.\" $NetBSD: ccd.4,v 1.5 1995/10/09 06:09:09 thorpej Exp $ .\" .\" Copyright (c) 1994 Jason Downs. .\" Copyright (c) 1994, 1995 Jason R. Thorpe. .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed for the NetBSD Project .\" by Jason Downs and Jason R. Thorpe. .\" 4. Neither the name of the author nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, .\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED .\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, .\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD: src/share/man/man4/ccd.4,v 1.11.2.8 2001/12/17 11:30:11 ru Exp $ .\" $DragonFly: src/share/man/man4/ccd.4,v 1.3 2006/02/17 19:37:09 swildner Exp $ .\" .Dd August 9, 1995 .Dt CCD 4 .Os .Sh NAME .Nm ccd .Nd Concatenated Disk driver .Sh SYNOPSIS .Cd "pseudo-device ccd 4" .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 should not be combined. The kernel will only allow component partitions of type FS_BSDFFS (type .Dq Bx 4.2 as shown as .Xr disklabel 8 ) . .Pp In order to compile in support for the ccd, 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 a boot time. In this example, no more than 4 .Nm Ns s may be configured. As of the .Fx 3.0 release, you do not need to configure your kernel with ccd but may instead use it as a kernel loadable module. Simply running .Nm ccdconfig will load the module into the kernel. .Pp A .Nm may be either serially concatenated or interleaved. To serially concatenate the partitions, specify the interleave factor of 0. Note that mirroring may not be used with an interleave factor of 0. .Pp There is a run-time utility that is used for configuring .Nm Ns s . See .Xr ccdconfig 8 for more information. .Ss The Interleave Factor If a .Nm is interleaved correctly, a .Dq striping effect is achieved, which can increase sequential read/write performance. The interleave factor is expressed in units of DEV_BSIZE (usually 512 bytes). For large writes, the optimum interleave factor is typically the size of a track, while for large reads, it is about a quarter of a track. (Note that this changes greatly depending on the number and speed of disks.) For instance, with eight 7,200 RPM drives on two Fast-Wide SCSI buses, this translates to about 128 for writes and 32 for reads. A larger interleave tends to work better when the disk is taking a multitasking load by localizing the file I/O from any given process onto a single disk. You lose sequential performance when you do this, but sequential performance is not usually an issue with a multitasking load. .Pp An interleave factor must be specified when using a mirroring configuration, even when you have only two disks (i.e. the layout winds up being the same no matter what the interleave factor). The interleave factor will determine how I/O is broken up, however, and a value 128 or greater is recommended. .Pp CCD has an option for a parity disk, but does not currently implement it. .Pp The best performance is achieved if all component disks have the same geometry and size. Optimum striping cannot occur with different disk types. .Pp For random-access oriented workloads, such as news servers, a larger interleave factor (e.g., 65,536) is more desirable. Note that there isn't much .Nm can do to speed up applications that are seek-time limited. Larger interleave factors will at least reduce the chance of having to seek two disk-heads to read one directory or a file. .Ss Disk Mirroring You can configure the .Nm to .Dq mirror any even number of disks. See .Xr ccdconfig 8 for how to specify the necessary flags. For example, if you have a ccd configuration specifying four disks, the first two disks will be mirrored with the second two disks. A write will be run to both sides of the mirror. A read will be run to either side of the mirror depending on what the driver believes to be most optimal. If the read fails, the driver will automatically attempt to read the same sector from the other side of the mirror. Currently .Nm uses a duel seek zone model to optimize reads for a multi-tasking load rather then a sequential load. .Pp In an event of a disk failure, you can use .Xr dd 1 to recover the failed disk. .Pp Note that a one-disk .Nm is not the same as the original partition. In particular, this means if you have a filesystem on a two-disk mirrored .Nm and one of the disks fail, you cannot mount and use the remaining partition as itself; you have to configure it as a one-disk .Nm . You cannot replace a disk in a mirrored .Nm partition without first backing up the partition, then replacing the disk, then restoring the partition. .Sh WARNINGS If just one (or more) of the disks in a .Nm fails, the entire file system will be lost unless you are mirroring the disks. .Pp If one of the disks in a mirror is lost, you should still be able to backup your data. If a write error occurs, however, data read from that sector may be non-deterministic. It may return the data prior to the write or it may return the data that was written. When a write error occurs, you should recover and regenerate the data as soon as possible. .Pp Changing the interleave or other parameters for a .Nm disk usually destroys whatever data previously existed on that disk. .Sh FILES .Bl -tag -width ".Pa /dev/ccd*" .It Pa /dev/ccd* .Nm device special files .El .Sh SEE ALSO .Xr dd 1 , .Xr ccdconfig 8 , .Xr config 8 , .Xr disklabel 8 , .Xr fsck 8 , .Xr MAKEDEV 8 , .Xr mount 8 , .Xr newfs 8 , .Xr vinum 8 .Sh HISTORY The concatenated disk driver was originally written at the University of Utah.