NetBSD/share/man/man4/ccd.4

.\"	$NetBSD: ccd.4,v 1.7 1997/01/05 22:53:08 cjs Exp $
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.\" Copyright (c) 1994 Jason Downs.
.\" Copyright (c) 1994, 1995, 1996 Jason R. Thorpe.
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.Dd August 9, 1995
.Dt CCD 4
.Os NetBSD
.Sh NAME
.Nm ccd
.Nd Concatenated Disk Driver
.Sh SYNOPSIS
.Cd "pseudo-device ccd 4"
.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 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's
disklabel.  The kernel will only allow component partitions of type 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 ccds
memory is allocated for a boot time.  In this example, no more than 4
.Nm ccds
may be configured.
.Pp
A
.Nm ccd
may be either serially concatenated or interleaved.  If a
.Nm ccd
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 DEV_BSIZE, one must account for sector sizes
other than 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 DEV_BSIZE.
.Pp
Note that best performance is achieved if all compent disks have the same
geometery and size.  Optimum striping cannot occur with different
disk types.
.Pp
The
.Nm ccd
also supports primitive data mirroring.  To enable this mirroring support,
the
.Nm ccd
must be configured with the
.Nm CCDF_MIRROR
flag set.  Note that the
.Nm CCDF_MIRROR
flag implies the
.Nm 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 ccd
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 ccd
should fail, the
.Nm ccd
can be reconfigured as a non-mirrored
.Nm ccd
until the failed component can be replaced.  For example, using the
hypothetical
.Nm ccd
above, the recovery process might look like this:
.Bd -literal -offset indent
* Component sd1g fails.  Reconfigure ccd without mirroring
  using the same interleave factor with components sd0g and sd3g.
  ccd can still be used until replacement drive arrives.
* Replace failed component.  Using dd(1), copy the contents of
  sd3g to the new sd1g.
* Restore ccd configuration to that of before the component failure.
.Ed
.Pp
There is a run-time utility that is used for configuring
.Nm ccds .
See
.Xr ccdconfig 8
for more information.
.Sh WARNINGS
If just one (or more) of the disks in a non-mirrored
.Nm ccd
fails, the entire
file system will be lost.
.Sh FILES
/dev/{,r}ccd* - ccd device special files.
.Pp
.Sh HISTORY
The concatenated disk driver was originally written at the University of
Utah.
.Sh SEE ALSO
.Xr MAKEDEV 8 ,
.Xr ccdconfig 8 ,
.Xr config 8 ,
.Xr config.old 8 ,
.Xr fsck 8 ,
.Xr mount 8 ,
.Xr newfs 8 .