAID array that is configured in RAID 5 and you lose one of the devices in the array (e.g., due to a hardware failure), you can recover from the lost device by rebuilding the array.
To rebuild the RAID 5 array, you will need to follow these steps:
Identify the lost device: You will need to determine which device in the array has failed or been lost. You can use the cat /proc/mdstat command to view the status of the RAID array, which will show you which devices are currently active and which ones are inactive or missing.
Replace the lost device: If the lost device is a physical hard drive, you will need to replace it with a new one. Make sure to use a drive that is compatible with the rest of the array in terms of capacity and performance.
Add the new device to the array: Once you have replaced the lost device, you will need to add it to the array. You can do this using the mdadm command. For example:
mdadm --add /dev/md0 /dev/sda
This command will add the device /dev/sda to the array /dev/md0. Make sure to replace /dev/sda with the correct device name for your system.
--rebuild flag with the mdadm command. For example:mdadm --rebuild /dev/md0 /dev/sda
This command will start the rebuilding process, which may take some time depending on the size of the array and the speed of the devices. You can monitor the progress of the rebuild using the cat /proc/mdstat command.
Once the rebuild is complete, the RAID 5 array should be fully functional again and you should be able to access the data that was stored on it.