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CCD(4)		       FreeBSD Kernel Interfaces Manual			CCD(4)

     ccd -- Concatenated Disk driver

     pseudo-device ccd 4

     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

     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

     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.

     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.

     /dev/ccd*	ccd device special files

     The concatenated disk driver was originally written at the	University of

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

FreeBSD	11.1			August 9, 1995			  FreeBSD 11.1


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