title: rrdtool update multiple datasources in two commands
body: <p><strong>RRD does not update the second datasource correctly, see:</strong></p>
  
  <p>First, I create the RRD file with two datasources (c1 and c2): </p>
  
  <pre><code>rdtool create test.rrd  --start N  --step 60 DS:c1:GAUGE:120:0:100 DS:c2:GAUGE:120:0:100 RRA:AVERAGE:0.5:1:1440
  </code></pre>
  
  <p>Then I do update the two datasources in two commands:</p>
  
  <pre><code>rrdtool update test.rrd -t c1 N:10 &amp;&amp; rrdtool update test.rrd -t c2 N:10
  </code></pre>
  
  <p>Wait for 60 seconds....
  Do again an update :</p>
  
  <pre><code>rdtool update test.rrd -t c1 N:20 &amp;&amp; rrdtool update test.rrd -t c2 N:20
  </code></pre>
  
  <p>Then lets see what we have:</p>
  
  <pre><code>rrdtool fetch test.rrd AVERAGE | tail -5
  1468409580: -nan -nan
  1468409640: -nan -nan
  1468409700: -nan -nan
  1468409760: 1,5988575517e+01 1,9266620475e-01
  1468409820: -nan -nan
  </code></pre>
  
  <p>The first datacource c1 works as expected, but the second c2 shows a value lower than 1 and I expect also a value close to 15.</p>
  
  <p>Yes, I know I can also update both datasources in ONE update command, but in my case a have a lot of datasources in one rrd file and its better to read and follow the mass of values. </p>
  
  <p><strong>Used rrd version : 1.6.0</strong></p>