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Linux: How to replace a bad disk on a Linux RAID configuration

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Replacing a failed disk in a Linux RAID configuration involves several steps to ensure that the array remains operational and data integrity is maintained. Below is a step-by-step guide on how to replace a bad disk in a Linux RAID configuration using the mdadm utility:

  1. Identify the Failed Disk:
    • Use the mdadm --detail /dev/mdX command to display detailed information about the RAID array.
    • Look for the state of each device in the array to identify the failed disk.
    • Note the device name (e.g., /dev/sdX) of the failed disk.
  2. Prepare the New Disk:
    • Insert the new disk into the system and ensure it is recognized by the operating system.
    • Partition the new disk using a partitioning tool like fdisk or parted. Create a Linux RAID (type FD) partition on the new disk.
  3. Add the New Disk to the RAID Array:
    • Use the mdadm --manage /dev/mdX --add /dev/sdX1 command to add the new disk to the RAID array.
    • Replace /dev/mdX with the name of your RAID array and /dev/sdX1 with the partition name of the new disk.
    • This command starts the process of rebuilding the RAID array onto the new disk.
  4. Monitor the Rebuild Process:
    • Monitor the rebuild process using the mdadm --detail /dev/mdX command.
    • Check the progress and status of the rebuild operation to ensure it completes successfully.
    • The rebuild process may take some time depending on the size of the RAID array and the performance of the disks.
  5. Verify RAID Array Status:
    • After the rebuild process completes, verify the status of the RAID array using the mdadm --detail /dev/mdX command.
    • Ensure that all devices in the array are in the “active sync” state and that there are no errors or warnings.
  6. Update Configuration Files:
    • Update configuration files such as /etc/mdadm/mdadm.conf to ensure that the new disk is recognized and configured correctly in the RAID array.
  7. Perform Testing and Verification:
    • Perform thorough testing to ensure that the RAID array is functioning correctly and that data integrity is maintained.
    • Test read and write operations on the array to verify its performance and reliability.
  8. Optional: Remove the Failed Disk:
    • Once the rebuild process is complete and the RAID array is fully operational, you can optionally remove the failed disk from the array using the mdadm --manage /dev/mdX --remove /dev/sdX1 command.
    • This step is optional but can help clean up the configuration and remove any references to the failed disk.

By following these steps, you can safely replace a bad disk in a Linux RAID configuration using the mdadm utility while maintaining data integrity and ensuring the continued operation of the RAID array.

Linux: systemd target units examples

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Here is a list of some systemd target units along with examples of how to use them:

  1. multi-user.target:
    • This target is used for a multi-user system without a graphical interface. It includes services required for a text-based or command-line environment.
    • Example: To switch to the multi-user target, you can use the following command: sudo systemctl isolate multi-user.target
  2. graphical.target:
    • Represents a multi-user system with a graphical interface (GUI). It includes services required for a graphical desktop environment.
    • Example: To switch to the graphical target, you can use the following command:sudo systemctl isolate graphical.target
  3. rescue.target:
    • Similar to runlevel 1 or single-user mode in traditional SysVinit systems. It provides a minimal environment with a root shell for system recovery and maintenance tasks.
    • Example: To switch to the rescue target, you can use the following command:sudo systemctl isolate rescue.target
  4. emergency.target:
    • Provides the most minimal environment possible, intended for emergencies where the system is in an unusable state. It drops the system into a single-user shell without starting any services.
    • Example: To switch to the emergency target, you can use the following command:sudo systemctl emergency
  5. shutdown.target:
    • Used to gracefully shut down the system. All services are stopped, and the system is powered off or rebooted, depending on the shutdown command used.
    • Example: To initiate a shutdown using this target, you can use the following command:sudo systemctl shutdown
  6. network.target:
    • Represents the availability of the network. Other services that depend on network connectivity may be started after this target is reached.
    • Example: To view the status of the network target, you can use the following command:systemctl status network.target
  7. sockets.target:
    • Represents the availability of system sockets. Services that provide network services via sockets may be started after this target is reached.
    • Example: To view the status of the sockets target, you can use the following command:systemctl status sockets.target

These are some of the systemd target units along with examples of how to use them. Depending on your specific distribution and configuration, there may be additional targets or custom targets defined. You can explore more targets and their usage by referring to the systemd documentation or using the systemctl list-units --type=target command.

Linux: Systemd target units

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Systemd target units are used to group and manage services and other units in Linux distributions that use systemd as the init system. Targets are similar to runlevels in traditional SysVinit systems but offer more flexibility and granularity in defining system states and dependencies between units.

Here are some common systemd target units:

  1. default.target:
    • This is the default target unit that the system boots into. It typically represents the normal operational state of the system.
  2. multi-user.target:
    • Represents a multi-user system without a graphical interface. It includes services required for a text-based or command-line environment.
  3. graphical.target:
    • Represents a multi-user system with a graphical interface (GUI). It includes services required for a graphical desktop environment.
  4. rescue.target:
    • Similar to runlevel 1 or single-user mode in traditional SysVinit systems. It provides a minimal environment with a root shell for system recovery and maintenance tasks.
  5. emergency.target:
    • Provides the most minimal environment possible, intended for emergencies where the system is in an unusable state. It drops the system into a single-user shell without starting any services.
  6. shutdown.target:
    • Used to gracefully shut down the system. All services are stopped, and the system is powered off or rebooted, depending on the shutdown command used.
  7. poweroff.target:
    • Initiates a system poweroff, shutting down the system and powering off the hardware.
  8. reboot.target:
    • Initiates a system reboot, shutting down the system and restarting the hardware.
  9. network.target:
    • Represents the network being available. Other services that depend on network connectivity may be started after this target is reached.
  10. basic.target:
    • A minimal target that is reached early during the boot process. It includes basic system initialization and dependency handling.
  11. sockets.target:
    • Represents the availability of system sockets. Services that provide network services via sockets may be started after this target is reached.
  12. timers.target:
    • Represents the availability of system timers. Services that depend on timers for scheduling tasks may be started after this target is reached.

These are some of the common systemd target units used in Linux distributions. Depending on the specific distribution and configuration, there may be additional targets or custom targets defined. You can view the available targets on your system using the systemctl list-units --type=target command.