Difference between revisions of "Installing with Fake RAID"

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{{Note|1=With dmraid >= 1.0.0.rc15-8, partitions are labeled "raidSet'''p1''', raidSet'''p2''', etc. instead of '''raidSet1''', '''raidSet2''', etc. If the setup command fails with "error 22: No such partition", temporary symlinks must be created.
{{Note|1=With dmraid >= 1.0.0.rc15-8, partitions are labeled "raidSet'''p1''', raidSet'''p2''', etc. instead of raidSet'''1''', raidSet'''2''', etc. If the setup command fails with "error 22: No such partition", temporary symlinks must be created.
The problem is that GRUB is looking at your {{Filename|menu.lst}} and sees {{Filename|/dev/mapper/raidSetp1}}, for example, but still uses an older detection algorithm, and is looking for {{Filename|/dev/mapper/raidSet1}}.
The problem is that GRUB is looking at your {{Filename|menu.lst}} and sees {{Filename|/dev/mapper/raidSetp1}}, for example, but still uses an older detection algorithm, and is looking for {{Filename|/dev/mapper/raidSet1}}.

Revision as of 21:16, 30 November 2009

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The purpose of this guide is to enable use of a RAID set created by the on-board BIOS RAID controller and thereby allow dual-booting of Linux and Windows from partitions inside the RAID set using GRUB. When using so-called "fake RAID" or "host RAID", the disc sets are reached from Template:Filename and not Template:Filename.

What is "fake RAID"

From Wikipedia:

Operating system-based RAID doesn't always protect the boot process and is generally impractical on desktop versions of Windows. Hardware RAID controllers are expensive and proprietary. To fill this gap, cheap "RAID controllers" were introduced that do not contain a RAID controller chip, but simply a standard disk controller chip with special firmware and drivers. During early stage boot-up, the RAID is implemented by the firmware. When a protected-mode operating system kernel such as Linux or a modern version of Microsoft Windows is loaded, the drivers take over.
These controllers are described by their manufacturers as RAID controllers, and it is rarely made clear to purchasers that the burden of RAID processing is borne by the host computer's central processing unit -- not the RAID controller itself -- thus introducing the aforementioned CPU overhead which hardware controllers don't suffer from. Firmware controllers often can only use certain types of hard drives in their RAID arrays (e.g. SATA for Intel Matrix RAID, as there is neither SCSI nor PATA support in modern Intel ICH southbridges; however, motherboard makers implement RAID controllers outside of the southbridge on some motherboards). Before their introduction, a "RAID controller" implied that the controller did the processing, and the new type has become known in technically knowledgeable circles as "fake RAID" even though the RAID itself is implemented correctly. Adaptec calls them "host RAID".[1]

See Wikipedia:RAID or FakeRaidHowto @ Community Ubuntu Documentation for more information.

Despite the terminology, "fake RAID" via Template:Package Official is a robust software RAID implementation that offers a solid system to mirror or stripe data across multiple disks with negligible overhead for any modern system. dmraid is comparable to mdraid (pure Linux software RAID) with the added benefit of being able to completely rebuild a drive after a failure before the system is ever booted.


In Linux 2.4, the ATARAID kernel framework provided support for fake RAID (software RAID assisted by the BIOS). For Linux 2.6 the device-mapper framework can, among other nice things like LVM and EVMS, do the same kind of work as ATARAID in 2.4. Whilst the new code handling the RAID I/O still runs in the kernel, device-mapper is generally configured by a userspace application. It was clear that when using the device-mapper for RAID, detection would go to userspace.

Heinz Maulshagen created the dmraid tool to detect RAID sets and create mappings for them. The controllers supported are (mostly cheap) fake RAID IDE/SATA controllers which contain BIOS functions. Common examples include: Promise FastTrak controllers; HighPoint HPT37x; Intel Matrix RAID; Silicon Image Medley; and NVIDIA nForce.

