1.\" $NetBSD: ccd.4,v 1.10 1997/09/20 03:56:38 mikel Exp $ 2.\" 3.\" Copyright (c) 1994 Jason Downs. 4.\" Copyright (c) 1994, 1995, 1996 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" Op Ar count 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. Each component partition should be offset at least one 58cylinder from the beginning of the component disk. This avoids potential 59conflicts between the component disk's disklabel and the 60.Nm ccd Ns 's 61disklabel. The kernel will only allow component partitions of type 62.Dv FS_BSDFFS . 63.Pp 64In order to compile in support for the ccd, you must add a line similar 65to the following to your kernel configuration file: 66.Bd -unfilled -offset indent 67pseudo-device ccd 4 # concatenated disk devices 68.Ed 69.Pp 70The count argument is how many 71.Nm ccd Ns s 72memory is allocated for at boot time. In this example, no more than 4 73.Nm ccd Ns s 74may be configured. 75.Pp 76A 77.Nm ccd 78may be either serially concatenated or interleaved. If a 79.Nm ccd 80is interleaved correctly, a 81.Dq striping 82effect is achieved, which can increase performance. The optimum interleave 83factor is typically the size of a track. Since the interleave factor 84is expressed in units of 85.Dv DEV_BSIZE , 86one must account for sector sizes other than 87.Dv DEV_BSIZE 88in order to calculate the correct interleave. 89The kernel will not allow an interleave factor less than the size 90of the largest component sector divided by 91.Dv DEV_BSIZE . 92.Pp 93Note that best performance is achieved if all component disks have the same 94geometry and size. Optimum striping cannot occur with different 95disk types. 96.Pp 97The 98.Nm ccd 99also supports primitive data mirroring. To enable this mirroring support, 100the 101.Nm ccd 102must be configured with the 103.Dv CCDF_MIRROR 104flag set. Note that the 105.Dv CCDF_MIRROR 106flag implies the 107.Dv CCDF_UNIFORM 108flag and requires an interleaved even number of components. 109.Pp 110Mirroring functions by making the second n/2 components exact duplicates 111of the first n/2. For example, in a mirrored 112.Nm ccd 113with components sd0g, sd1g, sd2g, and sd3g, sd2g would mirror sd0g and 114sd3g would mirror sd1g. Reads will come from the first n/2 components 115(in this example: sd0g and sd1g) while writes will go to all components. 116.Pp 117If a component of a mirrored 118.Nm ccd 119should fail, the 120.Nm ccd 121can be reconfigured as a non-mirrored 122.Nm ccd 123until the failed component can be replaced. For example, using the 124hypothetical 125.Nm ccd 126above, the recovery process might look like this: 127.Bd -literal -offset indent 128* Component sd1g fails. Reconfigure ccd without mirroring 129 using the same interleave factor with components sd0g and sd3g. 130 ccd can still be used until replacement drive arrives. 131* Replace failed component. Using dd(1), copy the contents of 132 sd3g to the new sd1g. 133* Restore ccd configuration to that of before the component failure. 134.Ed 135.Pp 136There is a run-time utility that is used for configuring 137.Nm ccd Ns s. 138See 139.Xr ccdconfig 8 140for more information. 141.Sh WARNINGS 142If just one (or more) of the disks in a non-mirrored 143.Nm ccd 144fails, the entire 145file system will be lost. 146.Sh FILES 147.Bl -tag -width /dev/XXrXccdX -compact 148.It Pa /dev/{,r}ccd* 149ccd device special files. 150.El 151.Pp 152.Sh HISTORY 153The concatenated disk driver was originally written at the University of 154Utah. 155.Sh SEE ALSO 156.Xr MAKEDEV 8 , 157.Xr ccdconfig 8 , 158.Xr config 8 , 159.Xr fsck 8 , 160.Xr mount 8 , 161.Xr newfs 8 . 162