.\"	$NetBSD: ccd.4,v 1.15 1998/06/07 09:31:31 enami Exp $
.\"
.\" Copyright (c) 1996, 1997 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Jason R. Thorpe.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"        This product includes software developed by the NetBSD
.\"        Foundation, Inc. and its contributors.
.\" 4. Neither the name of The NetBSD Foundation nor the names of its
.\"    contributors may be used to endorse or promote products derived
.\"    from this software without specific prior written permission.
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" Copyright (c) 1994 Jason Downs.
.\" All rights reserved.
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.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    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:
.\"	This product includes software developed for the NetBSD Project
.\"	by Jason Downs and Jason R. Thorpe.
.\" 4. Neither the name of the author nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" 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;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" 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.
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
The
.Nm
also supports primitive data mirroring.
To enable this mirroring support, the
.Nm
must be configured with the
.Dv CCDF_MIRROR
flag set.
Note that the
.Dv CCDF_MIRROR
flag implies the
.Dv CCDF_UNIFORM
flag and requires an interleaved even number of components.
.Pp
Mirroring functions by making the second n/2 components exact duplicates
of the first n/2.
For example, in a mirrored
.Nm
with components sd0g, sd1g, sd2g, and sd3g, sd2g would mirror sd0g and
sd3g would mirror sd1g.
Reads will come from the first n/2 components (in this example:
sd0g and sd1g) while writes will go to all components.
.Pp
If a component of a mirrored
.Nm
should fail, the
.Nm
can be reconfigured as a non-mirrored
.Nm
until the failed component can be replaced.
For example, using the hypothetical
.Nm
above, the recovery process might look like this:
.Bl -enum
.It
Component sd1g fails.
.It
Reconfigure
.Nm
without mirroring using the same interleave factor with components
sd0g and sd3g.
.Nm
can still be used until replacement drive arrives.
.It
Replace failed component.
.It
Using
.Xr dd 1 ,
copy the contents of sd3g to the new sd1g.
.It
Restore
.Nm
configuration to that of before the component failure.
.El
.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.