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diff --git a/share/man/man4/ccd.4 b/share/man/man4/ccd.4 new file mode 100644 index 0000000..4c95fb8 --- /dev/null +++ b/share/man/man4/ccd.4 @@ -0,0 +1,287 @@ +.\" $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. +.\" +.\" $FreeBSD$ +.\" +.Dd August 9, 1995 +.Dt CCD 4 +.Os +.Sh NAME +.Nm ccd +.Nd Concatenated Disk driver +.Sh SYNOPSIS +.Cd "device ccd" +.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 are familiar with how to generate kernels, +how to properly configure disks and devices in a kernel +configuration file, and how to partition disks. +.Pp +In order to compile in support for the +.Nm , +you must add a line similar +to the following to your kernel configuration file: +.Pp +.Dl "device ccd # concatenated disk devices" +.Pp +As of the +.Fx 3.0 +release, you do not need to +configure your kernel with +.Nm +but may instead use it as a kernel loadable +module. +Simply running +.Xr ccdconfig 8 +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 +.Dv 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 +.Nm +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 +is not 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 +.Nm +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 dual seek zone model to optimize reads for a multi-tasking load +rather than 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 file system 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. +.Ss Linux Compatibility +The +.Tn Linux +compatibility mode does not try to read the label that +.Tn Linux Ns ' +.Xr md 4 +driver leaves on the raw devices. +You will have to give the order +of devices and the interleave factor on your own. +When in +.Tn Linux +compatibility mode, +.Nm +will convert the interleave factor from +.Tn Linux +terminology. +That means you give the same interleave factor that you +gave as chunk size in +.Tn Linux . +.Pp +If you have a +.Tn Linux +.Xr md 4 +device in +.Dq legacy +mode, do not use the +.Dv CCDF_LINUX +flag in +.Xr ccdconfig 8 . +Use the +.Dv CCDF_NO_OFFSET +flag instead. +In that case you have to convert +the interleave factor on your own, usually it is +.Tn Linux Ns ' +chunk size multiplied by two. +.Pp +Using a +.Tn Linux +RAID this way is potentially dangerous and can destroy +the data in there. +Since +.Fx +does not read the label used by +.Tn Linux , +changes in +.Tn Linux +might invalidate the compatibility layer. +.Pp +However, using this is reasonably safe if you test the compatibility +before mounting a RAID read-write for the first time. +Just using +.Xr ccdconfig 8 +without mounting does not write anything to the +.Tn Linux +RAID. +Then you do a +.Nm fsck.ext2fs Pq Pa ports/sysutils/e2fsprogs +on the +.Nm +device using the +.Fl n +flag. +You can mount the file system read-only to check files in there. +If all this works, it is unlikely that there is a problem with +.Nm . +Keep in mind that even when the +.Tn Linux +compatibility mode in +.Nm +is working correctly, bugs in +.Fx Ap s +.Nm ex2fs +implementation would still destroy +your data. +.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 back up 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 gvinum 8 , +.Xr mount 8 , +.Xr newfs 8 +.Sh HISTORY +The concatenated disk driver was originally written at the University of +Utah. |
