man/man4/ccd.4

.\"	$OpenBSD: ccd.4,v 1.18 2004/03/21 19:47:59 miod Exp $
.\"	$NetBSD: ccd.4,v 1.5 1995/10/09 06:09:09 thorpej Exp $
.\"
.\" Copyright (c) 1994 Jason Downs.
.\" Copyright (c) 1994, 1995 Jason R. Thorpe.
.\" All rights reserved.
.\"
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.\"	This product includes software developed for the NetBSD Project
.\"	by Jason Downs and Jason R. Thorpe.
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.Dd August 9, 1995
.Dt CCD 4
.Os
.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
.Em should 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 ccd Ns 's
disklabel.
The kernel wants to only allow component partitions of type
.Dv FS_CCD ,
but due to disklabel restrictions on some architectures will also accept
component partitions of
.Dv FS_BSDFFS .
.Pp
In order to compile in support for the ccd, 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 ccd Ns 's
memory is allocated for at boot time.
In this example, no more than 4
.Nm ccd Ns 's
may be configured.
.Pp
A
.Nm
may be either serially concatenated or interleaved.
To serially concatenate partitions specify an interleave factor of 0.
.Pp
If a
.Nm
is interleaved correctly, a
.Dq striping
effect is achieved, which can increase performance.
Finding the optimum interleave factor is a hard problem.
Some things to think about are the number of disks in the ccd,
the typical read-ahead opportunities, the filesystem blocksize, and if
it's possible to use the optimized ccd I/O protocol.
The optimized protocol allows smaller interleave factors for a
comparably cheap price but requires that the factor be a multiple of the
software page size
.Pf ( Dv CLBYTES ) ,
and that mirroring is not requested.
So far the best policy is to test with different interleaves and
benchmark typical uses.
A rule of thumb might be to use the software pagesize with the optimized
I/O protocol (the default, unless the requirements mentioned above are not
fulfilled) or
.Dv MAXBSIZE /
#components with the unoptimized protocol.
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 ccd Ns 's .
See
.Xr ccdconfig 8
for more information.
.Sh WARNINGS
If just one (or more) of the disks in a
.Nm
fails, the entire
file system will be lost.
.Sh FILES
.Bl -tag -width tenletters+five -compact
.It Pa /dev/{,r}ccd*
ccd device special files
.El
.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.