Build a Software RAID Array on Oracle Linux

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Build a Software RAID Array on Oracle Linux

Introduction

A Redundant Array of Independent Disks or RAID device is a virtual device created from two or more real block devices. This functionality allows multiple devices (typically disk drives or partitions of a disk) to be combined into a single device to hold a single filesystem. Some RAID levels include redundancy, allowing the filesystem to survive some degree of device failure.

The Oracle Linux kernel uses the Multiple Device (MD) driver to support Linux software RAID and enable you to organize disk drives into RAID devices and implement different RAID levels.

For more information on these different RAID levels, see the Oracle documentation .

This tutorial will work with the MD utility (mdadm) to create a RAID1 device with a spare and then address a disk failure.

Objectives

  • Create a RAID1 device with a spare
  • Recover a failed RAID1 device

Prerequisites

Any Oracle Linux 8 system with the following configurations:

  • a non-root user with sudo permissions
  • additional block volumes for use with Linux Software RAID

Setup Lab Environment

Note: When using the free lab environment, see Oracle Linux Lab Basics for connection and other usage instructions.

  1. If not already connected, open a terminal and connect via ssh to each instance mentioned above.

    ssh oracle@<ip_address_of_instance>

  2. Verify the block volumes exist.

    sudo lsblk

    The output for the free lab environment should show the /dev/sda for the existing file system, and the available disks /dev/sdb, /dev/sdc, and /dev/sdd.

Install the MD Utility

  1. If not already available on the system, install MD utility.

    Check if mdadm installed.

    sudo dnf list --installed mdadm

    If not installed, install mdadm.

    sudo dnf -y install mdadm

    Note: The free lab environment already has the MD utility installed.

Create a RAID Device.

RAID1 provides data redundancy and resilience by writing identical data to each drive in the array. If one drive fails, a mirror can satisfy I/O requests. Mirroring is an expensive solution because the system writes the same information to all of the disks in the array.

Features of RAID1:

  • Includes redundancy.
  • Uses two or more disks with zero or more spare disks.
  • Maintains an exact mirror of the data written on each disk.
  • Disk devices should be of equal size.
    • If one disk device is larger than another, the RAID device will be the size of the smallest disk.
  • Allows up to n-1 disk devices to be removed or fail while all data remains intact.
  • Provided the system survives a crash and spare disks are available, recovery of the RAID1 mirror happens automatically and immediately upon detection of the fault.
  • Slower write performance occurs compared to a single disk due to writing the same data to multiple disks in the mirror set.

  1. List the options available to create a RAID device.

    Using mdadm --help shows to use the --create option to create a new array from unused devices.

    sudo mdadm --create --help

  2. Create a RAID1 device with one spare disk.

    sudo mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sdb /dev/sdc --spare-devices=1 /dev/sdd
    • --create: : Creates the new array
    • --level : The raid level
    • --raid-devices : The number of active devices in the array
    • --spare-devices: The number of spare (extra) devices in the initial array

    In this command, the device (array) is named /dev/md0 and uses /dev/sdb and /dev/sdc to create the RAID1 device. The device /dev/sdd is automatically used as a spare to recover from any active device's failure.

    Accept the Continue creating array? prompt by typing y and hitting ENTER.

    Example Output:

    [oracle@ol-mdadm-2022-06-04-180415 ~]$ sudo mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sdb /dev/sdc --spare-devices=1 /dev/sdd
mdadm: Note: this array has metadata at the start and
    may not be suitable as a boot device.  If you plan to
    store '/boot' on this device please ensure that
    your boot-loader understands md/v1.x metadata, or use
    --metadata=0.90
mdadm: size set to 52395008K
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.

Create a File System

  1. Create an ext4 filesystem on the RAID device and mount it.

    sudo mkfs.ext4 -F /dev/md0
sudo mkdir /u01
sudo mount /dev/md0 /u01

  2. Report the file system disk usage.

    df -h

    Example Output:

    [oracle@ol-mdadm-2022-06-04-180415 ~]$ df -h
Filesystem                  Size  Used Avail Use% Mounted on
...
/dev/md0                     49G   53M   47G   1% /u01

  3. Add an entry to /etc/fstab and make the mount point persistent across reboots.

    echo "/dev/md0    /data01    ext4    defaults    0 0" | sudo tee -a /etc/fstab > /dev/null

Verify RAID Device

  1. Get details about the array.

    sudo mdadm --detail /dev/md0

    Example Output:

    [oracle@ol-mdadm-2022-06-04-180415 ~]$ sudo mdadm --detail /dev/md0
/dev/md0:
           Version : 1.2
     Creation Time : Sat Jun  4 20:08:32 2022
        Raid Level : raid1
        Array Size : 52395008 (49.97 GiB 53.65 GB)
     Used Dev Size : 52395008 (49.97 GiB 53.65 GB)
      Raid Devices : 2
     Total Devices : 3
       Persistence : Superblock is persistent

