Import Replication docs

Replication.md

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+SeaweedFS can support replication. The replication is implemented not on file level, but on volume level.
+
+## How to use
+
+Basically, the way it works is:
+
+1. start weed master, and optionally specify the default replication type
+
+   ```bash
+   ./weed master -defaultReplication=001
+   ```
+
+2. start volume servers as this:
+
+   ```bash
+   ./weed volume -port=8081 -dir=/tmp/1 -max=100
+   ./weed volume -port=8082 -dir=/tmp/2 -max=100
+   ./weed volume -port=8083 -dir=/tmp/3 -max=100
+   ```
+
+Submitting, Reading, Deleting files has the same steps.
+
+## The meaning of replication type
+
+*Note: This subject to change.*
+
+Value | Meaning
+---|---
+000 | no replication, just one copy
+001 | replicate once on the same rack
+010 | replicate once on a different rack in the same data center
+100 | replicate once on a different data center
+200 | replicate twice on two other different data center
+110 | replicate once on a different rack, and once on a different data center
+... | ...
+
+So if the replication type is xyz
+
+Column | Meaning
+---|---
+**x** | number of replica in other data centers
+**y** | number of replica in other racks in the same data center
+**z** | number of replica in other servers in the same rack
+
+x,y,z each can be 0, 1, or 2. So there are 9 possible replication types, and can be easily extended. 
+Each replication type will physically create x+y+z+1 copies of volume data files.
+
+Example topology configuration 
+###################################
+
+The SeaweedFS master server tries to read the default topology configuration file are read from /etc/weedfs/weedfs.conf, if it exists. The topology setting to configure data center and racks file format is as this.
+
+```xml
+<Configuration>
+  <Topology>
+    <DataCenter name="dc1">
+      <Rack name="rack1">
+        <Ip>192.168.1.1</Ip>
+      </Rack>
+    </DataCenter>
+    <DataCenter name="dc2">
+      <Rack name="rack1">
+        <Ip>192.168.1.2</Ip>
+      </Rack>
+      <Rack name="rack2">
+        <Ip>192.168.1.3</Ip>
+        <Ip>192.168.1.4</Ip>
+      </Rack>
+    </DataCenter>
+  </Topology>
+</Configuration>
+```
+
+Allocate File Key on specific data center
+Volume servers can start with a specific data center name.
+
+```bash
+weed volume -dir=/tmp/1 -port=8080 -dataCenter=dc1
+weed volume -dir=/tmp/2 -port=8081 -dataCenter=dc2
+```
+
+Or the master server can determine the data center via volume server's IP address and settings in weed.conf file.
+
+Now when requesting a file key, an optional "dataCenter" parameter can limit the assigned volume to the specific data center. For example, this specify
+
+```bash
+http://localhost:9333/dir/assign?dataCenter=dc1
+```