Supported hardware

  • Tested with ICH10R on 2009.08 (x86_64) -- pointone 23:10, 29 November 2009 (EST)
  • Tested with Sil3124 on 2009.02 (i686) -- loosec
  • Tested with nForce4 on Core Dump (i686 and x86_64) -- loosec
  • Tested with Sil3512 on Overlord (x86_64) -- loosec

For more information on supported hardware, see RAID/Onboard @ Gentoo Linux Wiki


Warning: Backup all data before playing with RAID. What you do with your hardware is only your own fault. Data on RAID stripes is highly vulnerable to disc failures. Create regular backups or consider using mirror sets. Consider yourself warned!


  • Preparation
  • Boot the installer
  • Install dmraid
  • Perform traditional installation
  • Install GRUB


Configure RAID sets

Warning: If your drives are not already configured as RAID and Windows is already installed, switching to "RAID" may cause Windows to BSOD during boot.[2]
  • Enter your BIOS setup and enable the RAID controller.
    • The BIOS may contain an option to configure SATA drives as "IDE", "AHCI", or "RAID"; ensure "RAID" is selected.
  • Save and exit the BIOS setup. During boot, enter the RAID setup utility.
    • The RAID utility is usually either accessible via the boot menu (often F8 or F10) or whilst the RAID controller is initializing.
  • Use the RAID setup utility to create preferred stripe/mirror sets.
Tip: See your motherboard documentation for details. The exact procedure may vary.

Boot the installer

See Official Arch Linux Install Guide#Pre-Installation for details.

Tip: If your screen can handle it, consider adding Template:Codeline as a boot option for a higher framebuffer resolution; there are some long lines involved here.

Install dmraid

Load device-mapper; install dmraid package and find RAID sets:

# modprobe dm_mod
# pacman -S dmraid*
# dmraid -ay
# ls -la /dev/mapper/

Example output:

/dev/mapper/control            <- Created by device-mapper; if present, device-mapper is likely functioning
/dev/mapper/sil_aiageicechah   <- A RAID set on a Silicon Image SATA RAID controller
/dev/mapper/sil_aiageicechah1  <- First partition on this RAID Set

If there is only one file (Template:Filename), check if your controller chipset module is loaded with Template:Codeline. If it is, then dmraid does not support this controller or there are no RAID sets on the system (check RAID BIOS setup again). If correct, then you may be forced to use software RAID (this means no dual-booted RAID system on this controller).

If your chipset module is NOT loaded, load it now. For example:

# modprobe sata_sil

See Template:Filename for available drivers.

To test the RAID sets:

# dmraid -tay

Perform traditional installation

Switch to tty2 and start the installer:

# /arch/setup

Partition the RAID set

  • Under Prepare Hard Drive choose Manually partition hard drives since the Auto-prepare option will not find your RAID sets.
  • Choose OTHER and type in your RAID set's full path (e.g. Template:Filename). Switch back to tty1 to check your spelling.
  • Create the proper partitions the normal way.
Tip: This would be a good time to install the "other" OS if planning to dual-boot. If installing Windows XP to "C:" then all partitions before the Windows partition should be changed to type [1B] (hidden FAT32) to hide them during the Windows installation. When this is done, change them back to type [83] (Linux). Of course, a reboot unfortunately requires some of the above steps to be repeated.

Mounting the filesystem

If -- and this is probably the case -- you do not find your newly created partitions under Manually configure block devices, filesystems and mountpoints:

  • Switch back to tty1.
  • Deactivate all device-mapper nodes:
# dmsetup remove_all
  • Reactivate the newly-created RAID nodes:
# dmraid -ay
# ls -la /dev/mapper
  • Switch to tty2, re-enter the Manually configure block devices, filesystems and mountpoints menu and the partitions should be available.

Install and configure Arch

Tip: Utilize three consoles: the setup GUI to configure the system, a chroot to install GRUB, and finally a cfdisk reference since RAID sets have weird names.
  • tty1: chroot and grub-install
  • tty2: /arch/setup
  • tty3: cfdisk for a reference in spelling, partition table and geometry of the RAID set
Leave programs running and switch to when needed.

