title: mbuffer slower than dd if device block is size used
body: <p>I have an backup script which is generating a tar on the fly and 
  piping it to mbuffer. The data is then written to a tape drive (LTO3)</p>
  
  <p>I discovered that the mbuffer slows the bandwidth down and I can not figure out why.</p>
  
  <p>here the 2 commands and the average speeds below them</p>
  
  <pre><code>$tar -b 512 -cf - /data | \
   mbuffer -A "..." -P 90 -m 1G -f -o /dev/st0 -d 512
  
  in @ 21.8 MB/s, out @ 21.8 MB/s, 1287 MB total, buffer 100% full
  </code></pre>
  
  <p>and if I pipe the data again to dd it will result in a much higher bandwidth</p>
  
  <pre><code>$tar -b 512 -cf - /nas/homes/ /nas/photo/ | \
   mbuffer -P 90 -m 1G | \
   dd of=/dev/st0 bs=256k
  
  in @ 72.9 MB/s, out @ 64.0 MB/s, 2671 MB total, buffer  99% full
  </code></pre>
  
  <p>My question is if I am using mbuffer the wrong way or if its not supposed to be used with <code>-d</code>.
  Even if I'm not specifying the block-size with <code>-d</code> the speed stays the same.</p>
  
  <p>I would like to use mbuffer because of the <code>-A</code> flag but with this performance it would take triple the time.</p>