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CCD(4)                 FreeBSD Kernel Interfaces Manual                 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 configura-
     tion 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 ccd's memory is allocated for a boot time.
     In this example, no more than 4 ccds may be configured.  As of the
     FreeBSD 3.0 release, you do not need to configure your kernel with ccd
     but may instead use it as a kernel loadable module.  Simply running
     ccdconfig will load the module into the kernel.

     A ccd may be either serially concatenated or interleaved.  To serially
     concatenate the partitions, specify the interleave factor of 0.  Note
     that mirroring may not be used with an 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.  A larger interleave
     tends to work better when the disk is taking a multitasking load by
     localizing the file I/O from any given process onto a single disk.  You
     lose sequential performance when you do this, but sequential performance
     is not usually an issue with a multitasking load.

     An interleave factor must be specified when using a mirroring configura-
     tion, even when you have only two disks (i.e. the layout winds up being
     the same no matter what the interleave factor).  The interleave factor
     will determine how I/O is broken up, however, and a value 128 or greater
     is recommended.

     CCD has an option for a parity disk, but does not currently implement it.

     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 lim-
     ited.  Larger interleave factors will at least reduce the chance of hav-
     ing 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.   For example, if
     you have a ccd configuration specifying four disks, the first two disks
     will be mirrored with the second two disks.  A write will be run to both
     sides of the mirror.  A read will be run to either side of the mirror
     depending on what the driver believes to be most optimal.  If the read
     fails, the driver will automatically attempt to read the same sector from
     the other side of the mirror.  Currently ccd uses a duel seek zone model
     to optimize reads for a multi-tasking load rather then a sequential load.

     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.  You
     cannot replace a disk in a mirrored ccd partition without first backing
     up the partition, then replacing the disk, then restoring the partition.

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.

     If one of the disks in a mirror is lost, you should still be able to
     backup your data.  If a write error occurs, however, data read from that
     sector may be non-deterministic.  It may return the data prior to the
     write or it may return the data that was written.  When a write error
     occurs, you should recover and regenerate the data as soon as possible.

     Changing the interleave or other parameters for a ccd disk usually
     destroys whatever data previously existed on that disk.

FILES
     /dev/ccd*  ccd device special files

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

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

FreeBSD 4.10                    August 9, 1995                    FreeBSD 4.10

NAME | SYNOPSIS | DESCRIPTION | WARNINGS | FILES | HISTORY | SEE ALSO

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