Difference between revisions of "RAID"

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The following examples shows building a 3 device RAID5 array:
The following examples shows building a 3 device RAID5 array:
  # mdadm --create --verbose --level=5 --raid-devices=3 /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 --spare-devices=1 /dev/sde1
  # mdadm --create --verbose --level=5 --metadata=1.2 --chunk=256 --raid-devices=3 /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 --spare-devices=1 /dev/sde1
The array is created under the virtual device {{ic|/dev/mdX}}, assembled and ready to use (in degraded mode). One can directly start using it while mdadm resyncs the array in the background. It can take a long time to restore parity. Check the progress with:
The array is created under the virtual device {{ic|/dev/mdX}}, assembled and ready to use (in degraded mode). One can directly start using it while mdadm resyncs the array in the background. It can take a long time to restore parity. Check the progress with:

Revision as of 09:28, 3 October 2013

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Template:Wikipedia Redundant Array of Independent Disks (RAID) is a storage technology that combines multiple disk drive components (typically disk drives or partitions thereof) into a logical unit. Depending the RAID implementation, this logical unit can be a file system or an additional transparent layer that can hold several partitions. Data is distributed across the drives in one of several ways called "RAID levels", depending on the level of redundancy and performance required. The RAID level chosen can thus prevent data loss in the event of a hard disk failure, increase performance or be a combination of both.

Despite redundancy implied by most RAID levels, RAID does not guarantee that data is safe. A RAID will not protect data if there is a fire, the computer is stolen or multiple hard drives fail at once. Furthermore, installing a system with RAID is a complex process that may destroy data.

Warning: Therefore, be sure to backup all data before proceeding.

There are many different levels of RAID, please find hereafter the most commonly used ones.

Standard RAID levels

Uses striping to combine disks. Even if does not provide redundancy, it is anyway considered as a RAID. It does, however, provide a big speed benefit. If you think the speed increase is worth the possibility of data loss (for your swap partition for example), choose this RAID level. On a server, RAID 1 and RAID 5 arrays are more appropriate. The size of a RAID 0 array block device is the size of the smallest component partition times the number of component partitions.
The most straightforward RAID level: straight mirroring. As with other RAID levels, it only makes sense if the partitions are on different physical disk drives. If one of those drives fails, the block device provided by the RAID array will continue to function as normal. The example will be using RAID 1 for everything except swap and temporary data. Please note that with a software implementation, the RAID 1 level is the only option for the boot partition, because bootloaders reading the boot partition do not understand RAID, but a RAID 1 component partition can be read as a normal partition. The size of a RAID 1 array block device is the size of the smallest component partition.
Requires 3 or more physical drives, and provides the redundancy of RAID 1 combined with the speed and size benefits of RAID 0. RAID 5 uses striping, like RAID 0, but also stores parity blocks distributed across each member disk. In the event of a failed disk, these parity blocks are used to reconstruct the data on a replacement disk. RAID 5 can withstand the loss of one member disk.
Note: RAID 5 is a common choice due to its combination of speed and data redundancy. The caveat is that if one drive were to fail before and that drive was replaced another drive failed too, all data will be lost.

Nested RAID levels

RAID 1+0
Commonly referred to as RAID 10, is a nested RAID that combines two of the standard levels of RAID to gain performance and additional redundancy. It is the best alternative to RAID 5 when redundancy is crucial.

RAID level comparison

RAID level Data redundancy Physical drive utilization Read performance Write performance Min drives
0 No 100% nX




1 Yes 50% nX (theoretically)

1X (in practice)

1X 2
5 Yes 67% - 94% (n−1)X




6 Yes 50% - 88% (n−2)X (n−2)X 4
10 Yes 50% (n−2)X (n−2)X 4

* Where n is standing for the number of dedicated disks.


The RAID devices can be managed in different ways:

Software RAID
This is the easier implementation as it does not rely on obscure proprietary firmware and software to be used. The array is managed by the operating system either by:
  • by an abstraction layer (e.g. mdadm);
    Note: This is the method we will use later in this guide. If you want to use this one too, read on.
  • by a logical volume manager (e.g. LVM);
  • by a component of a file system (e.g. ZFS).
Hardware RAID
The array is directly managed by a dedicated hardware card installed in your computer to which the disks are directly connected. The RAID logic runs on an on-board processor independently of the host processor (CPU). Although this solution is independent of any operating system, the latter requires a driver in order to function properly with the hardware RAID controller. The RAID array can either be configured via an option rom interface or, depending on the manufacturer, with a dedicated application when the OS has been installed. The configuration is transparent for the Linux kernel: it doesn't see the disks separately.
This type of RAID is properly called BIOS or Onboard RAID, but is falsely advertised as hardware RAID. The array is managed by pseudo-RAID controllers where the RAID logic is implemented in an option rom or in the firmware itself with a EFI SataDriver (in case of UEFI), but are not full hardware RAID controllers with all RAID features implemented. Therefore, this type of RAID is sometimes called FakeRAID. dmraid from the official repositories, will be used to deal with these controllers. Some FakeRAID controller examples: Intel Rapid Storage, JMicron JMB36x RAID ROM, AMD RAID, ASMedia 106x,...

