title: Large mongo update queue burst issue
body: <p>I'm doing some user analytics tracking with mongo.  I'm averaging about 200 updates a second to documents (around 400k) based on a users email address.  There are 3 shards split along email alphabetically.  It works pretty well except for the daily user state change scripts.  It bursts the requests to about 6k per second.</p>
  
  <p>This causes a tail spin effect where it overloads the mongo queue and it never seems to catch up again.  Scripts fail, bosses get angry, etc.  They also won't allow the scripts to be throttled.  Since they are update operations and not insert they are not able to be submitted in bulk.  What I see for me options are.</p>
  
  <p>1:) Finding a way to allocate a large queue to mongo so it can wait for low points and get the data updated</p>
  
  <p>2:) Writing a custom throttling solution</p>
  
  <p>3:) Finding a more efficient indexing strategy (currently just indexing the email address)</p>
  
  <p>Pretty much anything is on the table.</p>
  
  <p>Any help is greatly appreciated</p>