293 lines
11 KiB
Groff
293 lines
11 KiB
Groff
.\" $NetBSD: raid.4,v 1.3 1999/02/04 14:50:31 oster Exp $
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.\"
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.\" Copyright (c) 1998 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Greg Oster
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\"
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.\" Copyright (c) 1995 Carnegie-Mellon University.
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.\" All rights reserved.
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.\"
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.\" Author: Mark Holland
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.\"
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.\" Permission to use, copy, modify and distribute this software and
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.\" its documentation is hereby granted, provided that both the copyright
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.\" notice and this permission notice appear in all copies of the
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.\" software, derivative works or modified versions, and any portions
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.\" thereof, and that both notices appear in supporting documentation.
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.\"
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.\" CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
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.\" CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
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.\" FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
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.\"
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.\" Carnegie Mellon requests users of this software to return to
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.\"
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.\" Software Distribution Coordinator or Software.Distribution@CS.CMU.EDU
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.\" School of Computer Science
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.\" Carnegie Mellon University
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.\" Pittsburgh PA 15213-3890
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.\"
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.\" any improvements or extensions that they make and grant Carnegie the
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.\" rights to redistribute these changes.
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.\"
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.Dd November 9, 1998
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.Dt RAID 4
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.Os NetBSD
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.Sh NAME
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.Nm raid
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.Nd RAIDframe Disk Driver
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.Sh SYNOPSIS
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.Cd "pseudo-device raid" Op Ar count
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.Sh DESCRIPTION
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The
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.Nm
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driver provides RAID 0, 1, 4, and 5 (and more!) capabilities to NetBSD. This
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document assumes that the reader has at least some familiarity with RAID
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and RAID concepts. The reader is also assumed to know how to configure
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disks and pseudo-devices into kernels, how to generate kernels, and how
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to partition disks.
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.Pp
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RAIDframe provides a number of different RAID levels including:
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.Bl -tag -width indent
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.It RAID 0
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provides simple data striping across the components.
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.It RAID 1
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provides mirroring.
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.It RAID 4
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provides data striping across the components, with parity
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stored on a dedicated drive (in this case, the last component).
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.It RAID 5
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provides data striping across the components, with parity
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distributed across all the components.
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.El
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.Pp
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There are a wide variety of other RAID levels supported by RAIDframe,
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including Even-Odd parity, RAID level 5 with rotated sparing, Chained
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declustering, and Interleaved declustering. The reader is referred
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to the RAIDframe documentation mentioned in the
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.Sx HISTORY
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section for more detail on these various RAID configurations.
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.Pp
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Depending on the parity level configured, the device driver can
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support the failure of component drives. The number of failures
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allowed depends on the parity level selected. If the driver is able
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to handle drive failures, and a drive does fail, then the system is
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operating in "degraded mode". In this mode, all missing data must be
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reconstructed from the data and parity present on the other
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components. This results in much slower data accesses, but
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does mean that a failure need not bring the system to a complete halt.
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.Pp
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The driver supports
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.Sq hot spares ,
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disks which are on-line, but are not
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actively used in an existing filesystem. Should a disk fail, the
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driver is capable of reconstructing the failed disk onto a hot spare.
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If the components are hot swapable, the failed disk can then be
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removed, a new disk put in it's place, and a copyback operation
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performed. The copyback operation, as it's name indicates, will copy
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the reconstructed data from the hot spare to the previously failed
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(and now replaced) disk.
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.Pp
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If a component cannot be detected when the RAID device is configured,
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that component will be simply marked as 'failed'.
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.Pp
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The user-land utility for doing all
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.Nm
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configuration and other operations
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is
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.Xr raidctl 8 .
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For any of the RAID flavours which have parity data,
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.Xr raidctl 8
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must be used with the
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.Fl r
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option to re-write the data when either a) a new RAID device is
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brought up for the first time or b) after an un-clean shutdown of a
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RAID device. By performing this on-demand recomputation of all parity
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before doing a
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.Xr fsck 8
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or a
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.Xr newfs 8
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filesystem integrity and parity integrity can be ensured. It bears
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repeating again that parity recomputation is
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.Ar required
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before any filesystems are created or used on the RAID device. If the
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parity is not correct, then missing data cannot be correctly recovered.
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.Pp
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RAID levels may be combined in a hierarchical fashion. For example, a RAID 0
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device can be constructed out of a number of RAID 5 devices (which, in turn,
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may be constructed out of the physical disks, or of other RAID devices).
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.Pp
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At the time of this writing, it is
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.Ar imperative
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that drives be
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.Sq nailed down
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at their respective addresses (i.e. not left free-floating, where a
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drive with SCSI ID of 4 can end up as /dev/sd0c). Consider a system
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with three SCSI drives at SCSI ID's 4, 5, and 6, and which map to
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components /dev/sd0e, /dev/sd1e, and /dev/sd2e. If the drive with
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SCSI ID 5 fails,
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and the system reboots, the old /dev/sd2e will show up as /dev/sd1e.