Which type of RAID do I have?

As the process to set up software RAID is completely user driven, determining if you're using this implementation is quite evident.

However, discerning between FakeRAID and true hardware RAID can be more difficult. As stated, manufacturers often incorrectly distinguish these two types of RAID and false advertising is always possible. The best solution in this instance is to run the lspci command and looking through the output to find your RAID controller. Then do a search to see what information you can find about your RAID controller. True hardware RAID controller are often rather expensive (~$400+), so if you customized a computer, it is very likely that choosing a hardware RAID setup made a very noticeable change in the computer's price.


Install mdadm which is used for administering pure software RAID using plain block devices: the underlying hardware does not provides any RAID logic, just a supply of disks. mdadm will work with any collection of block devices. Even if unusual. For example, one can thus make a RAID array from a collection of thumb drives.

Prepare the Devices

Warning: These steps erase everything on a device, so type carefully!

To prevent possible issues each device in the RAID should be securely wiped. If the device is being reused or re-purposed from an existing array, erase any old RAID configuration information:

# mdadm --zero-superblock /dev/<drive>

Create the Partition Table

It is highly recommended to pre-partition the disks to be used in the array. Since most RAID users are selecting HDDs >2 TB, GTP partition tables are required and recommended. Disks are easily partitioned using gptfdisk.

  • After created, the partition type should be assigned hex code FD00.
  • Creating partitions that are of the same size on each of the devices is preferred.
  • A good tip is to leave approx 100 MB at the end of the device when partitioning. See below for rationale.
Note: It is also possible to create a RAID directly on the raw disks (without partitions), but not recommended because it can cause problems when swapping a failed disk.

When replacing a failed disk of a RAID, the new disk has to be exactly the same size as the failed disk or bigger — otherwise the array recreation process will not work. Even hard drives of the same manufacturer and model can have small size differences. By leaving a little space at the end of the disk unallocated one can compensate for the size differences between drives, which makes choosing a replacement drive model easier. Therefore, it is good practice to leave about 100 MB of unallocated space at the end of the disk.

Build the Array

Use mdadm to build the array. Several examples are given below.

Warning: Do not simply copy/paste the examples below; use your brain and substitute the correct options/drive letters!
Note: If this is a RAID 1 array which you intend to boot from using Syslinux you need to change the metadata value to 1.0 (Syslinux as of version 4.07 does not understand md 1.2 metadata)

The following example shows building a 2 device RAID1 array:

# mdadm --create --verbose --level=1 --metadata=1.2 --chunk=64 --raid-devices=2 /dev/md0 /dev/sdb1 /dev/sdc1

The following examples shows building a 3 device RAID5 array:

# mdadm --create --verbose --level=5 --metadata=1.2 --chunk=256 --raid-devices=3 /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 --spare-devices=1 /dev/sde1

The array is created under the virtual device /dev/mdX, assembled and ready to use (in degraded mode). One can directly start using it while mdadm resyncs the array in the background. It can take a long time to restore parity. Check the progress with:

$ cat /proc/mdstat

Update Configuration file

After built, default configuration file,


needs to be updated like so:

# mdadm --detail --scan >> /etc/mdadm.conf
Note: If updating the RAID configuration from within the Arch Installer by swapping to another TTY, ensure that you are writing to the correct mdadm.conf file:
# mdadm --detail --scan > /mnt/etc/mdadm.conf

Once the configuration file has been updated the array can be assembled using mdadm:

# mdadm --assemble --scan

Format the RAID Filesystem

The array can now be formatted like any other disk, just keep in mind that:

  • Due to the large volume size not all filesystems are suited (see: File system limits).
  • The filesystem should support growing and shrinking while online (see: File system features).
  • Performance gains on a raid arrays can be had by formatting the volume aligned to your RAID stripe size (see: RAID Math).

