CCD(4)

NAME
ccd - Concatenated Disk Driver

SYNOPSIS
pseudo-device ccd 4

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 should not be combined. The kernel will only allow component partitions of type FS_BSDFFS (type “4.2BSD” as shown as disklabel(8) ).

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 a 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.

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

  The Interleave Factor
If a ccd is interleaved correctly, a “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.

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

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 ccd 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.

  Disk Mirroring
You can configure the ccd to “mirror” any even number of disks. See ccdconfig(8) for how to specify the necessary flags. In an event of a disk failure, you can use dd(1) to recover the failed disk.

Note that a one-disk ccd is not the same as the original partition. In particular, this means if you have a filesystem on a two-disk mirrored ccd 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 ccd.

WARNINGS
If just one (or more) of the disks in a ccd fails, the entire file system will be lost unless you are mirroring the disks.

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

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

SEE ALSO
dd(1), MAKEDEV(8), ccdconfig(8), config(8), disklabel(8), fsck(8), mount(8), newfs(8).