There are three possible RAID approaches: Firmware RAID, Hardware RAID and Software RAID.
Firmware RAID (also known as ATARAID) is a type of software RAID where the RAID sets can be configured using a firmware-based menu. The firmware used by this type of RAID also hooks into the BIOS, allowing you to boot from its RAID sets. Different vendors use different on-disk metadata formats to mark the RAID set members. The Intel Matrix RAID is a good example of a firmware RAID system.
The hardware-based array manages the RAID subsystem independently from the host. It presents a single disk per RAID array to the host.
A Hardware RAID device may be internal or external to the system, with internal devices commonly consisting of a specialized controller card that handles the RAID tasks tranparently to the operating system and with external devices commonly connecting to the system via SCSI, fiber channel, iSCSI, InfiniBand, or other high speed network interconnect and presenting logical volumes to the system.
RAID controller cards function like a SCSI controller to the operating system, and handle all the actual drive communications. The user plugs the drives into the RAID controller (just like a normal SCSI controller) and then adds them to the RAID controllers configuration, and the operating system won't know the difference.
Software RAID implements the various RAID levels in the kernel disk (block device) code. It offers the cheapest possible solution, as expensive disk controller cards or hot-swap chassis [] are not required. Software RAID also works with cheaper IDE disks as well as SCSI disks. With today's faster CPUs, Software RAID also generally outperforms Hardware RAID.
The Linux kernel contains a multi-disk (MD) driver that allows the RAID solution to be completely hardware independent. The performance of a software-based array depends on the server CPU performance and load.
Here are some of the key features of the Linux software RAID stack:
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Multi-threaded design
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Portability of arrays between Linux machines without reconstruction
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Backgrounded array reconstruction using idle system resources
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Hot-swappable drive support
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Automatic CPU detection to take advantage of certain CPU features such as streaming SIMD support
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Automatic correction of bad sectors on disks in an array
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Regular consistency checks of RAID data to ensure the health of the array
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Proactive monitoring of arrays with email alerts sent to a designated email address on important events
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Write-intent bitmaps which drastically speed resync events by allowing the kernel to know precisely which portions of a disk need to be resynced instead of having to resync the entire array
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Resync checkpointing so that if you reboot your computer during a resync, at startup the resync will pick up where it left off and not start all over again
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The ability to change parameters of the array after installation. For example, you can grow a 4-disk raid5 array to a 5-disk raid5 array when you have a new disk to add. This grow operation is done live and does not require you to reinstall on the new array.
