BIOS RAID support

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Environment

SUSE Linux Enterprise Desktop 10
SUSE Linux Enterprise Server 10

Situation

What is "BIOS RAID" and what support for BIOS RAID is available?

Resolution

Terminology

RAID

RAID, redundant array of inexpensive (or independent) disks, is a storage technology in which multiple disks are combined to act as a single storage unit in order to achieve better performance and/or reliability compared to single disks. A general introduction to RAID is beyond the scope of this TID; the Wikipedia article article "RAID" may serve as a starting point.

Hardware RAID

Hardware RAID is RAID which is implemented wholy inside a storage controller ("RAID card") through specialised hardware and firmware. The controller's BIOS provides an interface for managing the RAID setup. The controller presents the RAID as a single storage device, similar to a single SCSI disk, to the operating system for most purposes. Thus, regular operation of the RAID as storage can be supported by the operating system using a generic driver. Special drivers are only needed to make the RAID configuration and for detailed monitoring of the physical disks in the array.

Software RAID

Software RAID is RAID which is implemented in software outside the storage controller, by an OS driver. That driver takes care of configuration as well as operation of the RAID.

For Linux, software RAID is provided by the md (Multiple Devices) kernel driver and managed using the mdadm tool. Linux software RAID uses a format of its own to store RAID configuration information (the so-called "metadata").

BIOS RAID

BIOS RAID (also known as"quasi-hardware RAID") is a form of software RAID for which the RAID configuration is managed in part or in full by the storage controller's BIOS but which is not true hardware RAID. For BIOS RAID to work, a specific driver is needed at the operating system level.

BIOS RAID is often nicknamed "fake RAID" as it is easily mistaken for hardware RAID and vendor of controllers that offer BIOS RAID often do little to educate buyers about the fact that it is not hardware RAID and that specific OS drivers are necessary for its RAID functionality to function. A somewhat more neutral way of describing it would be "BIOS-assisted software RAID".

Comparing hardware, software and BIOS RAID

Hardware RAID versus software RAID

This comparison is outside the scope of this TID.

BIOS RAID versus hardware RAID

BIOS RAID requires special drivers for regular operation; hardware RAID does not. Special drivers can make the process of recovering from problems like boot failures more complicated.
With BIOS RAID, the CPU is used to maintain the RAID structure. With hardware RAID, this is offloaded to the storage controller and the main CPU is not involved in maintaining the RAID.

BIOS RAID versus Linux software RAID

BIOS RAID is restricted to the devices attached to the controller that offers BIOS RAID. With Linux software RAID, the RAID can be built from all kinds of block devices, including disks attached to controllers that do not provide RAID by themselves, or disks attached to different controllers.
For BIOS RAID, a driver specific to the controller is needed whereas Linux software RAID is supported by a generic driver (md).
The on-disk layout of a BIOS RAID is vendor-specific. If the storage controller fails, a replacement controller of the same make may be needed to regain access to data on the RAID. With Linux software RAID, the on-disk layout is independent of the storage controller, so when a storage controller hosting (part of) a Linux software RAID fails, access to the data can be regained using a replacement controller that does not need to be of the same make as the controller that is defect.
Data stored using a suitable file system on BIOS RAID can be shared easily with Microsoft Windows on dual boot Linux + Windows systems.

Support for BIOS RAID

The "dmraid" package ("A Device-Mapper Software RAID Support Tool") included in SUSE Linux Enterprise Server 10 and SUSE Linux Enterprise Desktop 10 can be used to discover, activate, deactivate and display properties of several types of BIOS RAID sets:

Highpoint HPT37X
Highpoint HPT45X
Intel Software RAID
Promise FastTrack
Silicon Image Medley

According to the README file included in the dmraid package, this software is not mature and should be used with care.

Disclaimer

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