NetBSD/share/man/man4/ccd.4

.\"	$NetBSD: ccd.4,v 1.3 1994/11/30 16:22:06 jtc Exp $
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.\" Copyright (c) 1994 Jason Downs
.\" Copyright (c) 1994 Jason R. Thorpe
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.Dd August 3, 1994
.Dt CCD 4
.Os BSD 4
.Sh NAME
.Nm ccd
.Nd procedure for configuring concatenated disk devices
.Sh DESCRIPTION
The
.Nm
driver provides the capability of `concatenating' two
or more disks/partitions into one virtual disk.
.Pp
This document assumes that you're familiar with how to generate kernels,
and how to properly set up a system configuration file.
.Pp
First, you need to add the disks that you wish to concatenate to your
kernel configuration.  They should probably have `hard-coded' id numbers
as opposed to wildcards, since you don't want the autoconfig sequence
to move these disks around.  For a hp300, your config might look something
like this:
.Pp
.Bd -unfilled -offset indent
master  hpib0   at scode7
master  hpib1   at scode?
master  hpib2   at scode?
disk    rd0     at hpib1 slave 0
disk    rd1     at hpib1 slave 1
disk    rd2     at hpib1 slave 2
disk    rd3     at hpib2 slave 0
disk    rd4     at hpib2 slave 1
tape    ct0     at hpib0 slave ?
.Ed
.Pp
In this example, the disks rd3 and rd4 will be concatenated.
.Pp
The `c' partitions of the disks
.Pa should not
be concatenated.  Rather, you should use some other partition, which is
defined in the individual labels of each disk, to use the entire disk
.Pa except the first cylinder.
This is to avoid corrupting the labels of component disks within the
concatenated disk.  In this example, the `h' partition will be used.
.Pp
The next piece is the pseudo-device line in your kernel configuration file
that actually configures the
.Nm
device.  It should look something like:
.Bd -unfilled -offset indent
pseudo-device	ccd0 on rd3h and rd4h interleave 8192
.Ed
.Pp
This will configure a virtual disk using rd3h and rd4h, interleaving the access
at every 8192 blocks.  An interleave is recommended, but not required; if it
is not specified, the driver will use each disk in sequence, which isn't as
efficient as interleaving the access.
.Pp
In this example, the disks to be concatenated are HP 7958s, which are 
approximately 120 megabytes in size.  The label on each of the 
example disks reads as follows:
.Bd -unfilled -offset indent
# /dev/rrd3c:
type: HP-IB
disk:
label:
flags:
bytes/sector: 512
sectors/track: 36
tracks/sector: 7
sectors/cylinder: 252
cylinders: 1013
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0           # milliseconds
track-to-track seek: 0  # milliseconds
drivedata: 0
.Pp
2 partitions:
#    size    offset  fstype  [fsize  bsize  cpg]
  c: 255276       0    boot    1024   4096   16  # (Cyl. 0 - 1012)
  h: 255024     252  4.2BSD    1024   4096   15  # (Cyl. 1 - 1012)
.Ed
.Pp
Of course, this label will not work for any other type of disk, so you
will have to create your own.  In the simple case, make the `h' (or whatever
partition you specified in your ccd pseudo-device specification) the size of
partition `c',
minus the number of sectors/cylinder, and set the offset to the same
number as your sectors/cylinder.  This leaves an extra cylinder for the label
to reside in.
.Pp
Once you have given each disk an appropriate label, you should reboot
with the newly generated, ccd-capable kernel.
.Pp
At boot time, shortly after the autoconfig sequence, you should see a message
like the following:
.Bd -unfilled -offset indent
ccd0:  2 components (rd3h, rd4h), 507904 blocks interleaved at 8192
ccd0 configured
.Ed
.Pp
If you do not see this message, then the
.Nm
device is
.Pa NOT
configured.
.Pp
Now it is time to make the device nodes for the concatenated disk, if you
have not done so already.  You can do this with the
.Xr MAKEDEV 8
script, as follows:
.Bd -unfilled -offset indent
# cd /dev
# sh MAKEDEV ccd0
.Ed
.Pp
Now you need to create 
.Xr disktab 5
entry for the virtual disk, since it does not 
support labels within itself.  The following is appropriate for this
example:
.Bd -unfilled -offset indent
rd7958-2:\\
        :ty=winchester:ns#36:nt#7:nc#2024:\\
        :pc#507904:bc#4096:fc#1024
.Ed
.Pp
All values are copied from the `standard' entry, taking into account any
differences from your label(s), except:
.Bd -unfilled -offset indent
nc#4048
This is the total number of cylinders.  This should be the
total number of cylinders from all of the partitions you
defined your ccd to encompass.
.Pp
pc#507904
This is the total size, which is not the sum of all of the
partitions configured as the ccd.  Use the number reported
by the autoconfig sequence.
.Ed
.Pp
If your
.Xr disktab 5
entry is set up correctly, you can now use
.Xr newfs 8
to create a file system on the virtual disk:
.Bd -unfilled -offset indent
# newfs /dev/rccd0c rd7958-2
.Ed
.Pp
where `/dev/rccd0c' is the raw device of the
.Nm
device, and `rd7958-2' is the name
of your
.Xr disktab 5
entry.
.Pp
You should now have a working file system on the concatenated disk, which you
can
.Xr fsck 8
and
.Xr mount 8
just like any other, normal, disk.
.Sh WARNINGS
Presumably, you must use disks of all of the same type.  The results of using
disks with different geometries would be unpredictable, at best.
.Pp
If just one (or more) of the disks in a concatenated disk fails, the entire
file system will be lost.
.Pp
There is a kernel-level limit of eight devices per one
.Nm
device.
.Sh BUGS
Currently, the
.Nm
driver works only on the hp300.  It should be fairly easy to add
support for other architectures using
.Xr config 8 ,
but support for architectures using
.Xr config.new 8
is more difficult.
.Sh FILES
/etc/disktab - disk description file
.Pp
.Sh SEE ALSO
.Xr MAKEDEV 8 ,
.Xr disktab 5 ,
.Xr newfs 8 ,
.Xr fsck 8 ,
.Xr mount 8 ,
.Xr config 8 ,
.Xr config.new 8