CCD(4)

NAME
ccd - Concatenated Disk Driver

SYNOPSIS
pseudo-device ccd [count]

DESCRIPTION
The ccd driver provides the capability of combining one or more disks/partitions into one virtual disk.

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.

Note that the ‘raw’ partitions of the disks must not be combined. Each component partition should be offset at least one cylinder from the beginning of the component disk. This avoids potential conflicts between the component disk's disklabel and the ccd's disklabel. The kernel will only allow component partitions of type FS_BSDFFS.

In order to compile in support for the, you must add a line similar to the following to your kernel configuration file:

    pseudo-device	ccd	4	# concatenated disk devices

The count argument is how many ccds memory is allocated for at boot time. In this example, no more than 4 ccds may be configured.

A ccd may be either serially concatenated or interleaved. To serially concatenate the partitions, specify the interleave factor of 0.

If a ccd is interleaved correctly, a “striping” effect is achieved, which can increase performance. The optimum interleave factor is typically the size of a track. Since the interleave factor is expressed in units of DEV_BSIZE, one must account for sector sizes other than DEV_BSIZE in order to calculate the correct interleave. The kernel will not allow an interleave factor less than the size of the largest component sector divided by DEV_BSIZE.

Note that best performance is achieved if all component disks have the same geometry and size. Optimum striping cannot occur with different disk types.

There is a run-time utility that is used for configuring ccds. See ccdconfig(8) for more information.

WARNINGS
If just one (or more) of the disks in a non-mirrored ccd fails, the entire file system will be lost.

FILES
/dev/{,r}ccd*
    ccd device special files.

SEE ALSO
MAKEDEV(8), ccdconfig(8), config(8), fsck(8), mount(8), newfs(8)

HISTORY
The concatenated disk driver was originally written at the University of Utah.