219 lines
7.3 KiB
Groff
219 lines
7.3 KiB
Groff
.\" $NetBSD: ccd.4,v 1.16 1998/07/09 20:53:42 thorpej Exp $
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.\"
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.\" Copyright (c) 1996, 1997 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 Jason R. Thorpe.
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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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.\" Copyright (c) 1994 Jason Downs.
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.\" All rights reserved.
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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 for the NetBSD Project
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.\" by Jason Downs and Jason R. Thorpe.
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.\" 4. Neither the name of the author nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
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.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd August 9, 1995
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.Dt CCD 4
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.Os NetBSD
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.Sh NAME
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.Nm ccd
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.Nd Concatenated Disk Driver
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.Sh SYNOPSIS
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.Cd "pseudo-device ccd" 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 the capability of combining one or more disks/partitions
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into one virtual disk.
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.Pp
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This document assumes that you're familiar with how to generate kernels,
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how to properly configure disks and pseudo-devices in a kernel
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configuration file, and how to partition disks.
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.Pp
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Note that 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.
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Each component partition should be offset at least one cylinder
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from the beginning of the component disk.
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This avoids potential conflicts between the component disk's
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disklabel and the
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.Nm Ns 's
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disklabel.
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The kernel will only allow component partitions of type
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.Dv FS_BSDFFS .
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.Pp
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In order to compile in support for the
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.Nm "" ,
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you must add a line similar
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to the following to your kernel configuration file:
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.Bd -unfilled -offset indent
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pseudo-device ccd 4 # concatenated disk devices
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.Ed
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.Pp
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The count argument is how many
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.Nm Ns s
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memory is allocated for at boot time.
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In this example, no more than 4
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.Nm Ns s
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may be configured.
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.Pp
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A
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.Nm
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may be either serially concatenated or interleaved.
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To serially concatenate the partitions, specify the interleave factor of 0.
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.Pp
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If a
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.Nm
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is interleaved correctly, a
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.Dq striping
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effect is achieved, which can increase performance.
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The optimum interleave factor is typically the size of a track.
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Since the interleave factor is expressed in units of
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.Dv DEV_BSIZE ,
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one must account for sector sizes other than
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.Dv DEV_BSIZE
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in order to calculate the correct interleave.
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The kernel will not allow an interleave factor less than the size
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of the largest component sector divided by
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.Dv DEV_BSIZE .
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.Pp
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Note that best performance is achieved if all component disks have the same
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geometry and size.
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Optimum striping cannot occur with different disk types.
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.Pp
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The
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.Nm
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also supports primitive data mirroring.
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To enable this mirroring support, the
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.Nm
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must be configured with the
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.Dv CCDF_MIRROR
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flag set.
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Note that the
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.Dv CCDF_MIRROR
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flag implies the
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.Dv CCDF_UNIFORM
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flag and requires an interleaved even number of components.
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.Pp
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Mirroring functions by making the second n/2 components exact duplicates
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of the first n/2.
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For example, in a mirrored
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.Nm
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with components sd0g, sd1g, sd2g, and sd3g, sd2g would mirror sd0g and
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sd3g would mirror sd1g.
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Reads will come from the first n/2 components (in this example:
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sd0g and sd1g) while writes will go to all components.
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.Pp
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If a component of a mirrored
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.Nm
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should fail, the
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.Nm
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can be reconfigured as a non-mirrored
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.Nm
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until the failed component can be replaced.
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For example, using the hypothetical
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.Nm
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above, the recovery process might look like this:
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.Bl -enum
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.It
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Component sd1g fails.
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.It
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Reconfigure
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.Nm
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without mirroring using the same interleave factor with components
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sd0g and sd3g.
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.Nm
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can still be used until replacement drive arrives.
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.It
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Replace failed component.
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.It
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Using
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.Xr dd 1 ,
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copy the contents of sd3g to the new sd1g.
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.It
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Restore
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.Nm
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configuration to that of before the component failure.
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.El
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.Pp
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There is a run-time utility that is used for configuring
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.Nm Ns s.
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See
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.Xr ccdconfig 8
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for more information.
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.Sh WARNINGS
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If just one (or more) of the disks in a non-mirrored
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.Nm
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fails, the entire file system will be lost.
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.Sh FILES
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.Bl -tag -width /dev/XXrXccdX -compact
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.It Pa /dev/{,r}ccd*
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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 ccdconfig 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 concatenated disk driver was originally written at the University of Utah.
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