Example formatting to ext4 with the correct stripe-width and stride:

  • Hypothetical RAID1 array is composed of 2 physical disks.
  • Chunk size is 64k.
  • Block size is 4k.

Stride = (chunk size/block size). In this example, the math is (256/4) so the stride = 16.

Stripe-width = (# of physical data disks * stride). In this example, the math is (2*16) so the stripe-width = 32.

# mkfs.ext4 -v -L myarray -m 0.5 -b 4096 -E stride=16,stripe-width=32 /dev/md0

Example formatting to ext4 with the correct stripe-width and stride:

  • Hypothetical RAID5 array is composed of 4 physical disks; 3 data discs and 1 parity disc.
  • Chunk size is 256k.
  • Block size is 4k.

Stride = (chunk size/block size). In this example, the math is 256/4) so the stride = 64.

Stripe-width = (# of physical data disks * stride). In this example, the math is (3*64) so the stripe-width = 192.

# mkfs.ext4 -v -L myarray -m 0.5 -b 4096 -E stride=64,stripe-width=192 /dev/md0

Add to Kernel Image

Add mdadm_udev to the HOOKS section of the Mkinitcpio file before the filesystems hook. This will add support for mdadm directly into the init image.

HOOKS="base udev autodetect block mdadm_udev filesystems usbinput fsck"

Add the raid456 module and the filesystem module created on the RAID (e.g. ext4) to the MODULES section. This will build these modules into the kernel image. For example,

MODULES="ext4 raid456"

Next regenerate the initramfs image (see Image creation and activation).

Mounting from a Live CD

If you want to mount your RAID partition from a Live CD, use

# mdadm --assemble /dev/<disk1> /dev/<disk2> /dev/<disk3> /dev/<disk4>
Note: Live CDs like SystemrescueCD assemble the RAIDs automatically at boot time if you used the partition type fd at the install of the array.

Removing device, stop using the array

You can remove a device from the array after you mark it as faulty.

# mdadm --fail /dev/md0 /dev/sdxx

Then you can remove it from the array.

# mdadm -r /dev/md0 /dev/sdxx

Remove device permanently (for example in the case you want to use it individally from now on). Issue the two commands described above then:

# mdadm --zero-superblock /dev/sdxx

After this you can use the disk as you did before creating the array.

Warning: If you reuse the removed disk without zeroing the superblock you will LOSE all your data next boot. (After mdadm will try to use it as the part of the raid array). DO NOT issue this command on linear or RAID0 arrays or you will LOSE all your data on the raid array.

Stop using an array:

  1. Umount target array
  2. Stop the array with: mdadm --stop /dev/md0
  3. Repeat the three command described in the beginning of this section on each device.
  4. Remove the corresponding line from /etc/mdadm.conf

Adding a device to the array

Adding new devices with mdadm can be done on a running system with the devices mounted. Partition the new device /dev/sdx using the same layout as one of those already in the arrays /dev/sda.

# sfdisk -d /dev/sda > table
# sfdisk /dev/sdx < table

Assemble the RAID arrays if they are not already assembled:

# mdadm --assemble /dev/md1 /dev/sda1 /dev/sdb1 /dev/sdc1
# mdadm --assemble /dev/md2 /dev/sda2 /dev/sdb2 /dev/sdc2
# mdadm --assemble /dev/md0 /dev/sda3 /dev/sdb3 /dev/sdc3

First, add the new device as a Spare Device to all of the arrays. We will assume you have followed the guide and use separate arrays for /boot RAID 1 (/dev/md1), swap RAID 1 (/dev/md2) and root RAID 5 (/dev/md0).

# mdadm --add /dev/md1 /dev/sdx1
# mdadm --add /dev/md2 /dev/sdx2
# mdadm --add /dev/md0 /dev/sdx3

This should not take long for mdadm to do. Check the progress with:

# cat /proc/mdstat

Check that the device has been added with the command:

# mdadm --misc --detail /dev/md0

It should be listed as a Spare Device.

Tell mdadm to grow the arrays from 3 devices to 4 (or however many devices you want to use):

# mdadm --grow -n 4 /dev/md1
# mdadm --grow -n 4 /dev/md2
# mdadm --grow -n 4 /dev/md0

This will probably take several hours. You need to wait for it to finish before you can continue. Check the progress in /proc/mdstat. The RAID 1 arrays should automatically sync /boot and swap but you need to install Grub on the MBR of the new device manually. Installing_with_Software_RAID_or_LVM#Install_Grub_on_the_Alternate_Boot_Drives

The rest of this guide will explain how to resize the underlying LVM and filesystem on the RAID 5 array.

