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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 Ta ccd Ta # 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 Ns. See ccdconfig(8) for more information.
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.
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.
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, usually 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
on the
ccd
device using the
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 | |
CCD (4) | August 9, 1995 |
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