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CCD(4)                    NetBSD Programmer's Manual                    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 configura-
     tion 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 be-
     ginning 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.  The optimum interleave factor is typi-
     cally the size of a track, see ``sectors/track'' value in disklabel(8)
     output.  Since the interleave factor is expressed in units of DEV_BSIZE,
     one must account for sector sizes other than DEV_BSIZE in order to calcu-
     late 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 en-
     tire 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.

NetBSD 1.6.2                    August 9, 1995                               1