RAID(4)

NAME
raid - RAIDframe disk driver

SYNOPSIS
pseudo-device raid [count]

DESCRIPTION
The raid driver provides RAID 0, 1, 4, and 5 (and more!) capabilities to OpenBSD. This document assumes that the reader has at least some familiarity with RAID and RAID concepts. The reader is also assumed to know how to configure disks and pseudo-devices into kernels, how to generate kernels, and how to partition disks.

RAIDframe provides a number of different RAID levels including:

RAID 0
    provides simple data striping across the components.

RAID 1
    provides mirroring.

RAID 4
    provides data striping across the components, with parity stored on a dedicated drive (in this case, the last component).

RAID 5
    provides data striping across the components, with parity distributed across all the components.

There are a wide variety of other RAID levels supported by RAIDframe, including Even-Odd parity, RAID level 5 with rotated sparing, Chained declustering, and Interleaved declustering. The reader is referred to the RAIDframe documentation mentioned in the HISTORY section for more detail on these various RAID configurations.

Depending on the parity level configured, the device driver can support the failure of component drives. The number of failures allowed depends on the parity level selected. If the driver is able to handle drive failures, and a drive does fail, then the system is operating in "degraded mode". In this mode, all missing data must be reconstructed from the data and parity present on the other components. This results in much slower data accesses, but does mean that a failure need not bring the system to a complete halt.

The RAID driver supports and enforces the use of ‘component labels’. A ‘component label’ contains important information about the component, including a user-specified serial number, the row and column of that component in the RAID set, and whether the data (and parity) on the component is ‘clean’. If the driver determines that the labels are very inconsistent with respect to each other (e.g. two or more serial numbers do not match) or that the component label is not consistent with its assigned place in the set (e.g., the component label claims the component should be the 3rd one of a 6-disk set, but the RAID set has it as the 3rd component in a 5-disk set) then the device will fail to configure. If the driver determines that exactly one component label seems to be incorrect, and the RAID set is being configured as a set that supports a single failure, then the RAID set will be allowed to configure, but the incorrectly labeled component will be marked as ‘failed’, and the RAID set will begin operation in degraded mode. If all of the components are consistent among themselves, the RAID set will configure normally.

Component labels are also used to support the auto-detection and auto-configuration of RAID sets. A RAID set can be flagged as auto-configurable, in which case it will be configured automatically during the kernel boot process. RAID filesystems which are automatically configured are also eligible to be the root filesystem. There is currently no support for booting a kernel directly from a RAID set. To use a RAID set as the root filesystem, a kernel is usually obtained from a small non-RAID partition, after which any auto-configuring RAID set can be used for the root filesystem. See raidctl(8) for more information on auto-configuration of RAID sets.

The driver supports ‘hot spares’, disks which are on-line, but are not actively used in an existing filesystem. Should a disk fail, the driver is capable of reconstructing the failed disk onto a hot spare or back onto a replacement drive. If the components are hot swapable, the failed disk can then be removed, a new disk put in its place, and a copyback operation performed. The copyback operation, as its name indicates, will copy the reconstructed data from the hot spare to the previously failed (and now replaced) disk. Hot spares can also be hot-added using raidctl(8).

If a component cannot be detected when the RAID device is configured, that component will be simply marked as 'failed'.

The user-land utility for doing all raid configuration and other operations is raidctl(8). Most importantly, raidctl(8) must be used with the -i option to initialize all RAID sets. In particular, this initialization includes re-building the parity data. This rebuilding of parity data is also required when either a) a new RAID device is brought up for the first time or b) after an un-clean shutdown of a RAID device. By using the -P option to raidctl(8), and performing this on-demand recomputation of all parity before doing a fsck(8) or a newfs(8), filesystem integrity and parity integrity can be ensured. It bears repeating again that parity recomputation is required before any filesystems are created or used on the RAID device. If the parity is not correct, then missing data cannot be correctly recovered.

RAID levels may be combined in a hierarchical fashion. For example, a RAID 0 device can be constructed out of a number of RAID 5 devices (which, in turn, may be constructed out of the physical disks, or of other RAID devices).

