2.11BSD/src/man/man4/ram.4

.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)ram.4	1.2 (2.11BSD) 1996/1/27
.\"
.TH RAM 4 "Januray 27, 1996"
.UC 2
.SH NAME
ram \- ram disk driver
.SH SYNOPSIS
.ft B
.nf
/sys/conf/SYSTEM:
	NRAM	\fIram_size\fP	# RAM disk size (512-byte blocks)

major device number(s):
	block: 3
minor device encoding:
	must be zero (0)
.fi
.ft R
.SH DESCRIPTION
The
.I ram
pseudo-device provides a very fast extended memory store.  It's use is
intended for file systems like
.I /tmp
and applications which need to access a reasonably large amount of data
quickly.
.PP
The amount of memory dedicated to the ram device is controlled by the NRAM
definition in units of 512-byte blocks.  This is also patchable in the
system binary through the variable
.IR ram_size
(though a patched system would have to be rebooted before any change took
effect; see
.IR adb (1)).
This makes it easy to test the effects of different ram disk sizes on
system performance.  It's important to note that any space given to the ram
device is permanently allocated at system boot time.  Dedicating too much
memory can adversely affect system performance by forcing the system to
swap heavily as in a memory poor environment.
.PP
The block file accesses the ram disk via the system's buffering mechanism
through a buffer sharing arrangement with the buffer cache.  It may be read
and written without regard to physical disk records.  There is no `raw'
interface since no speed advantage is gained by such an interface with
the ram disk.
.SH "DISK SUPPORT"
The
.I ram
driver does not support pseudo-disks (partitions).  The special files
refer to the entire `drive' as a single sequentially addressed file.
.PP
A typical use for the ram disk would be to mount
.I /tmp
on it.  Note that if this arrangement is recorded in
.I /etc/fstab
then
.I /etc/rc
will have to be modified slightly to do a
.IR mkfs (8)
on the ram disk
.B before
the standard file system checks are done.
.SH FILES
.ta \w'/dev/MAKEDEV.local  'u
/dev/ram	block file
.br
/dev/MAKEDEV	script to create special files
.br
/dev/MAKEDEV.local	script to localize special files
.DT
.SH "SEE ALSO"
hk(4),
ra(4),
rl(4),
rk(4),
rp(4),
rx(4),
si(4),
xp(4)
dtab(5),
autoconfig(8)
.SH DIAGNOSTICS
\fBram: no space\fR.  There is not enough memory to allocate the space needed
by the ram disk.  The ram disk is disabled.  Any attempts to access
it will return an error.
.PP
\fBram: not allocated\fR.  No memory was allocated to the ram disk and an
attempt was made to open it.  Either not enough memory was available at
boot time or the kernel variable
.I ram_size
was set to zero.
.SH BUGS
The
.I ram
driver is only available under 2.11BSD.