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NAME    [Toc]    [Back]

     ccd - concatenated disk driver

SYNOPSIS    [Toc]    [Back]

     pseudo-device ccd [count]

DESCRIPTION    [Toc]    [Back]

     The ccd driver provides the capability of combining  one  or
     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.  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 wants
     to  only  allow component partitions of type FS_CCD, but due
to disklabel
     restrictions on some architectures will also  accept  component partitions
     of FS_BSDFFS.

     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 ccd's memory is allocated for
at boot
     time.   In this example, no more than 4 ccd's may be configured.

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

     If  a ccd is interleaved correctly, a ``striping'' effect is
     which can increase performance.  Finding the optimum  interleave factor is
     a  hard  problem.  Some things to think about are the number
of disks in
     the ccd, the typical read-ahead opportunities, the  filesystem blocksize,
     and  if it's possible to use the optimized ccd I/O protocol.
The optimized
 protocol allows smaller interleave factors for a  comparably cheap
     price  but  requires  that  the  factor be a multiple of the
software page
     size (CLBYTES), and that mirroring is not requested.  So far
the best
     policy  is  to test with different interleaves and benchmark
typical uses.
     A rule of thumb might be to use the software  pagesize  with
the optimized
     I/O protocol (the default, unless the requirements mentioned
above are
     not fulfilled) or MAXBSIZE / #components  with  the  unoptimized protocol.
     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

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

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

WARNINGS    [Toc]    [Back]

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

FILES    [Toc]    [Back]

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

SEE ALSO    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

     The concatenated disk driver was originally written  at  the
University of

OpenBSD      3.6                          August      9,     1995
[ Back ]
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