document distributed with 4.2BSD

SCCS-vsn: usr.sbin/config/SMM.doc/b.t 5.1

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+.\" Copyright (c) 1983 Regents of the University of California.
+.\" All rights reserved.  The Berkeley software License Agreement
+.\" specifies the terms and conditions for redistribution.
+.\"
+.\"    @(#)b.t 5.1 (Berkeley) %G%
+.\"
+.ds LH "Building Systems With Config
+.ds RH "Device Defaulting Rules
+.ds CF July 27, 1983
+.LP
+.nr H1 1
+.ds CH "
+.bp
+.ds CH "\(hy \\n(PN \(hy
+.LG
+.B
+.ce
+APPENDIX B. RULES FOR DEFAULTING SYSTEM DEVICES
+.sp 2
+.R
+.NL
+.PP
+When \fIconfig\fP processes a ``config'' rule which does
+not fully specify the location of the root file system,
+paging area(s), device for system dumps, and device for
+argument list processing it applies a set of rules to
+define those values left unspecified.  The following list
+of rules are used in defaulting system devices.
+.IP 1) 3
+If a root device is not specified, the swap
+specification must indicate a ``generic'' system is to be built.
+.IP 2) 3
+If the root device does not specify a unit number, it
+defaults to unit 0.
+.IP 3) 3
+If the root device does not include a partition specification,
+it defaults to the ``a'' partition.
+.IP 4) 3
+If no swap area is specified, it defaults to the ``b''
+partition of the root device.
+.IP 5) 3
+If no device is specified for processing argument lists, the
+first swap partition is selected.
+.IP 6) 3
+If no device is chosen for system dumps, the first swap
+partition is selected (see below to find out where dumps are
+placed within the partition).
+.PP
+The following table summarizes the default partitions selected
+when a device specification is incomplete, e.g. ``hp0''.
+.DS
+.TS
+l l.
+Type   Partition
+_
+root   ``a''
+swap   ``b''
+args   ``b''
+dumps  ``b''
+.TE
+.DE
+.SH
+Multiple swap/paging areas
+.PP
+When multiple swap partitions are specified, the system treats the
+first specified as a ``primary'' swap area which is always used.
+The remaining partitions are then interleaved into the paging
+system at the time a
+.IR swapon (2)
+system call is made.  This is normally done at boot time with
+a call to
+.IR swapon (8)
+from the /etc/rc file.
+.SH
+System dumps
+.PP
+System dumps are automatically taken after a system crash,
+provided the device driver for the ``dumps'' device supports
+this.  The dump contains the contents of memory, but not
+the swap areas.  Normally the dump device is a disk in
+which case the information is copied to a location near the
+back of the partition.  The dump is placed in the back of the
+partition because the primary swap and dump device are commonly
+the same device and this allows the system to be rebooted without
+immediately overwriting the saved information.  When a dump has
+occurred, the system variable \fIdumpsize\fP 
+is set to a non-zero value indicating the size (in bytes) of
+the dump.  The \fIsavecore\fP\|(8)
+program then copies the information from the dump partition to
+a file in a ``crash'' directory and also makes a copy of the
+system which was running at the time of the crash (usually
+``/vmunix'').  The offset to the system dump is defined in the
+system variable \fIdumplo\fP (a sector offset from
+the front of the dump partition). The 
+.I savecore
+program operates by reading the contents of \fIdumplo\fP, \fIdumpdev\fP,
+and \fIdumpmagic\fP from /dev/kmem, then comparing the value
+of \fIdumpmagic\fP read from /dev/kmem to that located in
+corresponding location in the dump area of the dump partition.
+If a match is found, 
+.I savecore
+assumes a crash occurred and reads \fIdumpsize\fP from the dump area
+of the dump partition.  This value is then used in copying the
+system dump.  Refer to 
+\fIsavecore\fP\|(8)
+for more information about its operation.
+.PP
+The value \fIdumplo\fP is calculated to be 
+.DS
+\fIdumpdev-size\fP \- DUMPDEV
+.DE
+where \fIdumpdev-size\fP is the size of the disk partition
+where system dumps are to be placed, and DUMPDEV is 10 Megabytes.
+If the disk partition is not large enough to hold a 10 Megabyte
+dump, \fIdumplo\fP is set to 0 (the front of the partition).
+For sites with more than 10 Megabytes of memory the definition
+of DUMPDEV in /sys/vax/autoconf.c will have to be changed.