Contents: Overview of mdadm and RAID5
mdadm is a tool created by Neil Brown that allows for easy administration of multiple storage devices. One task that can be accomplished with mdadm is the ability to create a RAID array such as a RAID level 5 with a hot spare which will be demonstrated in this writeup.

RAID 5 arrays increase the fault tolerance of the data on the array, and depending on the configuration and hardware used it may increase (or decrease) the performance of read/writes. These traits are achieved by slicing data in to chunks, striping the data across multiple devices and then performing an exclusive OR on the data to create parity data. RAID 5 can handle one device failure at a time and can rebuild the data from the failed drive with parity and the regular data. The steps below show how to create a RAID5 array with one hot spare comprised of 5 disks from start to finish and also some administrative best practices.

These steps were ran on a Debian 6.0 "Squeeze" system (Linux debian 2.6.32-5-amd64) virtual machine on ESXi 4.1.0 build 348481. The hardware is one Intel E5620, 24GB of Kingston RAM, and two 1TB Western Digital EARS's all connected through a Supermicro X8DT3. root access is required and it is assumed you have either an Internet connection or the Debian package DVD's available.

The steps outlined below will damage any data on the selected disks. Do not run them on a production system without fully understanding the process and testing in a development environment.

These instructions are not meant to be exhaustive and may not be appropriate for your environment. Always check with your hardware and software vendors for the appropriate steps to manage your infrastructure.

Steps to create a RAID5 device
  1. Log in to your system and open a command prompt.

  2. Change your shell to run as root:
    user@debian~$: su -

  3. Update your package list so the latest software versions are installed:
    root@debian~$: apt-get update
    Hit http://security.debian.org squeeze/updates Release.gpg
    [data omited for length]
    Reading package lists... Done

  4. Install the mdadm software. During the installation if it asks to configure any current arrays, type "none" and hit enter. If it asks to automatically monitor arrays, hit yes.
    root@debian~$: apt-get install mdadm
    Reading package lists.. Done
    [data omited for length]
    Setting up mdadm (3.1.3-1+8efb9d1)...
    Generating array device nodes... done.
    Generating mdadm.conf... done.
    Starting MD monitoring service: mdadm --monitor.
    Assembling MD arrays... done (no arrays found in config file or automatically).
    update-initramfs: Generating /boot/initrd.img-2.6.32-5-amd64
    W: mdadm: /etc/mdadm/mdadm.conf defines no arrays.
    W: mdadm: no arrays defined in configuration file.

  5. Verify mdadm was installed and works:
    root@debian~$: mdadm
    Usage: mdadm --help
      for help

  6. Add your media devices to the machine. This process will be different depending on system setup. Plug in your hard drives, thumb drives, SSD's, floppies and etc to be used for the RAID array. Shutting down the system to plug in interfaces such as SATA is recommended and will not interfere with this process, just remember to perform steps 1 and 2 before continuing. The system used in this example is a VM and new hard drives were added while it was powered on.

  7. Rescan for new media devices. This step can be skipped if the system was powered off to add the storage devices.
    root@debian~$: echo "- - -" > /sys/class/scsi_host/host0/scan
    If successful there will be no output.

    Note that just running the scan command will not work, the dashes sent to the command are wildcards (that I believe tell the controller to scan all buses). Unfortunately I could not find enough information on this command to fully understand how it is being used, if you have details please contact me.

    If you are using an HBA for fiber channel, iSCSI, or other protocols it is possible you may have to use vendor supplied programs to scan for new devices.

  8. Verify the disks have been detected:
    root@debian~$: fdisk -l

    Disk /dev/sda: 21.5 GB, 21474836480 bytes
    [data removed for length]
    Device   Boot   Start   End   Blocks   Id   System
    /dev/sda1   *   1   2499   20067328   83   Linux
    /dev/sda2         2499   2611   901121   5   Extended
    /dev/sda5         2499   2611   901121   82   Linux swap / Solaris

    Disk /dev/sdb: 5368 MB, 5368709120 bytes
    255 heads, 63 sectors/track, 652 cylinders
    Units = cylinders of 16065 * 512 = 8225280 bytes
    Sector size (logical/physical): 512 bytes / 512 bytes
    I/O size (minimum/optimal): 512 bytes / 512 bytes
    Disk identifier: 0x00000000

    Disk /dev/sdb doesn't contain a valid partition table

    Disk /dev/sdc: 5368 MB, 5368709120 bytes
    [data removed for length]
    Disk /dev/sdc doesn't contain a valid partition table

    Disk /dev/sdd: 5368 MB, 5368709120 bytes
    [data removed for length]
    Disk /dev/sdd doesn't contain a valid partition table

    Disk /dev/sde: 5368 MB, 5368709120 bytes
    [data removed for length]
    Disk /dev/sde doesn't contain a valid partition table

    Disk /dev/sdf: 5368 MB, 5368709120 bytes
    [data removed for length]
    Disk /dev/sdf doesn't contain a valid partition table
    The output shown here describes 6 total disks, /dev/sda sdb sdc sdd sde and sdf. The first, /dev/sda is where my boot, root and swap partitions are installed. Each partition is assigned an identifier based on its disk and partition number, /dev/sda1 /dev/sda2 and /dev/sda5, which will not be used to create the array. The other 5 disks have a size of 5GB and have no meaningful data on them, such as partitions or filesystems (see footnote 1).

