title: What is the default mongod write concern in which version?
body: <p>I could not find in <a href="https://docs.mongodb.com/manual/reference/write-concern/" rel="noreferrer">mongodb's documentation</a> what's the default write concern and how an "acknowledged write" is defined.
  It seems that this have changed throughout the different mongodb versions as shows the <a href="https://docs.mongodb.com/v3.2/reference/write-concern/" rel="noreferrer">v3.2 documentation</a> for example:</p>
  
  <blockquote>
    <p>In 3.2-versions earlier than 3.2.6, w: "majority" implies j: true if journaling is enabled. With earlier versions of MongoDB, w: majority" does not imply journaling.</p>
  </blockquote>
  
  <p>Or:</p>
  
  <blockquote>
    <p>Changed in version 3.0: Prior to MongoDB 3.0, w: "majority" refers to the majority of the replica set’s members.</p>
  </blockquote>
  
  <p>Or:</p>
  
  <blockquote>
    <p>Changed in version 2.6: In Master/Slave deployments, MongoDB treats w: "majority" as equivalent to w: 1. In earlier versions of MongoDB, w: "majority" produces an error in master/slave deployments.</p>
  </blockquote>
  
  <p>Also, I'm wondering that "majority" refers to all voting nodes in <a href="https://docs.mongodb.com/v3.2/reference/write-concern/" rel="noreferrer">v3.2 documentation</a> :</p>
  
  <blockquote>
    <p>Requests acknowledgment that write operations have propagated to the majority of voting nodes [1], including the primary.</p>
  </blockquote>
  
  <p>Does this mean that even arbiters count since they are voting nodes? So, for example, if I have a replSet consisting of 2 data bearing nodes plus 1 arbiter, a write operation with write concern "majority" would succeed even if 1 data bearing node went down because the write was acknowledged by the remaining data bearing node and the arbiter, hence the majority?</p>