1. each file can choose the replication factor
2. replication granularity is in volume level
3. if not enough spaces, we can automatically decrease some volume's the replication factor, especially for cold data
4. support migrating data to cheaper storage
5. manual volume placement, access-based volume placement, auction based volume placement

When a new volume server is started, it reports 
  1. how many volumes it can hold
  2. current list of existing volumes
Each volume server remembers:
  1. current volume ids, replica locations

The master assign volume ids based on
  1. replication factor
     data center, rack
  2. concurrent write support
On master, stores the replication configuration
{
  replication:{
    {factor:1, min_volume_count:3, weight:10},
    {factor:2, min_volume_count:2, weight:20},
    {factor:3, min_volume_count:3, weight:30}
  },
  port:9333,
  
}
Or manually via command line
  1. add volume with specified replication factor
  2. add volume with specified volume id


If duplicated volume ids are reported from different volume servers, 
the master determines the replication factor of the volume,
if less than the replication factor, the volume is in readonly mode
if more than the replication factor, the volume will purge the smallest/oldest volume
if equal, the volume will function as usual

maybe use gossip to send the volumeServer~volumes information


Use cases:
  on volume server
    1. weed volume -mserver="xx.xx.xx.xx:9333" -publicUrl="good.com:8080" -dir="/tmp" -volumes=50
  on weed master
    1. weed master -port=9333
      generate a default json configuration file if doesn't exist
      
Bootstrap
  1. at the very beginning, the system has no volumes at all.
  2. if maxReplicationFactor==1, always initialize volumes right away
  3. if nServersHasFreeSpaces >= maxReplicationFactor, auto initialize
  4. if maxReplicationFactor>1
     weed shell
     > disable_auto_initialize
     > enable_auto_initialize
     > assign_free_volume vid "server1:port","server2:port","server3:port"
     > status
  5.