Migrate from mongodb master/slave to replica sets

replica_set_migration.rb

#!/bin/env ruby

# Caveat: your must ensure no writes are going into the system
# Total time for this migration, including a restart of mongo
# is less than 10s

require 'mongo'

replica_set    = 'replica_set_name'
primary_host   = 'primary.local'
primary_port   = 27017
secondary_host = 'secondary.local'
secondary_port = 27017

puts 'initialize replica set'
primary = Mongo::Connection.new(primary_host, primary_port, { safe: true })
begin
  puts primary['admin'].command(replSetInitiate: nil)
rescue
end
primary.close
primary = Mongo::Connection.new(primary_host, primary_port, { safe: true })

print 'writing something'
begin
  primary['test']['replset'].insert({write: 1})
rescue
  print '.'
  sleep(2)
  retry
end
puts ''

puts 'pulling oplog for write'
fake_write = primary['local']['oplog.rs'].find().to_a.last

print 'clone oplog entry'
secondary = Mongo::Connection.new(secondary_host, secondary_port, { safe: true })
secondary['local'].create_collection('oplog.rs', { capped: true, size: 4003196912 }) # 3.72GB
begin
  secondary['local']['oplog.rs'].insert(fake_write)
rescue
  print '.'
  sleep(2)
  retry
end
puts ''

puts 'add secondary member'
replset_config = primary['local']['system.replset'].find().to_a.first
replset_config['members'] << { _id: 1, host: [ secondary_host, secondary_port.to_s ].join(':') }
replset_config['version'] += 1

primary['admin'].command(replSetReconfig: replset_config)

puts 'done'

You could have two different data dirs, but I guess moving within a datacenter isn't too bad.