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If the RAID driver is automatically configured, it will only detect
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that /dev/sd2e has failed, and will not notice that /dev/sd2e has
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actually become /dev/sd1e. Hopefully this will change within a few
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days of this writing with the addition of MD5 checksums to each of the
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components.
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.Pp
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The first step to using the
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.Nm
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driver is to ensure that it is suitably configured in the kernel. This is
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done by adding a line similar to:
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.Bd -unfilled -offset indent
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pseudo-device raid 4 # RAIDframe disk device
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.Ed
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.Pp
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to the kernel configuration file. The
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.Sq count
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argument (
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.Sq 4 ,
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in this case), specifies the number of RAIDframe drivers to configure.
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At the time of this writing, 4 is the MAXIMUM of
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.Nm
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devices which are supported. This will change as soon as kernel threads
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are available.
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.Pp
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In all cases the
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.Sq raw
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partitions of the disks
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.Pa must not
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be combined. Rather, each component parition should be offset by at least one
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cylinder from the beginning of that component disk. This ensures that
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the disklabels for the component disks do not conflict with the
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disklabel for the
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.Nm
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device.
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As well, all component paritions must be of the type
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.Dv FS_BSDFFS .
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.Pp
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A more detailed treatment of actually using a
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.Nm
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device is found in
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.Xr raidctl 8 .
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It is highly recommended that the steps to reconstruct, copyback, and
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re-compute parity are well understood by the system administrator(s)
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.Ar before
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a component failure. Doing the wrong thing when a component fails may
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result in data loss.
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.Pp
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.Sh WARNINGS
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Certain RAID levels (1, 4, 5, 6, and others) can protect against some
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data loss due to component failure. However the loss of two
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components of a RAID 4 or 5 system, or the loss of a single component
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of a RAID 0 system, will result in the entire filesystems on that RAID
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device being lost.
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RAID is
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.Ar NOT
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a substitute for good backup practices.
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.Pp
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Recomputation of parity
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.Ar MUST
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be performed whenever there is a chance that it may have been
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compromised. This includes after system crashes, or before a RAID
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device has been used for the first time. Failure to keep parity
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correct will be catastrophic should a component ever fail -- it is
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better to use RAID 0 and get the additional space and speed, than it
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is to use parity, but not keep the parity correct. At least with RAID
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0 there is no perception of increased data security.
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.Pp
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.Sh FILES
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.Bl -tag -width /dev/XXrXraidX -compact
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.It Pa /dev/{,r}raid*
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.Nm
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device special files.
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.El
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.Pp
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.Sh SEE ALSO
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.Xr MAKEDEV 8 ,
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.Xr raidctl 8 ,
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.Xr config 8 ,
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.Xr fsck 8 ,
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.Xr mount 8 ,
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.Xr newfs 8
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.Sh HISTORY
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The
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.Nm
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driver in
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.Nx
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is a port of RAIDframe, a framework for rapid prototyping of RAID
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structures developed by the folks at the Parallel Data Laboratory at
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Carnegie Mellon University (CMU). RAIDframe, as originally distributed
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by CMU, provides a RAID simulator for a number of different
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architectures, and a user-level device driver and a kernel device
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driver for for Digital Unix. The
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.Nm
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driver is a kernelized version of RAIDframe v1.1.
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.Pp
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A more complete description of the internals and functionality of
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RAIDframe is found in the paper "RAIDframe: A Rapid Prototyping Tool
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for RAID Systems", by William V. Courtright II, Garth Gibson, Mark
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Holland, LeAnn Neal Reilly, and Jim Zelenka, and published by the
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Parallel Data Laboratory of Carnegie Mellon University.
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The
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.Nm
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driver first appeared in
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.Nx 1.4 .
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.Sh COPYRIGHT
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.Bd -unfilled
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The RAIDframe Copyright is as follows:
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Copyright (c) 1994-1996 Carnegie-Mellon University.
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All rights reserved.
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Permission to use, copy, modify and distribute this software and
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its documentation is hereby granted, provided that both the copyright
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notice and this permission notice appear in all copies of the
|
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software, derivative works or modified versions, and any portions
|
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thereof, and that both notices appear in supporting documentation.
|
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|
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CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
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CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
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FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
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|
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Carnegie Mellon requests users of this software to return to
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|
|
Software Distribution Coordinator or Software.Distribution@CS.CMU.EDU
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School of Computer Science
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Carnegie Mellon University
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Pittsburgh PA 15213-3890
|
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|
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any improvements or extensions that they make and grant Carnegie the
|
|
rights to redistribute these changes.
|
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|
|
.Ed
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