ccd(4)

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 gen

erate kernels,
how to properly configure disks and devices in a kernel con

figuration
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 mirror

ing configuration, even when you have only two disks (i.e., the layout

winds up being
the same no matter what the interleave factor). The inter

leave 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 dif

ferent 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 en

tire 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 possi

ble.

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.

BSD August 9, 1995

BSD