  9. Create the RAID array called /dev/md0 comprised of the 5 new disks:
    root@debian~$: mdadm --create --verbose /dev/md0 --level=5 --raid-devices=4 /dev/sdb /dev/sdc /dev/sdd /dev/sde --spare-devices=1 /dev/sdf

    mdadm: layout defaults to left-symmetric
    mdadm: chunk size defaults to 512K
    mdadm: layout defaults to left-symmetric
    mdadm: layout defaults to left-symmetric
    mdadm: layout defaults to left-symmetric
    mdadm: layout defaults to left-symmetric
    mdadm: layout defaults to left-symmetric
    mdadm: layout defaults to left-symmetric
    mdadm: size set to 5241344K
    mdadm: Defaulting to version 1.2 metadata
    mdadm: array /dev/md0 started.
    The mdadm application was called, placed in create mode, told to print verbose output, the device to create is defined as /dev/md0, RAID level is set to 5, the number of RAID devices is set to 4 and they are defined, and the number of spare devices is set to 1 with it defined.

    mdadm responds with its status of RAID parity alignment is set to left-symmetric alignment, the chunk size is 512K (see footnote 2), size to use from each disk is 5241344K, the superblock version is 1.2, and the device has been started.

  10. Check the status of the array:
    root@debian~$: cat /proc/mdstat
    Personalities : [raid6] [raid5] [raid4]

    md0 : active raid5 sde[5] sdf[4](S) sdd[2] sdc[1] sdb[0]
        15724032 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UUU_]
        [=>...................] recovery = 5.1% (269312/5241344) finish=2.1min speed=38473K/sec

    unused devices: <none>
    The first line states what RAID levels are supported by mdadm.

    The line starting with md0 describes all device information regarding the newly created /dev/md0. The device is active, running in raid5, has 5 member devices, their device numbers, and /dev/sdf is marked as a spare.

    The next line lists configuration and status of the device /dev/md0. The total number of blocks in the array, it is using superblock version 1.2, RAID level 5, 512K chunk size and using algorithm 2, the total number of devices in the array is 4 and 3 are active. The last part is the status of the devices in the array, the first 3 devices are up and one is down as the 4th is being rebuilt.

    The final line in the group is the activity line which shows that the array is currently in recovery mode, 5.1% complete, the amount of data written of the total, the estimated finish time, and the I/O speed.

    The last line of the file confirms that all devices have been assigned.

  11. Wait until the array rebuild is completed. Though you can use the device while it is rebuilding, the performance will be slow and it is a general best practice to allow it to complete.
    root@debian~$: cat /proc/mdstat
    Personality : [raid6] [raid5] [raid4]
    md0 : active raid5 sde[5] sdf[4](S) sdd[2] sdc[1] sdb[0]     15724032 blocks super 1.2 level 5, 512k chunk, algorith 2 [4/4] [UUUU]

    unused devices: <none>
    Above is the output when the array is completely leveled. Notice that the total number of devices is 4 and the total active is now 4 from the "[4/4]" text. Also all of the devices are marked as up since the [UUUU] has none marked down.

  12. Create a backup of the new device's configuration, the status of the device and information about each disk:
    root@debian~$: mdadm --detail /dev/md0 > /etc/mdadm/md0.info
    root@debian~$: cat /proc/mdstat >> /etc/mdadm/md0.info
    root@debian~$: mdadm --examine /dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf > /etc/mdadm/md0.disks
    These steps create two files: /etc/mdadm/md0.info and /etc/mdadm/md0.disks. The first is general information over the new device and the later is detailed information on each disk comprising the device. Data from these two files can later be used to re-create a damaged device. View them to become familiar with the data.

  13. The device /dev/md0 is now ready for use. It is possible to create partitions, put file systems on it, dd images to it, delete the device, or even use it with other devices to create a nested RAID device.

Following the steps from above should allow you to successfully create a device running RAID level 5 with a single hot spare from five drives using the powerful mdadm tool. These steps can be modified to configure a device to use a different RAID level, such as 0, 1, 10, 4, or 6, and possibly use a different number of disks like 4 or 24. Be sure to test different configurations for the best performance and fault tolerance levels.

  1. The devices used to create the new RAID device /dev/md0 do not have any data on them, they are completely null. It is possible to create a device out of specific partitions on drives, though that was not done here. There are benefits to both and without introducing bias it should be researched before deciding which to use.
  2. Chunk size is the amount of data, in bytes, written to each independent device of a stripe-set. The default is 512KB which may or may not be well suited for your configuration. The chunk size is a value that should be tested in your environment before using in production as it can directly impact I/O performance.


Written by Eric Wamsley
Posted: July 26th, 2011 7:17pm.
Topic: mdadm RAID 5
Tags: mdadm, RAID5, Debian, Linux, storage

 Eric Wamsley - ewams.net