It is important that drives be hard-coded at their respective addresses (i.e., not left free-floating, where a drive with SCSI ID of 4 can end up as /dev/sd0c) for well-behaved functioning of the RAID device. This is true for all types of drives, including IDE, HP-IB, etc. For normal SCSI drives, for example, the following can be used to fix the device addresses:

    sd0     at scsibus0 target 0       # SCSI disk drives
    sd1     at scsibus0 target 1       # SCSI disk drives
    sd2     at scsibus0 target 2       # SCSI disk drives
    sd3     at scsibus0 target 3       # SCSI disk drives
    sd4     at scsibus0 target 4       # SCSI disk drives
    sd5     at scsibus0 target 5       # SCSI disk drives
    sd6     at scsibus0 target 6       # SCSI disk drives

See sd(4) for more information. The rationale for fixing the device addresses is as follows: Consider a system with three SCSI drives at SCSI ID's 4, 5, and 6, and which map to components /dev/sd0e, /dev/sd1e, and /dev/sd2e of a RAID 5 set. If the drive with SCSI ID 5 fails, and the system reboots, the old /dev/sd2e will show up as /dev/sd1e. The RAID driver is able to detect that component positions have changed, and will not allow normal configuration. If the device addresses are hard coded, however, the RAID driver would detect that the middle component is unavailable, and bring the RAID 5 set up in degraded mode. Note that the auto-detection and auto-configuration code does not care about where the components live. The auto-configuration code will correctly configure a device even after any number of the components have been re-arranged.

The first step to using the raid driver is to ensure that it is suitably configured in the kernel. This is done by adding a line similar to:

    pseudo-device   raid   4       # RAIDframe disk device

to the kernel configuration file. The ‘count’ argument ( ‘4’, in this case), specifies the number of RAIDframe drivers to configure. To turn on component auto-detection and auto-configuration of RAID sets, simply add:

    option	RAID_AUTOCONFIG

to the kernel configuration file.

All component partitions must be of the type FS_BSDFFS (e.g., 4.2BSD) or FS_RAID (e.g., RAID). The use of the latter is strongly encouraged, and is required if auto-configuration of the RAID set is desired. Since RAIDframe leaves room for disklabels, RAID components can be simply raw disks, or partitions which use an entire disk. Note that some platforms (such as SUN) do not allow using the FS_RAID partition type. On these platforms, the raid driver can still auto-configure from FS_BSDFFS partitions.

A more detailed treatment of actually using a raid device is found in raidctl(8). It is highly recommended that the steps to reconstruct, copyback, and re-compute parity are well understood by the system administrator(s) before a component failure. Doing the wrong thing when a component fails may result in data loss.

Additional debug information can be sent to the console by specifying:

    option	RAIDDEBUG

FILES
/dev/{,r}raid*
    raid device special files.

SEE ALSO
ccd(4), sd(4), wd(4), config(8), fsck(8), MAKEDEV(8), mount(8), newfs(8), raidctl(8)

HISTORY
The raid driver in OpenBSD is a port of RAIDframe, a framework for rapid prototyping of RAID structures developed by the folks at the Parallel Data Laboratory at Carnegie Mellon University (CMU). RAIDframe, as originally distributed by CMU, provides a RAID simulator for a number of different architectures, and a user-level device driver and a kernel device driver for Digital UNIX. The raid driver is a kernelized version of RAIDframe v1.1.

A more complete description of the internals and functionality of RAIDframe is found in the paper "RAIDframe: A Rapid Prototyping Tool for RAID Systems", by William V. Courtright II, Garth Gibson, Mark Holland, LeAnn Neal Reilly, and Jim Zelenka, and published by the Parallel Data Laboratory of Carnegie Mellon University. The raid driver first appeared in NetBSD 1.4 from where it was ported to OpenBSD 2.5.

CAVEATS
Certain RAID levels (1, 4, 5, 6, and others) can protect against some data loss due to component failure. However the loss of two components of a RAID 4 or 5 system, or the loss of a single component of a RAID 0 system, will result in the entire filesystems on that RAID device being lost. RAID is NOT a substitute for good backup practices.

Recomputation of parity MUST be performed whenever there is a chance that it may have been compromised. This includes after system crashes, or before a RAID device has been used for the first time. Failure to keep parity correct will be catastrophic should a component ever fail -- it is better to use RAID 0 and get the additional space and speed, than it is to use parity, but not keep the parity correct. At least with RAID 0 there is no perception of increased data security.