fix issue:426 (Numbers are crazy in diskstats plugin after reboot)

 * munin-monitoring#426

 * check /proc/uptime to detect system reboot
 * use uptime second instead of interval if uptime < interval
 * reset all previous status values to zero on reboot

plugins/node.d.linux/diskstats.in

@@ -14,6 +14,8 @@ use Munin::Plugin;
 use MIME::Base64;
 use Storable qw(nfreeze thaw);
 
+use Data::Dumper;
+
 # Hardcoded pollinterval of 300 seconds
 my $poll_interval = 300;
 
@@ -30,10 +32,14 @@ need_multigraph();
 
 # Handle munin 'autoconf' command
 do_autoconf() if ( $ARGV[0] && $ARGV[0] eq 'autoconf' );
+my ($debug) = 1 if ( $ARGV[0] && $ARGV[0] eq 'debug' );
 
 # Fetch current counter values
 my %cur_diskstats = fetch_device_counters();
 
+# Fetch uptime to detect system reboot
+my ($uptime) = fetch_uptime();
+
 # Weed out unwanted devices
 filter_device_list( \%cur_diskstats );
 
@@ -54,6 +60,10 @@ eval {
 # Persist state from current run
 add_new_state( time(), %cur_diskstats );
 
+print   "uptime     => $uptime \n",
+        "prev_time  => $prev_time \n",
+        if ($debug);
+
 # Probably the first run for the given device, we need state to do our job,
 # so let's wait for the next run.
 exit if ( not defined $prev_time or not %prev_diskstats );
@@ -67,6 +77,21 @@ exit 0;
 # SUBS #
 ########
 
+
+# fetch_uptime
+#
+# read /proc/uptime and return it
+
+sub fetch_uptime {
+    open my $FH, "<", '/proc/uptime' or return undef;
+    my $line = <$FH>;
+    chomp($line);
+    my @row = split(/\s+/, $line);
+    close $FH;
+
+    return @row;
+}
+
 # generate_multigraph_data
 #
 # Creates the data which is needed by munin's fetch command
@@ -180,6 +205,23 @@ sub calculate_values {
     my $bytes_per_sector = 512;
 
     my $interval = time() - $prev_time;
+
+print   "interval     => $interval \n",
+        "prev_stats => ", Dumper( $prev_stats ),
+        if ($debug);
+    if ($uptime < $interval) {
+        # system has rebooted
+
+        $interval = $uptime;
+
+        # all values will be zero at system reboot
+        for my $entry ( keys %$prev_stats ) {
+            $prev_stats->{$entry} = 0;
+        }
+    }
+print   "interval     => $interval \n",
+        "prev_stats => ", Dumper( $prev_stats ),
+        if ($debug);
 
     my $read_ios  = subtract_wrapping_numbers($cur_stats->{'rd_ios'}, $prev_stats->{'rd_ios'});
     my $write_ios = subtract_wrapping_numbers($cur_stats->{'wr_ios'}, $prev_stats->{'wr_ios'});