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.\" $FreeBSD$ 36.\" 37.Dd August 9, 1995 38.Dt CCD 4 39.Os NetBSD 40.Sh NAME 41.Nm ccd 42.Nd Concatenated Disk Driver 43.Sh SYNOPSIS 44.Cd "pseudo-device ccd 4" 45.Sh DESCRIPTION 46The 47.Nm 48driver provides the capability of combining one or more disks/partitions 49into one virtual disk. 50.Pp 51This document assumes that you're familiar with how to generate kernels, 52how to properly configure disks and pseudo-devices in a kernel 53configuration file, and how to partition disks. 54.Pp 55Note that the 56.Sq raw 57partitions of the disks 58.Pa should not 59be combined. The kernel will only allow component partitions of type 60FS_BSDFFS (type 61.Dq 4.2BSD 62as shown as 63.Xr disklabel 8 ). 64.Pp 65In order to compile in support for the ccd, you must add a line similar 66to the following to your kernel configuration file: 67.Bd -unfilled -offset indent 68pseudo-device ccd 4 # concatenated disk devices 69.Ed 70.Pp 71The count argument is how many 72.Nm ccds 73memory is allocated for a boot time. In this example, no more than 4 74.Nm ccds 75may be configured. As of the 76.Fx 3.0 77release, you do not need to 78configure your kernel with ccd but may instead use it as a kernel loadable 79module. Simply running 80.Nm ccdconfig 81will load the module into the kernel. 82.Pp 83A 84.Nm ccd 85may be either serially concatenated or interleaved. To serially 86concatenate the partitions, specify the interleave factor of 0. 87Note that mirroring may not be used with an interleave factor of 0. 88.Pp 89There is a run-time utility that is used for configuring 90.Nm ccds . 91See 92.Xr ccdconfig 8 93for more information. 94.Ss The Interleave Factor 95If a 96.Nm ccd 97is interleaved correctly, a 98.Dq striping 99effect is achieved, which can increase sequential read/write 100performance. The interleave factor is expressed in units of DEV_BSIZE 101(usually 512 bytes). For large writes, the optimum interleave factor 102is typically the size of a track, while for large reads, it is about a 103quarter of a track. (Note that this changes greatly depending on the 104number and speed of disks.) For instance, with eight 7,200 RPM drives 105on two Fast-Wide SCSI buses, this translates to about 128 for writes 106and 32 for reads. A larger interleave tends to work better when the 107disk is taking a multitasking load by localizing the file I/O from 108any given process onto a single disk. You lose sequential performance when 109you do this, but sequential performance is not usually an issue with a 110multitasking load. 111.Pp 112An interleave factor must be specified when using a mirroring configuration, 113even when you have only two disks (i.e. the layout winds up being the same 114no matter what the interleave factor). The interleave factor will determine 115how I/O is broken up, however, and a value 128 or greater is recommended. 116.Pp 117CCD has an option for a parity disk, but does not currently implement it. 118.Pp 119The best performance is achieved if all component disks have the same 120geometry and size. Optimum striping cannot occur with different 121disk types. 122.Pp 123For random-access oriented workloads, such as news servers, a larger 124interleave factor (e.g., 65,536) is more desirable. Note that there 125isn't much 126.Nm ccd 127can do to speed up applications that are seek-time limited. Larger 128interleave factors will at least reduce the chance of having to seek 129two disk-heads to read one directory or a file. 130.Ss Disk Mirroring 131You can configure the 132.Nm ccd 133to 134.Dq mirror 135any even number of disks. See 136.Xr ccdconfig 8 137for how to specify the necessary flags. For example, if you have a ccd 138configuration specifying four disks, the first two disks will be mirrored with 139the second two disks. A write will be run to both sides of 140the mirror. A read will be run to either side of the mirror depending 141on what the driver believes to be most optimal. If the read fails, 142the driver will automatically attempt to read the same sector from the 143other side of the mirror. Currently 144.Nm ccd 145uses a duel seek zone model to optimize reads for a multi-tasking load 146rather then a sequential load. 147.Pp 148In an event of a disk 149failure, you can use 150.Xr dd 1 151to recover the failed disk. 152.Pp 153Note that a one-disk 154.Nm ccd 155is not the same as the original partition. In particular, this means 156if you have a filesystem on a two-disk mirrored 157.Nm ccd 158and one of the disks fail, you cannot mount and use the remaining 159partition as itself; you have to configure it as a one-disk 160.Nm ccd. 161You cannot replace a disk in a mirrored 162.Nm ccd 163partition without first backing up the partition, then replacing the disk, 164then restoring the partition. 165.Sh WARNINGS 166If just one (or more) of the disks in a 167.Nm ccd 168fails, the entire 169file system will be lost unless you are mirroring the disks. 170.Pp 171If one of the disks in a mirror is lost, you should still 172be able to backup your data. If a write error occurs, however, data 173read from that sector may be non-deterministic. It may return the data 174prior to the write or it may return the data that was written. When a 175write error occurs, you should recover and regenerate the data as soon 176as possible. 177.Pp 178Changing the interleave or other parameters for a 179.Nm ccd 180disk usually destroys whatever data previously existed on that disk. 181.Sh FILES 182/dev/{,r}ccd* - ccd device special files. 183.Pp 184.Sh HISTORY 185The concatenated disk driver was originally written at the University of 186Utah. 187.Sh SEE ALSO 188.Xr dd 1 , 189.Xr ccdconfig 8 , 190.Xr config 8 , 191.Xr disklabel 8 , 192.Xr fsck 8 , 193.Xr MAKEDEV 8 , 194.Xr mount 8 , 195.Xr newfs 8 , 196.Xr vinum 8 . 197