关于node-mongodb的参数

collection.update(criteria, objNew, options, [callback]);

Useful options:

• safe:true Should always set if you have a callback.

• multi:true If set, all matching documents are updated, not just the first.

• upsert:true Atomically inserts the document if no documents matched.

  db.collection('test').update({hi: 'here'}, {$set: {hi: 'there'}}, {w:1},
  function(err) { if (err) console.warn(err.message); else console.log('successfully updated'); });

README文档里给的是safe multi upsert这几个选项。 但这个{w:1}是啥意思，哪里能看到相关文档说明？

地址： node-mongodb