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_CCD. But for now, it allows partition of all types since some port lacks support of an on-disk BSD disklabel. The partition of FS_UNUSED may be rejected because device driver of component disk will refuse it.

In order to compile in support for the ccd, 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. 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.

Also note that the total size of concatenated disk may vary depending on the interleave factor even if the exact same components are concatenated. And an old on-disk disklabel may be read after interleave factor change. As a result, the disklabel may contain wrong partition geometry and will cause an error when doing I/O near the end of concatenated disk.

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

config(1), MAKEDEV(8), ccdconfig(8), fsck(8), mount(8), newfs(8)

HISTORY

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