xref: /freebsd/share/man/man4/ccd.4 (revision 380a989b3223d455375b4fae70fd0b9bdd43bafb)
1.\"	$NetBSD: ccd.4,v 1.5 1995/10/09 06:09:09 thorpej Exp $
2.\"
3.\" Copyright (c) 1994 Jason Downs.
4.\" Copyright (c) 1994, 1995 Jason R. Thorpe.
5.\" All rights reserved.
6.\"
7.\" Redistribution and use in source and binary forms, with or without
8.\" modification, are permitted provided that the following conditions
9.\" are met:
10.\" 1. Redistributions of source code must retain the above copyright
11.\"    notice, this list of conditions and the following disclaimer.
12.\" 2. Redistributions in binary form must reproduce the above copyright
13.\"    notice, this list of conditions and the following disclaimer in the
14.\"    documentation and/or other materials provided with the distribution.
15.\" 3. All advertising materials mentioning features or use of this software
16.\"    must display the following acknowledgement:
17.\"	This product includes software developed for the NetBSD Project
18.\"	by Jason Downs and Jason R. Thorpe.
19.\" 4. Neither the name of the author nor the names of its contributors
20.\"    may be used to endorse or promote products derived from this software
21.\"    without specific prior written permission.
22.\"
23.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
24.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
25.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
26.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
27.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
28.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
29.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
30.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
31.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
33.\" SUCH DAMAGE.
34.\"
35.Dd August 9, 1995
36.Dt CCD 4
37.Os NetBSD
38.Sh NAME
39.Nm ccd
40.Nd Concatenated Disk Driver
41.Sh SYNOPSIS
42.Cd "pseudo-device ccd 4"
43.Sh DESCRIPTION
44The
45.Nm
46driver provides the capability of combining one or more disks/partitions
47into one virtual disk.
48.Pp
49This document assumes that you're familiar with how to generate kernels,
50how to properly configure disks and pseudo-devices in a kernel
51configuration file, and how to partition disks.
52.Pp
53Note that the
54.Sq raw
55partitions of the disks
56.Pa should not
57be combined.  The kernel will only allow component partitions of type
58FS_BSDFFS (type
59.Dq 4.2BSD
60as shown as
61.Xr disklabel 8 ).
62.Pp
63In order to compile in support for the ccd, you must add a line similar
64to the following to your kernel configuration file:
65.Bd -unfilled -offset indent
66pseudo-device	ccd	4	# concatenated disk devices
67.Ed
68.Pp
69The count argument is how many
70.Nm ccds
71memory is allocated for a boot time.  In this example, no more than 4
72.Nm ccds
73may be configured.
74.Pp
75A
76.Nm ccd
77may be either serially concatenated or interleaved.  To serially
78concatenate the partitions, specify the interleave factor of 0.
79.Pp
80There is a run-time utility that is used for configuring
81.Nm ccds .
82See
83.Xr ccdconfig 8
84for more information.
85.Ss The Interleave Factor
86If a
87.Nm ccd
88is interleaved correctly, a
89.Dq striping
90effect is achieved, which can increase sequential read/write
91performance.  The interleave factor is expressed in units of DEV_BSIZE
92(usually 512 bytes).  For large writes, the optimum interleave factor
93is typically the size of a track, while for large reads, it is about a
94quarter of a track. (Note that this changes greatly depending on the
95number and speed of disks.)  For instance, with eight 7,200 RPM drives
96on two Fast-Wide SCSI buses, this translates to about 128 for writes
97and 32 for reads.
98.Pp
99The best performance is achieved if all component disks have the same
100geometry and size.  Optimum striping cannot occur with different
101disk types.
102.Pp
103For random-access oriented workloads, such as news servers, a larger
104interleave factor (e.g., 65,536) is more desirable.  Note that there
105isn't much
106.Nm ccd
107can do to speed up applications that are seek-time limited.  Larger
108interleave factors will at least reduce the chance of having to seek
109two disk-heads to read one directory or a file.
110.Ss Disk Mirroring
111You can configure the
112.Nm ccd
113to
114.Dq mirror
115any even number of disks.  See
116.Xr ccdconfig 8
117for how to specify the necessary flags.  In an event of a disk
118failure, you can use
119.Xr dd 1
120to recover the failed disk.
121.Pp
122Note that a one-disk
123.Nm ccd
124is not the same as the original partition.  In particular, this means
125if you have a filesystem on a two-disk mirrored
126.Nm ccd
127and one of the disks fail, you cannot mount and use the remaining
128partition as itself; you have to configure it as a one-disk
129.Nm ccd.
130.Sh WARNINGS
131If just one (or more) of the disks in a
132.Nm ccd
133fails, the entire
134file system will be lost unless you are mirroring the disks.
135.Sh FILES
136/dev/{,r}ccd* - ccd device special files.
137.Pp
138.Sh HISTORY
139The concatenated disk driver was originally written at the University of
140Utah.
141.Sh SEE ALSO
142.Xr dd 1 ,
143.Xr ccdconfig 8 ,
144.Xr config 8 ,
145.Xr disklabel 8 ,
146.Xr fsck 8 ,
147.Xr MAKEDEV 8 ,
148.Xr mount 8 ,
149.Xr newfs 8 .
150