       Update Time : Sat Jun  4 20:28:58 2022
             State : clean, resyncing 
    Active Devices : 2
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 1

Consistency Policy : resync

     Resync Status : 59% complete

              Name : ol-mdadm-2022-06-04-180415:0  (local to host ol-mdadm-2022-06-04-180415)
              UUID : f6c35144:66a24ae9:5b96e616:f7252a9f
            Events : 9

    Number   Major   Minor   RaidDevice State
       0       8       16        0      active sync   /dev/sdb
       1       8       32        1      active sync   /dev/sdc

       2       8       48        -      spare   /dev/sdd

    In the output, the State shows the array is clean and resyncing. The resysc always occurs after initial creation of the array, or after recovery. The output shows the resync is 59% complete.

  2. Check real time information from the kernel.

    sudo cat /proc/mdstat

    Example Output:

    [oracle@ol-mdadm-2022-06-04-180415 ~]$ cat /proc/mdstat 
Personalities : [raid1] 
md0 : active raid1 sdd[2](S) sdc[1] sdb[0]
      52395008 blocks super 1.2 [2/2] [UU]
      [==================>..]  resync = 92.2% (48341824/52395008) finish=2.7min speed=24677K/sec
      
unused devices: <none>

Create RAID Configuration File

  1. (Optional) Add the RAID configuration to /etc/mdadm.conf

    The configuration file identifies which devices are RAID devices and to which array a specific device belongs. Based on this configuration file, mdadm is able to assemble the arrays at boot time.

    sudo mdadm --examine --scan | sudo tee -a /etc/mdadm.conf

    Example Output:

    [oracle@ol-mdadm-2022-06-04-180415 ~]$ sudo mdadm --examine --scan | sudo tee -a /etc/mdadm.conf
ARRAY /dev/md/0  metadata=1.2 UUID=f6c35144:66a24ae9:5b96e616:f7252a9f name=ol-mdadm-2022-06-04-180415:0
   spares=1

Manage RAID Devices

This options is for managing the component devices within an array, such as adding, removing, or faulting a device.

  1. List the options available to manage a RAID device.

    sudo mdadm --manage --help
    • --add : Hotadd subsequent devices.
    • --remove : Remove subsequent non-active devices.
    • --fail : Mark subsequent devices as faulty.

  2. Synchronize cached writes to persistent storage.

    This is required before running any disk managment to ensure all disk caches are written to disk.

    sudo sync

  3. Mark a disk as failed.

    sudo mdadm --manage /dev/md0 --fail /dev/sdb

  4. Get array details.

    sudo mdadm --detail /dev/md0

    Example Output:

    [oracle@ol-mdadm-2022-06-04-180415 ~]$ sudo mdadm --detail /dev/md0 
/dev/md0:
           Version : 1.2
     Creation Time : Sat Jun  4 20:08:32 2022
        Raid Level : raid1
        Array Size : 52395008 (49.97 GiB 53.65 GB)
     Used Dev Size : 52395008 (49.97 GiB 53.65 GB)
      Raid Devices : 2
     Total Devices : 3
       Persistence : Superblock is persistent

       Update Time : Sat Jun  4 21:34:19 2022
             State : clean, degraded, recovering 
    Active Devices : 1
   Working Devices : 2
    Failed Devices : 1
     Spare Devices : 1

Consistency Policy : resync

    Rebuild Status : 1% complete

              Name : ol-mdadm-2022-06-04-180415:0  (local to host ol-mdadm-2022-06-04-180415)
              UUID : f6c35144:66a24ae9:5b96e616:f7252a9f
            Events : 19

    Number   Major   Minor   RaidDevice State
       2       8       48        0      spare rebuilding   /dev/sdd
       1       8       32        1      active sync   /dev/sdc

       0       8       16        -      faulty   /dev/sdb

    The array is marked as degraded and recovering. The output also shows that the spare device /dev/sdd is automatically rebuilding the array, while /dev/sdb is faulty.

  5. Remove the failed disk.

    sudo mdadm --manage /dev/md0 --remove /dev/sdb

  6. Replace the failed disk.

    Replace the server's failed physical disk with a new one.

    Note: There is nothing to do for this step when using the free lab environment. The lab will repurpose the existing dev/sdb device.

  7. Remove previous linux_raid_member signature.

    A signature, metadata, is written a disk when used in a RAID array, and the disk cannot be moved to another system or repurposed until removing those signatures.

    sudo wipefs -a -f /dev/sdb

    Warning: The wipefs command is destructive and removes the entire partition table on the target disk (/dev/sdb) and any signatures.

  8. Add new spare to the array.

    sudo mdadm --manage /dev/md0 --add /dev/sdb

  9. Verify the spare disk exists.

    sudo mdadm --detail /dev/md0

    At the bottom of the output, the device /dev/sdb should appear in the list with the State set to spare.

For More Information:

SSR