NetBSD/share/man/man4/ccd.4

.\"	$NetBSD: ccd.4,v 1.15 1998/06/07 09:31:31 enami Exp $
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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" 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.