Add note about how one-volume-server-per-disk affects replication

Production-Setup.md

@@ -110,6 +110,7 @@ Additional notes:
 * If the disk space is huge and there will be a lot of volumes, configure `-index=leveldb` to reduce memory load.
 * For busy volume servers, `-compactionMBps` can help to throttle the background jobs, e.g., compaction, balancing, encoding/decoding,etc.
 * After adding volume servers, there will not be data rebalancing. It is generally not a good idea to actively rebalance data, which cost network bandwidth and slows down other servers. Data are written to new servers after new volumes are created on them. You can use `weed shell` and run `volume.balance -force` to manually balance them.
+* Multiple volume servers on the same physical host count as separate servers for replication purposes. So if you have two physical hosts with multiple volume servers each, replication `001` (one replica in the same rack) does not guarantee that each copy will be stored on different physical hosts.
 
 ## Check the object store setup