.TH VD 4
.SH NAME
vd \- virtual disk
.SH DESCRIPTION
The
.B "vd*"
devices are virtual disks made up by concatenating one or more regular disks
or partitions.  A virtual disk will appear to have a size equal to the sum
of the sizes of the disks it is made of.  The blocks in a virtual disk are
all the blocks in the real disks in the same order as the order in which the
disks were attached to the virtual disk.  So if virtual disk
.B vd
is made up of disks
.BR d0 ,
.BR d1 ", ..."
.BI d n
then the following command will return true:
.PP
.RS
.BI "cat d0 d1 ... d" n " | cmp \- vd"
.RE
.PP
Virtual disks devices can be formed by using the following
.B ioctl
call on an open virtual disk file descriptor:
.sp
.RS
.ft B
ioctl(\fIfd\fP, DIOCVDATTACH, &struct vdisk_attach);
.ft R
.RE
.sp
The third argument is a pointer to a
.B struct vdisk_attach
as defined in <minix/vdisk.h>:
.sp
.nf
.ta +4n +6n +16n
struct vdisk_attach {
	int	index;		/* Position in the list of subdisks */
	int	task_nr;	/* Subdisk = <task, minor> */
	int	minor;
};
.fi
.sp
The
.I index
field is the position that the subdisk must have in the list of disks that
make up the virtual disk.  Attaching the first disk with index 0 (re)sets
the list of subdisks to just that one disk.  Each next disk must have an
index number that is one higher.
.PP
The
.I task_nr
and
.I minor
fields specify the disk device to use as a subdisk.  These numbers can be
obtained by translating a device number with the
.B FSDEVMAP svrctl
call.
.PP
One of the subdisks may be another virtual disk device.  There is little
reason to do this now, but this may change if other methods of combining
disks are implemented.
.PP
The overhead of the virtual disk driver is so small it is hard to
measure.  The main cost is a context switch to the task that handles the
proper subdisk.  So there is no reason to shun the use of virtual disks
for performance reasons.
.SH FILES
/dev/vd*
.SH "SEE ALSO"
.BR ioctl (2),
.BR svrctl (2),
.BR vdisk.conf (5),
.BR vdisk_ctrl (8).
.SH NOTES
The driver allows up to 8 virtual disks.  They can have 32 subdisks total.
.PP
Virtual disks do not have partitions, so
.B vd1
is
.B not
the first partition of
.BR vd0 ,
they are separate virtual disks.
.SH BUGS
The "cat|cmp" example will fail if you use block devices that are not a
whole number of kilobytes in length.  Raw devices always work.
.PP
Other interesting ways of combining disks, like striping, may well be slower
than concatenation under Minix.  So don't hold your breath.
.PP
Disk devices used as subdisks should be made unusable to prevent aliasing
problems.  For now,
.B beware!
.SH AUTHOR
Kees J. Bot (kjb@cs.vu.nl)