title: Two MySQL requests at the same time - Performance issue
body: <p>I have a MySQL server with many innodb tables.
  I have a background script that does A LOT a delete/insert with one request : it deletes many millions of rows from table 2, then insert many millions of rows to table 2 using data from table 1 : </p>
  
  <pre><code>INSERT INTO table 2 (date)
  SELECT date from table 1 GROUP BY date
  </code></pre>
  
  <p>(The request is actually more complex but it is to shown what kind of request I am doing).</p>
  
  <p>At the same time, I am going to run a second background script, that does about a million INSERT or UPDATE requests, but separately (I mean, I execute the first update query, then I execute an insert query, etc...) in table 3.</p>
  
  <p>My issue is that when a script is running, it is fast, like let's say it takes 30minutes each, so 1h total. But when the two scripts are running at the same time, it is VERY slow, like it will take 5h, instead of 1h. </p>
  
  <p>So first, I would like to know what can cause this ? Is it because of IO performance ? (like mysql is writing in two different tables so it is slow to switch between the two ?)</p>
  
  <p>And how could I fix this ? If I could say that the big INSERT query is paused while my second background script is running, it would be great, for example... But I can't find a way to do something like this.</p>
  
  <p>I am not an expert at MySQL administration.. If you need more information, please let me know !</p>
  
  <p>Thank you !!</p>