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Import Replication docs

Stuart P. Bentley 2015-03-04 13:23:33 -08:00
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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
```