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

     ccd -- Concatenated Disk driver

     device ccd

     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 run-
     ning 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 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 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 opti-
     mize 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.

   Linux Compatibility
     The Linux compatibility mode does not try to read the label that Linux'
     md(4) driver leaves on the	raw devices.  You will have to give the	order
     of	devices	and the	interleave factor on your own.	When in	Linux compati-
     bility mode, ccd will convert the interleave factor from Linux terminol-
     ogy.  That	means you give the same	interleave factor that you gave	as
     chunk size	in Linux.

     If	you have a Linux md(4) device in ``legacy'' mode, do not use the
     CCDF_LINUX	flag in	ccdconfig(8).  Use the CCDF_NO_OFFSET flag instead.
     In	that case you have to convert the interleave factor on your own, usu-
     ally it is	Linux' chunk size multiplied by	two.

     Using a Linux RAID	this way is potentially	dangerous and can destroy the
     data in there.  Since FreeBSD does	not read the label used	by Linux,
     changes in	Linux might invalidate the compatibility layer.

     However, using this is reasonably safe if you test	the compatibility
     before mounting a RAID read-write for the first time.  Just using
     ccdconfig(8) without mounting does	not write anything to the Linux	RAID.
     Then you do a fsck.ext2fs (ports/sysutils/e2fsprogs) on the ccd device
     using the -n flag.	 You can mount the file	system read-only to check
     files in there.  If all this works, it is unlikely	that there is a	prob-
     lem with ccd.  Keep in mind that even when	the Linux compatibility	mode
     in	ccd is working correctly, bugs in FreeBSD's ex2fs implementation would
     still destroy your	data.

     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 sec-
     tor 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), gvinum(8),
     mount(8), newfs(8)

     The concatenated disk driver was originally written at the	University of

FreeBSD	Ports 11.2		August 9, 1995		    FreeBSD Ports 11.2


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