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NetBSD 6.1.5 - man page for ccd (netbsd section 4)

CCD(4)				   BSD Kernel Interfaces 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 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 par-
     tition of all types since some port lacks support of an on-disk BSD disklabel.  The parti-
     tion 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 par-
     titions, 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 fac-
     tor 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.

BSD					  March 5, 2004 				      BSD


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