Standard levels
A number of standard schemes have evolved which are referred to as levels. There were five RAID levels originally conceived, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary).
Many storage controllers allow RAID levels to be nested: the elements of a RAID may be either individual disks or RAIDs themselves. Nesting more than two deep is unusual.
As there is no basic RAID level numbered larger than 10, nested RAIDs are usually unambiguously described by concatenating the numbers indicating the RAID levels, sometimes with a "+" in between. For example, RAID 10 (or RAID 1+0) consists of several level 1 arrays of physical drives, each of which is one of the "drives" of a level 0 array striped over the level 1 arrays. It is not called RAID 01, to avoid confusion with RAID 1, or indeed, RAID 01. When the top array is a RAID 0 (such as in RAID 10 and RAID 50) most vendors omit the "+", though RAID 5+0 is clearer.
- RAID 0+1: striped sets in a mirrored set (minimum four disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost.
- RAID 1+0: mirrored sets in a striped set (minimum four disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. In a failed disk situation, RAID 1+0 performs better because all the remaining disks continue to be used. The array can sustain multiple drive losses so long as no mirror loses all its drives.
- RAID 5+0: stripe across distributed parity RAID systems.
- RAID 5+1: mirror striped set with distributed parity (some manufacturers label this as RAID 53).
[edit] Non-standard levels
Many configurations other than the basic numbered RAID levels are possible, and many companies, organizations, and groups have created their own non-standard configurations, in many cases designed to meet the specialised needs of a small niche group. Most of these non-standard RAID levels are proprietary.
Some of the more prominent modifications are:
- Storage Computer Corporation uses RAID 7, which adds caching to RAID 3 and RAID 4 to improve I/O performance.
- EMC Corporation offered RAID S as an alternative to RAID 5 on their Symmetrix systems (which is no longer supported on the latest releases of Enginuity, the Symmetrix's operating system).
- The ZFS filesystem, available in Solaris, OpenSolaris, FreeBSD and Mac OS X, offers RAID-Z, which solves RAID 5's write hole problem.
- Hewlett-Packard's Advanced Data Guarding (ADG) is a form of RAID 6.
- NetApp's Data ONTAP uses RAID-DP (also referred to as "double", "dual" or "diagonal" parity), which is a form of RAID 6, but unlike many RAID 6 implementations, does not use distributed parity as in RAID 5. Instead, two unique parity disks with separate parity calculations are used. This is a modification of RAID 4 with an extra parity disk.
- Accusys Triple Parity (RAID TP) implements three independent parities by extending RAID 6 algorithms on its FC-SATA and SCSI-SATA RAID controllers to tolerate three-disk failure.
- Linux MD RAID10 (RAID10) implements a general RAID driver that defaults to a standard RAID 1+0 with 4 drives, but can have any number of drives. MD RAID10 can run striped and mirrored with only 2 drives with the f2 layout (mirroring with striped reads, normal Linux software RAID 1 does not stripe reads, but can read in parallel).[4]
- Infrant (Now part of Netgear) X-RAID offers dynamic expansion of a RAID5 volume without having to backup/restore the existing content. Just add larger drives one at a time, let it resync, then add the next drive until all drives are installed. The resulting volume capacity is increased without user downtime. (It should be noted that this is also possible in Linux, when utilizing Mdadm utility. It has also been possible in the EMC Clariion for several years.)
- BeyondRAID created by Data Robotics and used in the Drobo series of products, implements both mirroring and striping simultaneously or individually dependent on disk and data context. It offers expandability without reconfiguration, the ability to mix and match drive sizes and the ability to reorder disks. It supports NTFS, HFS+, FAT32, and EXT3 file systems[5]. It also utilizes Thin provisioning to allow for single volumes up to 16TB depending on the host operating system support.
[edit] Implementations
 | It has been suggested that Vinum volume manager be merged into this article or section. (Discuss) |
(Specifically, the section comparing hardware / software raid)
The distribution of data across multiple drives can be managed either by dedicated hardware or by software. When done in software the software may be part of the operating system or it may be part of the firmware and drivers supplied with the card.
[edit] Operating system based ("software RAID")
Software implementations are now provided by many operating systems. A software layer sits above the (generally block-based) disk device drivers and provides an abstraction layer between the logical drives (RAIDs) and physical drives. Most common levels are RAID 0 (striping across multiple drives for increased space and performance) and RAID 1 (mirroring two drives), followed by RAID 1+0, RAID 0+1, and RAID 5 (data striping with parity) are supported.
