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 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 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
ccd
s. 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 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.
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.
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 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
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
(
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 problem 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.
WARNINGS¶
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
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),
gvinum(8),
mount(8),
newfs(8)
HISTORY¶
The concatenated disk driver was originally written at the University of
Utah.