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ccd - Concatenated Disk driver
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
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.
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
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.
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.
/dev/ccd* ccd device special files
dd(1), ccdconfig(8), config(8), disklabel(8), fsck(8), mount(8),
The concatenated disk driver was originally written at the University of