1.\" $NetBSD: ccd.4,v 1.6 1996/02/01 20:47:46 thorpej 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 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. Each component partition should be offset at least one 58cylinder from the beginning of the component disk. This avoids potential 59conflicts between the compoent disk's disklabel and the 60.Nm ccd's 61disklabel. The kernel will only allow component partitions of type FS_BSDFFS. 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. If a 78.Nm ccd 79is interleaved correctly, a 80.Dq striping 81effect is achieved, which can increase performance. The optimum interleave 82factor is typically the size of a track. Since the interleave factor 83is expressed in units of DEV_BSIZE, one must account for sector sizes 84other than DEV_BSIZE in order to calculate the correct interleave. 85The kernel will not allow an interleave factor less than the size 86of the largest component sector divided by DEV_BSIZE. 87.Pp 88Note that best performance is achieved if all compent disks have the same 89geometery and size. Optimum striping cannot occur with different 90disk types. 91.Pp 92The 93.Nm ccd 94also supports primitive data mirroring. To enable this mirroring support, 95the 96.Nm ccd 97must be configured with the 98.Nm CCDF_MIRROR 99flag set. Note that the 100.Nm CCDF_MIRROR 101flag implies the 102.Nm CCDF_UNIFORM 103flag and requires an interleaved even number of components. 104.Pp 105Mirroring functions by making the second n/2 components exact duplicates 106of the first n/2. For example, in a mirrored 107.Nm ccd 108with components sd0g, sd1g, sd2g, and sd3g, sd2g would mirror sd0g and 109sd3g would mirror sd1g. Reads will come from the first n/2 components 110(in this example: sd0g and sd1g) while writes will go to all components. 111.Pp 112If a component of a mirrored 113.Nm ccd 114should fail, the 115.Nm ccd 116can be reconfigured as a non-mirrored 117.Nm ccd 118until the failed component can be replaced. For example, using the 119hypothetical 120.Nm ccd 121above, the recovery process might look like this: 122.Bd -literal -offset indent 123* Component sd1g fails. Reconfigure ccd without mirroring 124 using the same interleave factor with components sd0g and sd3g. 125 ccd can still be used until replacement drive arrives. 126* Replace failed component. Using dd(1), copy the contents of 127 sd3g to the new sd1g. 128* Restore ccd configuration to that of before the component failure. 129.Ed 130.Pp 131There is a run-time utility that is used for configuring 132.Nm ccds . 133See 134.Xr ccdconfig 8 135for more information. 136.Sh WARNINGS 137If just one (or more) of the disks in a non-mirrored 138.Nm ccd 139fails, the entire 140file system will be lost. 141.Sh FILES 142/dev/{,r}ccd* - ccd device special files. 143.Pp 144.Sh HISTORY 145The concatenated disk driver was originally written at the University of 146Utah. 147.Sh SEE ALSO 148.Xr MAKEDEV 8 , 149.Xr ccdconfig 8 , 150.Xr config 8 , 151.Xr config.old 8 , 152.Xr fsck 8 , 153.Xr mount 8 , 154.Xr newfs 8 . 155