freebsd-skq/share/man/man4/ccd.4

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.\" Copyright (c) 1994 Jason Downs.
.\" Copyright (c) 1994, 1995 Jason R. Thorpe.
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.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.
.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 HISTORY
The concatenated disk driver was originally written at the University of
Utah.
.Sh SEE ALSO
.Xr dd 1 ,
.Xr ccdconfig 8 ,
.Xr config 8 ,
.Xr disklabel 8 ,
.Xr fsck 8 ,
.Xr mount 8 ,
.Xr newfs 8 ,
.Xr vinum 8