154 lines
5.3 KiB
Groff
154 lines
5.3 KiB
Groff
.\" $NetBSD: ccd.4,v 1.9 1997/06/20 20:42:36 pk Exp $
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.\"
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.\" Copyright (c) 1994 Jason Downs.
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.\" Copyright (c) 1994, 1995, 1996 Jason R. Thorpe.
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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 4"
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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 should not
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be combined. Each component partition should be offset at least one
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cylinder from the beginning of the component disk. This avoids potential
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conflicts between the component disk's disklabel and the
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.Nm ccd's
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disklabel. The kernel will only allow component partitions of type FS_BSDFFS.
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.Pp
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In order to compile in support for the ccd, 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 ccds
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memory is allocated for at boot time. In this example, no more than 4
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.Nm ccds
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may be configured.
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.Pp
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A
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.Nm ccd
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may be either serially concatenated or interleaved. If a
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.Nm ccd
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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. The optimum interleave
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factor is typically the size of a track. Since the interleave factor
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is expressed in units of DEV_BSIZE, one must account for sector sizes
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other than DEV_BSIZE 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 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. Optimum striping cannot occur with different
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disk types.
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.Pp
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The
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.Nm ccd
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also supports primitive data mirroring. To enable this mirroring support,
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the
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.Nm ccd
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must be configured with the
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.Nm CCDF_MIRROR
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flag set. Note that the
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.Nm CCDF_MIRROR
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flag implies the
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.Nm 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. For example, in a mirrored
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.Nm ccd
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with components sd0g, sd1g, sd2g, and sd3g, sd2g would mirror sd0g and
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sd3g would mirror sd1g. Reads will come from the first n/2 components
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(in this example: 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 ccd
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should fail, the
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.Nm ccd
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can be reconfigured as a non-mirrored
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.Nm ccd
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until the failed component can be replaced. For example, using the
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hypothetical
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.Nm ccd
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above, the recovery process might look like this:
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.Bd -literal -offset indent
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* Component sd1g fails. Reconfigure ccd without mirroring
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using the same interleave factor with components sd0g and sd3g.
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ccd can still be used until replacement drive arrives.
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* Replace failed component. Using dd(1), copy the contents of
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sd3g to the new sd1g.
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* Restore ccd configuration to that of before the component failure.
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.Ed
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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 ccds .
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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 ccd
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fails, the entire
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file system will be lost.
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.Sh FILES
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/dev/{,r}ccd* - ccd device special files.
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.Pp
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.Sh HISTORY
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The concatenated disk driver was originally written at the University of
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Utah.
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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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