title: MongoDB 3.0 write lock on oplog collection in replica mode
body: <p>In replica mode each write operation to any collection in any DB, also writes to the oplog collection.</p>
  
  <p>Now, when writing to multiple DBs in parallel, all these write operations also write to the oplog.
  My question: do these write operations require locking the oplog ? (I'm using w:1 write concern). If they do, this is kind of similar to having a global lock between all the write operations to all the different DBs, isn't it ?</p>
  
  <p>I'd be happy to get any hints on this.</p>