NetBSD/share/man/man4/ccd.4

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.\" $NetBSD: ccd.4,v 1.17 1998/11/13 00:28:48 thorpej Exp $
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.\" Copyright (c) 1996, 1997 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Jason R. Thorpe.
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.\" Copyright (c) 1994 Jason Downs.
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.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
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.\" by Jason Downs and Jason R. Thorpe.
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.\" may be used to endorse or promote products derived from this software
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd August 9, 1995
.Dt CCD 4
.Os NetBSD
.Sh NAME
.Nm ccd
.Nd Concatenated Disk Driver
.Sh SYNOPSIS
.Cd "pseudo-device ccd" Op Ar count
.Sh DESCRIPTION
The
.Nm
driver provides the capability of combining one or more disks/partitions
into one virtual disk.
.Pp
This document assumes that you're familiar with how to generate kernels,
how to properly configure disks and pseudo-devices in a kernel
configuration file, and how to partition disks.
.Pp
Note that the
.Sq raw
partitions of the disks
.Pa must not
be combined.
Each component partition should be offset at least one cylinder
from the beginning of the component disk.
This avoids potential conflicts between the component disk's
disklabel and the
.Nm Ns 's
disklabel.
The kernel will only allow component partitions of type
.Dv FS_BSDFFS .
.Pp
In order to compile in support for the
.Nm "" ,
you must add a line similar
to the following to your kernel configuration file:
.Bd -unfilled -offset indent
pseudo-device ccd 4 # concatenated disk devices
.Ed
.Pp
The count argument is how many
.Nm Ns s
memory is allocated for at boot time.
In this example, no more than 4
.Nm Ns s
may be configured.
.Pp
A
.Nm
may be either serially concatenated or interleaved.
To serially concatenate the partitions, specify the interleave factor of 0.
.Pp
If a
.Nm
is interleaved correctly, a
.Dq striping
effect is achieved, which can increase performance.
The optimum interleave factor is typically the size of a track.
Since the interleave factor is expressed in units of
.Dv DEV_BSIZE ,
one must account for sector sizes other than
.Dv DEV_BSIZE
in order to calculate the correct interleave.
The kernel will not allow an interleave factor less than the size
of the largest component sector divided by
.Dv DEV_BSIZE .
.Pp
Note that best performance is achieved if all component disks have the same
geometry and size.
Optimum striping cannot occur with different disk types.
.Pp
There is a run-time utility that is used for configuring
.Nm Ns s.
See
.Xr ccdconfig 8
for more information.
.Sh WARNINGS
If just one (or more) of the disks in a non-mirrored
.Nm
fails, the entire file system will be lost.
.Sh FILES
.Bl -tag -width /dev/XXrXccdX -compact
.It Pa /dev/{,r}ccd*
.Nm
device special files.
.El
.Pp
.Sh SEE ALSO
.Xr MAKEDEV 8 ,
.Xr ccdconfig 8 ,
.Xr config 8 ,
.Xr fsck 8 ,
.Xr mount 8 ,
.Xr newfs 8
.Sh HISTORY
The concatenated disk driver was originally written at the University of Utah.