Note: I am not sure if this can be done with the volumes mounted and will assume you are booting from a live-cd/usb

If you are have encrypted your LVM volumes with LUKS, you need resize the LUKS volume first. Otherwise, ignore this step.

# cryptsetup luksOpen /dev/md0 cryptedlvm
# cryptsetup resize cryptedlvm

Activate the LVM volume groups:

# vgscan
# vgchange -ay

Resize the LVM Physical Volume /dev/md0 (or e.g. /dev/mapper/cryptedlvm if using LUKS) to take up all the available space on the array. You can list them with the command "pvdisplay".

# pvresize /dev/md0

Resize the Logical Volume you wish to allocate the new space to. You can list them with "lvdisplay". Assuming you want to put it all to your /home volume:

# lvresize -l +100%FREE /dev/array/home

To resize the filesystem to allocate the new space use the appropriate tool. If using ext2 you can resize a mounted filesystem with ext2online. For ext3 you can use resize2fs or ext2resize but not while mounted.

You should check the filesystem before resizing.

# e2fsck -f /dev/array/home
# resize2fs /dev/array/home

Read the manuals for lvresize and resize2fs if you want to customize the sizes for the volumes.


A simple one-liner that prints out the status of your Raid devices:

awk '/^md/ {printf "%s: ", $1}; /blocks/ {print $NF}' </proc/mdstat
md1: [UU]
md0: [UU]

Watch mdstat

watch -t 'cat /proc/mdstat'

Or preferably using tmux

tmux split-window -l 12 "watch -t 'cat /proc/mdstat'"

Track IO with iotop

The iotop package lets you view the input/output stats for processes. Use this command to view the IO for raid threads.

iotop -a -p $(sed 's, , -p ,g' <<<`pgrep "_raid|_resync|jbd2"`)

Track IO with iostat

The iostat package lets you view input/output statistics for devices and partitions.

 iostat -dmy 1 /dev/md0
 iostat -dmy 1 # all

Mailing on events

You need a smtp mail server (sendmail) or at least an email forwarder (ssmtp/msmtp). Be sure you have configured an email in /etc/mdadm.conf

# mdadm --monitor --scan --test

When it is ready you can enable the service

# systemctl enable mdadm.service


If you are getting error when you reboot about "invalid raid superblock magic" and you have additional hard drives other than the ones you installed to, check that your hard drive order is correct. During installation, your RAID devices may be hdd, hde and hdf, but during boot they may be hda, hdb and hdc. Adjust your kernel line accordingly. This is what happened to me anyway.

Start arrays read-only

When an md array is started, the superblock will be written, and resync may begin. To start read-only set the kernel module md_mod parameter start_ro. When this is set, new arrays get an 'auto-ro' mode, which disables all internal io (superblock updates, resync, recovery) and is automatically switched to 'rw' when the first write request arrives.

Note: The array can be set to true 'ro' mode using mdadm -r before the first write request, or resync can be started without a write using mdadm -w.

To set the parameter at boot, add md_mod.start_ro=1 to your kernel line.

Or set it at module load time from /etc/modprobe.d/ file or from directly from /sys/.

echo 1 > /sys/module/md_mod/parameters/start_ro

Recovering from a broken or missing drive in the raid

You might get the above mentioned error also when one of the drives breaks for whatever reason. In that case you will have to force the raid to still turn on even with one disk short. Type this (change where needed):

# mdadm --manage /dev/md0 --run

Now you should be able to mount it again with something like this (if you had it in fstab):

# mount /dev/md0

Now the raid should be working again and available to use, however with one disk short! So, to add that one disc partition it the way like described above in Prepare the device. Once that is done you can add the new disk to the raid by doing:

# mdadm --manage --add /dev/md0 /dev/sdd1

If you type:

# cat /proc/mdstat

you probably see that the raid is now active and rebuilding.

You also might want to update your configuration (see: #Update configuration file).


There are several tools for benchmarking a RAID. The most notable improvement is the speed increase when multiple threads are reading from the same RAID volume.

Tiobench specifically benchmarks these performance improvements by measuring fully-threaded I/O on the disk.

Bonnie++ tests database type access to one or more files, and creation, reading, and deleting of small files which can simulate the usage of programs such as Squid, INN, or Maildir format e-mail. The enclosed ZCAV program tests the performance of different zones of a hard drive without writing any data to the disk.

hdparm should NOT be used to benchmark a RAID, because it provides very inconsistent results.

See also


Forum threads

RAID with encryption