Index: src/libexec/rc/rc.d/dumpon
===================================================================
--- src/libexec/rc/rc.d/dumpon	(revision 368353)
+++ src/libexec/rc/rc.d/dumpon	(working copy)
@@ -35,6 +35,50 @@
 
 dumpon_start()
 {
+	local n=6 camlist=`camcontrol devlist -v`
+
+	# Wait for cam(4) devices attaching, 6 times at max by increasing
+	# 1s each (21s max in total)
+	# With root-MFS, it's likely that root_hold_wait() shortcuts
+	# before cam(4) probing finishes.  In that case, we have /, but
+	# probably not /usr, so we don't utilize grep(1) e.g.
+	# Run unconditionally of dumpdev_enable setting,
+	# rcorder(8) dictates to do the delay at this place.
+	#
+
+	while [ -n "${camlist}" ]; do
+
+		echo "${camlist}" | while read line; do
+			# Skip line without "target 0 lun 0" pattern
+			[ X"${line}" != \
+			X"${line#*target [[:xdigit:]]* lun [[:xdigit:]]*}" ] \
+			|| continue
+
+			# If prefilterd line doesn't end with  () or probe), we
+			# continue checking next line
+			[ X"${line% ()}" == X"${line}" ] &&
+				[ X"${line%[apr][pro][rob][obe][be0-9][e0-9])}"\
+				  == X"${line}" ] &&
+					continue
+
+			return $n # no need to check remaining lines
+		done
+		camlist=$? # recycle to be refreshed camlist variable
+
+		# $camlist is 0 if every line was no match obove, so simply
+		# break before delaying rc(8), since cam seems to be ready.
+		[ $camlist -gt 0 -a $camlist -le 6 ] || break
+
+		# Initialize counter once only
+		[ $camlist -eq 6 ] && n=0
+
+		sleep $((n+=1))
+		[ $n -lt 6 ] || break
+		camlist=`camcontrol devlist -v`
+
+	done
+
+
 	# Enable dumpdev so that savecore can see it. Enable it
 	# early so a crash early in the boot process can be caught.
 	#