Re-activate the installer (tty2) and proceed as normal with the following exceptions:

  • Select Packages
    • Ensure dmraid is marked for installation
  • Configure System
    • Add dm_mod to the MODULES line in Template:Filename. If using a mirrored (RAID 1) array, additionally add dm_mirror
    • Add chipset_module_driver to the MODULES line if necessary
    • Add dmraid to the HOOKS line in Template:Filename; preferably after sata but before filesystems

Install GRUB

Warning: You can normally specify default saved instead of a number in Template:Filename so that the default entry is the entry saved with the command savedefault. If you are using dmraid do not use savedefault or your array will de-sync and will not let you boot your system.

Please read GRUB for more information about configuring GRUB. Installation is begun by selecting Install Bootloader from the Arch installer.

Note: For an unknown reason, the default Template:Filename will likely be incorrectly populated when installing via fake RAID. Double-check the root lines (e.g. Template:Codeline). Additionally, if you did not create a separate Template:Filename partition, ensure the kernel/initrd paths are correct (e.g. Template:Filename and Template:Filename instead of Template:Filename and Template:Filename.

For example, if you created logical partitions (creating the equivalent of sda5, sda6, sda7, etc.) that were mapped as:

  /dev/mapper     |    Linux    GRUB Partition
                  |  Partition      Number
nvidia_fffadgic   |
nvidia_fffadgic5  |    /              4
nvidia_fffadgic6  |    /boot          5
nvidia_fffadgic7  |    /home          6

The correct root designation would be (hd0,5) in this example.

Note: If you use more than one set of dmraid arrays or multiple Linux distributions installed on different dmraid arrays (for example 2 disks in nvidia_fdaacfde and 2 disks in nvidia_fffadgic and you are installing to the second dmraid array (nvidia_fffadgic)), you will need designate the second array's Template:Filename partition as the GRUB root. In the example above, if nvidia_fffadgic was the second dmraid array you were installing to, your root designation would be root (hd1,5).

After saving the configuration file, the GRUB installer will FAIL. However it will still copy files to Template:Filename. DO NOT GIVE UP AND REBOOT -- just follow the directions below:

  • Switch to tty1 and chroot into our installed system:
# mount -o bind /dev /mnt/dev
# mount -t proc none /mnt/proc
# mount -t sysfs none /mnt/sys
# chroot /mnt /bin/bash
  • Switch to tty3 and look up the geometry of the RAID set:
    • The number of Cylinders, Heads and Sectors on the RAID set should be written at the top of the screen inside cfdisk.
Example: 18079 255 63 for a RAID stripe of two 74GB Raptor discs.
Example: 38914 255 63 for a RAID stripe of two 160GB laptop discs.
  • GRUB will fail to properly read the drives; the geometry command must be used to manually direct GRUB:
# dmsetup mknodes
# grub --device-map=/dev/null

grub> device (hd0) /dev/mapper/raidSet
grub> geometry (hd0) C H S

Exchange C H S above with the proper numbers (be aware: they are not entered in the same order as they are read from cfdisk).

If geometry is entered properly, GRUB will list partitions found on this RAID set. Then, continue to install the bootloader into the Master Boot Record, changing "hd0" to "hd1" if required.

grub> root (hd0,0)
grub> setup (hd0)
grub> quit
Note: With dmraid >= 1.0.0.rc15-8, partitions are labeled "raidSetp1, raidSetp2, etc. instead of raidSet1, raidSet2, etc. If the setup command fails with "error 22: No such partition", temporary symlinks must be created.

The problem is that GRUB is looking at your Template:Filename and sees Template:Filename, for example, but still uses an older detection algorithm, and is looking for Template:Filename.

The solution is to create a symlink from Template:Filename to Template:Filename (changing the partition number as needed).

Lastly, if you have multiple dmraid devices with multiple sets of arrays set up (say: nvidia_fdaacfde and nvidia_fffadgic), then create the Template:Filename file to help GRUB retain its sanity when working with the arrays. All the file does is map the dmraid device to a traditional hd#. Using these dmraid devices, your device.map file will look like this:

(hd0) /dev/mapper/nvidia_fdaacfde
(hd1) /dev/mapper/nvidia_fffadgic

And now you are finished with the installation!

# reboot