Update instructions for setting the dump device prior to multi-user

mode.

Reported by:	dcs
Submitted by:	asmodai

1 files changed, 21 insertions, 10 deletions

diff --git a/sbin/dumpon/dumpon.8 b/sbin/dumpon/dumpon.8
index 61359f4..1ee8717 100644
--- a/sbin/dumpon/dumpon.8
+++ b/sbin/dumpon/dumpon.8
@@ -71,16 +71,6 @@ to be verbose about its activity.
 One of the system swap devices is usually chosen as the dump device, as
 swap devices are the most likely to have the required space at dump time.
 .Pp
-The dump device can also be specified at kernel compile time using
-the
-.Dq dumps on
-clause in the kernel configuration file (see
-.Xr config 8 ).  
-This is useful when the kernel panics before multi-user mode 
-is reached.  Subsequent successful invocations of 
-.Nm
-will override the compiled in value.
-.Pp
 The 
 .Nm
 program operates by setting the
@@ -95,6 +85,27 @@ or to
 .Ar special_file
 is the text string:
 .Dq Li off .
+.Pp
+The
+.Xr ddb 4
+interactive kernel debugger may be used
+to specify a device for crash dumps
+that occur in single-user mode.
+For example,
+the following command at the
+.Xr ddb 4
+prompt would return the device number for the swap device
+.Pa /dev/ad0s1b :
+.Bd -literal -offset ident
+show disk/ad0s1b
+.Ed
+.Pp
+The value returned could then be written to the
+.Va dumpdev
+kernel variable as follows:
+.Bd -literal -offset indent
+write dumpdev 0xc0b65800
+.Ed
 .Sh SEE ALSO
 .Xr sysctl 3 ,
 .Xr fstab 5 ,