.TH RA 4
.CT 2 sa
.SH NAME
ra \- DEC MSCP disks (RA60, RA80, RA81, RA90)
.SH DESCRIPTION
.I Ra
devices occupy disk drives conforming to
DEC's Mass Storage Control Protocol standard:
drives such as the RA81
connected via controllers such as the UDA50.
Files with minor device numbers 0 through 7
refer to different sections of drive 0,
minor devices 8 through 16 refer to drive 1,
and so on up to 63 (8 drives).
.PP
Normally the disk is accessed in 1024-byte blocks (1K).
If 64 is added to the minor device number,
4096-byte blocks (4K) are used instead.
A 4K device mounted as a file system is bitmapped; see
.IR filsys (5).
.PP
Conventionally the files are given names like
.L ra37
for section 7 of drive 3.
There are no name rules distinguishing 1024-byte files
from 4096-byte files;
in practice the files are almost always the 4096-byte kind.
.PP
The start and size
of the sections of each drive
are as follows.
Sizes are measured in
512-byte hardware sectors.
.PP
.nf
.ta .5i +\w'000000    'u +\w'000000    'u
	disk	start	length
	0	0	10240
	1	10240	20480
	2	30720	249848
	3	280568	249848
	4	530416	249848
	5	780264	arbitrarily large
	6	30720	749544
	7	0	arbitrarily large
.DT
.fi
.PP
The `arbitrarily large'
sections reach to the end of the disk.
.I Rarct
will display disk sizes; see
.IR rarepl (8).
For example,
an RA81 has 891072 sectors,
so section 7 is that size,
and section 5 is
891072\(mi780264=110808 sectors.
An RA90 has 2376153 sectors;
section 7 is that size,
section 5 is
2376153\(mi780264=1595889 sectors.
For other disks,
run
.I rarct
and do the arithmetic.
.PP
The
.I ra
files
discussed above access the disk via the system's normal
buffering mechanism
and may be read and written without regard to
physical disk records.
There is also a `raw' interface
which provides for direct transmission between the disk
and the user's read or write buffer.
A single read or write call results in exactly one I/O operation
and therefore raw I/O is considerably more efficient when
many words are transmitted.
The names of the raw files
begin with
.L rra
and end with a number which selects the same disk
as the corresponding
.L ra
file.
.PP
In raw I/O the buffer must begin on a word boundary,
and counts should be a multiple of 512 bytes
(a disk block).
Likewise
.IR lseek (2)
calls should specify a multiple of 512 bytes.
.PP
Several
.IR ioctl (2)
calls
apply to the raw devices.
.TF UIORRCT
.TP
.B UIOCHAR
The third argument to
.I ioctl
points to an object 
to be filled with drive parameters:
.EX
.ta .5i +\w'daddr_t 'u +\w'radsize   'u
struct ud_unit {
	daddr_t	radsize;	/* disk size, sectors */
	daddr_t	rctsize;	/* RCT size, including pad */
	long	medium;	/* medium id */
	short	tracksz;	/* sectors per track */
	short	groupsz;	/* tracks per group */
	short	cylsz;	/* groups per cylinder */
	char	rbns;	/* RBNs per track */
	char	copies;	/* number of RCT copies */
};
.EE
.TF UIORRCT
.TP
.B UIORRCT
The third argument points to an object of type
.EX
struct ud_rctbuf {
	caddr_t	buf;
	int	lbn;
};
.EE
.IP
.B buf
points to a 512-byte buffer,
into which block
.B lbn
of the replacement and caching table
(RCT)
is read.
As many copies of the RCT
as necessary
are examined to find a readable copy
of the block.
.TP
.B UIOWRCT
The third argument is like that of
.BR UIORRCT .
Block
.B lbn
of the RCT is written
in all copies.
.TP
.B UIOREPL
The third argument points to an object of type:
.EX
struct ud_repl {
	daddr_t	replbn;	/* good block */
	daddr_t	lbn;	/* bad block */
	short	prim;	/* nonzero if primary replacement */
};
.EE
.IP
A `replace' command
is sent to the controller,
requesting that attempts to access logical block
.I lbn
henceforth be revectored to replacement block
.IR replbn .
.I Prim
should be set nonzero
if and only if
.I replbn
is the primary replacement block for
.IR lbn .
.TP
.B UIOSPDW
Arrange that the disk drive will spin down
when the last file using it is closed.
.TP
.B UIORST
Reset the controller
to which this disk is connected.
Any pending operations are abandoned and return an error.
.SH FILES
.F /dev/ra*
.br
.F /dev/rra*
.SH SEE ALSO
.IR rarepl (8)
.br
MSCP Basic Disk Functions Manual
.br
DEC Standard Disk Format Specification
.SH BUGS
In raw I/O
.IR read (2)
and
.IR write 
truncate file offsets to 512-byte block boundaries,
and
.I write
scribbles on the tail of incomplete blocks.
Thus,
in programs that are likely to access raw devices,
.I read, write
and
.IR lseek (2)
should always deal in 512-byte multiples.
.PP
.B UIORRCT
and
.B UIOWRCT
will misbehave if invoked on a section
that doesn't start
at the beginning of the disk.
Section 7
(the whole disk)
is the best choice.
.PP
The 1K/4K flag bit in the device number is unfortunate.