Updated Erasure coding for warm storage (markdown)

Erasure-coding-for-warm-storage.md

@@ -5,4 +5,30 @@ Warm data are accessed less frequently. To store them more efficiently, you can
 * SeaweedFS uses continuous 1GB block layout with 1MB block sizes for edge cases, optimized for both small file reads and storage efficiency.
 * Efficient memory usage and faster startup time. The volume server does not load index data into memory.
 
-## How to enable it?
+## How to enable it?
+Run `weed scaffold -conf=master` to generate a `master.toml` file, put it in current directory, `~/.seaweedfs/`, or `/etc/seaweedfs/`.
+
+It will add a list of commands executed periodically. Actually the commands can also be executed via `weed shell` with exactly the same effect. The scripts stored in the `master.toml` file is to make the deployment convenient.
+
+## How the scripts works?
+The scripts has 3 steps.
+
+### 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 has no updates for 1 hour.
+
+### Data Repair
+The default command is `ec.rebuild -force`. If disk fails or server fails, some data shards are lost. With erasure coding, we can recover the lost data shards from remaining data shards.
+
+### EC data balancing
+With servers added or removed, some data shards may not be laid out optimally. For example, one volume's 5 data shards could be on the same server. If the server goes down, the volume would be unrepairable or part of the data is lost permanently. 
+
+The default command is `ec.balance -force`. It will try to spread the data shards to minimize the data loss risk.
+
+## How the read works?
+When all data shards are online, the read are randomly assigned to one volume server (A) that has at least one data shard. Server A will read its copy of index file, and locate the volume server (B), and read from server B for the file.
+
+
+
+## Read Performance