create_data.js

var lvlup = require("levelup")

var db = lvlup("/tmp/source_db")

var batch = 10000

function makeBatch(num) {
  var b = []
  for (var i = num; i < num + batch; i++) {
    b.push({type: "put", key: "z~" + i, value: Math.random()})
  }   
  return b
} 

function populate(start) {
  db.batch(makeBatch(start), function (err) {
      if (err) throw err
      start += batch
      if (start < 1000000) {
        console.log(start)
        populate(start)
      }
  })
}

populate(0)

streaming_copy.js

var lvlup = require("levelup")

var src = lvlup("/tmp/source_db")

var tgt = lvlup("/tmp/target_db")

var start = Date.now()
src.createReadStream()
  .pipe(tgt.createWriteStream()
    .on("close", function () {
      console.log("Done, in %s", Date.now() - start)
    })
  )