- Apple's Mac OS X Server supports RAID 0, RAID 1, RAID 5 and RAID 1+0.[6]
- FreeBSD and MidnightBSD support RAID 0, RAID 1, RAID 3, and RAID 5 and all layerings of the above via GEOM modules[7][8] and ccd.[9], as well as supporting RAID 0, RAID 1, RAID-Z, and RAID-Z2 (similar to RAID-5 and RAID-6 respectively), plus nested combinations of those via ZFS.
- Microsoft's server operating systems support 3 RAID levels; RAID 0, RAID 1, and RAID 5. Some of the Microsoft desktop operating systems support RAID such as Windows XP Professional which supports RAID level 0 in addition to spanning multiple disks but only if using dynamic disks and volumes. Windows XP supports RAID 0, 1, and 5 with a simple file patch [12].
- NetBSD supports RAID 0, RAID 1, RAID 4 and RAID 5 (and any nested combination of those like 1+0) via its software implementation, named raidframe.
- OpenBSD supports RAID 0 and RAID 1 via its software implementation softraid.
- OpenSolaris and Solaris 10 supports RAID 0, RAID 1, RAID 5 (or the similar "RAID Z" found only on ZFS), and RAID 6 (and any nested combination of those like 1+0) via ZFS and now has the ability to boot from a ZFS volume on both x86 and ultrasparc. Through SVM, Solaris 10 and earlier versions support RAID 0, RAID 1, and RAID 5 on both system and data drives
Software RAID has advantages and disadvantages compared to hardware RAID. The software must run on a host server attached to storage, and server's processor must dedicate processing time to run the RAID software. This is negligible for RAID 0 and RAID 1, but may become significant when using parity-based arrays and either accessing several arrays at the same time or running many disks. Furthermore all the busses between the processor and the disk controller must carry the extra data required by RAID which may cause congestion.
Another concern with operating system-based RAID is the boot process. It can be difficult or impossible to set up the boot process such that it can fail over to another drive if the usual boot drive fails. Such systems can require manual intervention to make the machine bootable again after a failure. There are exceptions to this, such as the LILO bootloader for Linux, loader for FreeBSD[13] , and some configurations of the GRUB bootloader natively understand RAID-1 and can load a kernel. If the BIOS recognizes a broken first disk and refers bootstrapping to the next disk, such a system will come up without intervention, but the BIOS might or might not do that as intended. A hardware RAID controller typically has explicit programming to decide that a disk is broken and fall through to the next disk.
Hardware RAID controllers can also carry battery-powered cache memory. For data safety in modern systems the user of software RAID might need to turn the write-back cache on the disk off (but some drives have their own battery/capacitors on the write-back cache, a UPS, and/or implement atomicity in various ways, etc). Turning off the write cache has a performance penalty that can, depending on workload and how well supported command queuing in the disk system is, be significant. The battery backed cache on a RAID controller is one solution to have a safe write-back cache.
Finally operating system-based RAID usually uses formats specific to the operating system in question so it cannot generally be used for partitions that are shared between operating systems as part of a multi-boot setup. However, this allows RAID disks to be moved from one computer to a computer with an operating system or file system of the same type, which can be more difficult when using hardware RAID (e.g. #1: When one computer uses a hardware RAID controller from one manufacturer and another computer uses a controller from a different manufacturer, drives typically cannot be interchanged. e.g. #2: If the hardware controller 'dies' before the disks do, data may become unrecoverable unless a hardware controller of the same type is obtained, unlike with firmware-based or software-based RAID).
Most operating system-based implementations allow RAIDs to be created from partitions rather than entire physical drives. For instance, an administrator could divide an odd number of disks into two partitions per disk, mirror partitions across disks and stripe a volume across the mirrored partitions to emulate IBM's RAID 1E configuration and more (see [Linux mdadm section] of Non-standard RAID levels). Using partitions in this way also allows mixing reliability levels on the same set of disks. For example, one could have a very robust RAID 1 partition for important files, and a less robust RAID 5 or RAID 0 partition for less important data. (Some BIOS-based controllers offer similar features, e.g. Intel Matrix RAID.) Using two partitions on the same drive in the same RAID is, however, dangerous. (e.g. #1: Having all partitions of a RAID-1 on the same drive will, obviously, make all the data inaccessible if the single drive fails. e.g. #2: In a RAID 5 array composed of four drives 250 + 250 + 250 + 500 GB, with the 500-GB drive split into two 250 GB partitions, a failure of this drive will remove two partitions from the array, causing all of the data held on it to be lost).
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