Document new topology-aware EC rebalancing logic.

Erasure-Coding-for-warm-storage.md

@@ -47,7 +47,7 @@ The scripts have 3 steps related to erasure coding.
 ### Erasure Encode Sealed Data
 `ec.encode` command will find volumes that are almost full and has been stale for a period of time.
 
-The default command is `ec.encode -fullPercent=95 -quietFor=1h`. It will find volumes at least 95% of the maximum volume size, which is usually 30GB, and have no updates for 1 hour.
+The default command is `ec.encode -fullPercent=95 -quietFor=1h -rebalance`. It will find volumes at least 95% of the maximum volume size, which is usually 30GB, and have no updates for 1 hour. Once the encoding is completed, EC shards are re-balanced; see [EC data balancing](#ec-data-balancing) below.
 
 Note that if you have any collections, i.e. because you're using s3 where every bucket is a collection you explicitly need to specify the collection for the ec.encode command for them to be erasure coded `ec.encode -collection="collection" -fullPercent=95 -quietFor=1h`.
 
@@ -67,6 +67,14 @@ With servers added or removed, some data shards may not be laid out optimally. F
 
 The default command is `ec.balance -force`. It will try to spread the data shards evenly to minimize the data shard loss risk.
 
+EC shard re-balancing happens in three steps:
+
+  1. Duplicate EC shards for the same volume + server are deleted.
+  2. EC shards are balanced across volumes for all racks.
+  3. EC shards are balanced across volumes for individual racks.
+
+Destination volume/racks for EC shards are selected based on capacity, favoring those with less preexisting shards in order to ensure an uniform distribution. Additionally, EC shards selection obey default [replica placement settings](Replication#the-meaning-of-replication-type) for the master server.
+
 ## How the read works?
 When all data shards are online, the read for one file key are assigned to one volume server (A) that has at least one data shard for the volume. Server A will read its copy of index file, and locate the volume server (B), and read from server B for the file key.