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NAME

     ccd - Concatenated Disk driver

SYNOPSIS

     device ccd

DESCRIPTION

     The ccd driver provides the capability of combining one or more
     disks/partitions into one virtual disk.

     This document assumes that you are familiar with how to generate kernels,
     how to properly configure disks and devices in a kernel configuration
     file, and how to partition disks.

     In order to compile in support for the ccd, you must add a line similar
     to the following to your kernel configuration file:

           device    ccd       # concatenated disk devices

     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(8) 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
     configuration, 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 is
     not 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.  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 dual seek zone model to
     optimize reads for a multi-tasking load rather than 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 file system 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 back
     up 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

SEE ALSO

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

HISTORY

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