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 NetBSD. 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 it's assigned place in the set (e.g. the component label claims the component should be the 3rd one 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.

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 replacment drive. If the components are hot swapable, the failed disk can then be removed, a new disk put in it's place, and a copyback operation performed. The copyback operation, as it's 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). For any of the RAID flavours which have parity data, raidctl(8) must be used with the -i option to re-write the data 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 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).

At the time of this writing, it is imperative that drives be ‘nailed down’ at their respective addresses (i.e. not left free-floating, where a drive with SCSI ID of 4 can end up as /dev/sd0c). 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. If the drive with SCSI ID 5 fails, and the system reboots, the old /dev/sd2e will show up as /dev/sd1e. If the RAID driver is automatically configured, it will only detect that /dev/sd2e has failed, and will not notice that /dev/sd2e has actually become /dev/sd1e. Hopefully this will change within a few days of this writing with the addition of MD5 checksums to each of the components.

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. At the time of this writing, 4 is the MAXIMUM of raid devices which are supported. This will change as soon as kernel threads are available.

In all cases the ‘raw’ partitions of the disks must not be combined. Rather, each component parition should be offset by at least one cylinder from the beginning of that component disk. This ensures that the disklabels for the component disks do not conflict with the disklabel for the raid device. As well, all component paritions must be of the type FS_BSDFFS.

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.

WARNINGS
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.

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

SEE ALSO
MAKEDEV(8), raidctl(8), config(8), fsck(8), mount(8), newfs(8)

HISTORY
The raid driver in NetBSD 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 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.

COPYRIGHT

    The RAIDframe Copyright is as follows:
    Copyright (c) 1994-1996 Carnegie-Mellon University.
    All rights reserved.
    Permission to use, copy, modify and distribute this software and
    its documentation is hereby granted, provided that both the copyright
    notice and this permission notice appear in all copies of the
    software, derivative works or modified versions, and any portions
    thereof, and that both notices appear in supporting documentation.
    CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
    CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
    FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
    Carnegie Mellon requests users of this software to return to
     Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
     School of Computer Science
     Carnegie Mellon University
     Pittsburgh PA 15213-3890
    any improvements or extensions that they make and grant Carnegie the
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