— Concatenated Disk driver
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
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)
load the module into the kernel.
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 ccd
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
(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.
has an option for a parity disk, but does not currently
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
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
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)
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
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
. You cannot replace a disk in a mirrored
partition without first backing up the partition, then
replacing the disk, then restoring the partition.
The Linux compatibility mode does not try to read the label that Linux'
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
compatibility mode, ccd
will convert the interleave factor
from Linux terminology. 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
. Use the
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
without mounting does not write anything to the
Linux RAID. Then you do a fsck.ext2fs
) 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 problem with ccd
. Keep in mind that even when the
Linux compatibility mode in ccd
is working correctly, bugs
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 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
usually destroys whatever data previously existed on that disk.
- ccd device special files
The concatenated disk driver was originally written at the University of