Configure Software RAID on Linux

5 min to read Contributors

It's a common scenario to use software RAID on Linux virtual machines in Azure to present multiple attached data disks as a single RAID device. Typically this can be used to improve performance and allow for improved throughput compared to using just a single disk.

Attaching data disks

Two or more empty data disks are needed to configure a RAID device. The primary reason for creating a RAID device is to improve performance of your disk IO. Based on your IO needs, you can choose to attach disks that are stored in our Standard Storage, with up to 500 IO/ps per disk or our Premium storage with up to 5000 IO/ps per disk. This article does not go into detail on how to provision and attach data disks to a Linux virtual machine. See the Microsoft Azure article attach a disk for detailed instructions on how to attach an empty data disk to a Linux virtual machine on Azure.

Install the mdadm utility

  • Ubuntu

    sudo apt-get update
    sudo apt-get install mdadm
    
  • CentOS & Oracle Linux

    sudo yum install mdadm
    
  • SLES and openSUSE

    zypper install mdadm
    

Create the disk partitions

In this example, we create a single disk partition on /dev/sdc. The new disk partition will be called /dev/sdc1.

  1. Start fdisk to begin creating partitions

     sudo fdisk /dev/sdc
     Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
     Building a new DOS disklabel with disk identifier 0xa34cb70c.
     Changes will remain in memory only, until you decide to write them.
     After that, of course, the previous content won't be recoverable.
    
     WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
                     switch off the mode (command 'c') and change display units to
                     sectors (command 'u').
    
  2. Press 'n' at the prompt to create a new partition:

     Command (m for help): n
    
  3. Next, press 'p' to create a primary partition:

     Command action
             e   extended
             p   primary partition (1-4)
    
  4. Press '1' to select partition number 1:

     Partition number (1-4): 1
    
  5. Select the starting point of the new partition, or press <enter> to accept the default to place the partition at the beginning of the free space on the drive:

     First cylinder (1-1305, default 1):
     Using default value 1
    
  6. Select the size of the partition, for example type '+10G' to create a 10 gigabyte partition. Or, press <enter> create a single partition that spans the entire drive:

     Last cylinder, +cylinders or +size{K,M,G} (1-1305, default 1305): 
     Using default value 1305
    
  7. Next, change the ID and type of the partition from the default ID '83' (Linux) to ID 'fd' (Linux raid auto):

     Command (m for help): t
     Selected partition 1
     Hex code (type L to list codes): fd
    
  8. Finally, write the partition table to the drive and exit fdisk:

     Command (m for help): w
     The partition table has been altered!
    

Create the RAID array

  1. The following example will "stripe" (RAID level 0) three partitions located on three separate data disks (sdc1, sdd1, sde1). After running this command a new RAID device called /dev/md127 is created. Also note that if these data disks we previously part of another defunct RAID array it may be necessary to add the --force parameter to the mdadm command:

     sudo mdadm --create /dev/md127 --level 0 --raid-devices 3 \
         /dev/sdc1 /dev/sdd1 /dev/sde1
    
  2. Create the file system on the new RAID device

    a. CentOS, Oracle Linux, SLES 12, openSUSE, and Ubuntu

     sudo mkfs -t ext4 /dev/md127
    

    b. SLES 11

     sudo mkfs -t ext3 /dev/md127
    

    c. SLES 11 & openSUSE - enable boot.md and create mdadm.conf

     sudo -i chkconfig --add boot.md
     sudo echo 'DEVICE /dev/sd*[0-9]' >> /etc/mdadm.conf
    
    Note

    A reboot may be required after making these changes on SUSE systems. This step is not required on SLES 12.

Add the new file system to /etc/fstab

Important

Improperly editing the /etc/fstab file could result in an unbootable system. If unsure, refer to the distribution's documentation for information on how to properly edit this file. It is also recommended that a backup of the /etc/fstab file is created before editing.

  1. Create the desired mount point for your new file system, for example:

     sudo mkdir /data
    
  2. When editing /etc/fstab, the UUID should be used to reference the file system rather than the device name. Use the blkid utility to determine the UUID for the new file system:

     sudo /sbin/blkid
     ...........
     /dev/md127: UUID="aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee" TYPE="ext4"
    
  3. Open /etc/fstab in a text editor and add an entry for the new file system, for example:

     UUID=aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee  /data  ext4  defaults  0  2
    

    Or on SLES 11 & openSUSE:

     /dev/disk/by-uuid/aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee  /data  ext3  defaults  0  2
    

    Then, save and close /etc/fstab.

  4. Test that the /etc/fstab entry is correct:

     sudo mount -a
    

    If this command results in an error message, please check the syntax in the /etc/fstab file.

    Next run the mount command to ensure the file system is mounted:

     mount
     .................
     /dev/md127 on /data type ext4 (rw)
    
  5. (Optional) Failsafe Boot Parameters

    fstab configuration

    Many distributions include either the nobootwait or nofail mount parameters that may be added to the /etc/fstab file. These parameters allow for failures when mounting a particular file system and allow the Linux system to continue to boot even if it is unable to properly mount the RAID file system. Refer to your distribution's documentation for more information on these parameters.

    Example (Ubuntu):

     UUID=aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee  /data  ext4  defaults,nobootwait  0  2
    

    Linux boot parameters

    In addition to the above parameters, the kernel parameter "bootdegraded=true" can allow the system to boot even if the RAID is perceived as damaged or degraded, for example if a data drive is inadvertently removed from the virtual machine. By default this could also result in a non-bootable system.

    Please refer to your distribution's documentation on how to properly edit kernel parameters. For example, in many distributions (CentOS, Oracle Linux, SLES 11) these parameters may be added manually to the "/boot/grub/menu.lst" file. On Ubuntu this parameter can be added to the GRUB_CMDLINE_LINUX_DEFAULT variable on "/etc/default/grub".

TRIM/UNMAP support

Some Linux kernels support TRIM/UNMAP operations to discard unused blocks on the disk. These operations are primarily useful in standard storage to inform Azure that deleted pages are no longer valid and can be discarded. Discarding pages can save cost if you create large files and then delete them.

Note

RAID may not issue discard commands if the chunk size for the array is set to less than the default (512KB). This is because the unmap granularity on the Host is also 512KB. If you modified the array's chunk size via mdadm's --chunk= parameter, then TRIM/unmap requests may be ignored by the kernel.

There are two ways to enable TRIM support in your Linux VM. As usual, consult your distribution for the recommended approach:

  • Use the discard mount option in /etc/fstab, for example:

      UUID=aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee  /data  ext4  defaults,discard  0  2
    
  • In some cases the discard option may have performance implications. Alternatively, you can run the fstrim command manually from the command line, or add it to your crontab to run regularly:

    Ubuntu

      # sudo apt-get install util-linux
      # sudo fstrim /data
    

    RHEL/CentOS

      # sudo yum install util-linux
      # sudo fstrim /data