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port RackDataCenterAwareReplication from Google
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Rack-Datacenter-Aware-Replication.md
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Rack-Datacenter-Aware-Replication.md
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How to use Replication
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## Introduction
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SeaweedFS can support replication. The replication is implemented not on file level, but on volume level.
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## How to use Replication
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Basically, the way it works is:
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1. start weed master, and optionally specify the default replication type
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```
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./weed master -defaultReplicationType=001
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```
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2. start volume servers as this:
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```
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./weed volume -port=8081 -dir=/tmp/1 -max=100
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./weed volume -port=8082 -dir=/tmp/2 -max=100
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./weed volume -port=8083 -dir=/tmp/3 -max=100
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```
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Submitting, Reading, Deleting files has the same steps.
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## The meaning of replication type
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Note: This subject to change.
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```
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000: no replication, just one copy
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001: replicate once on the same rack
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010: replicate once on a different rack in the same data center
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100: replicate once on a different data center
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200: replicate twice on two other different data center
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110: replicate once on a different rack, and once on a different data center
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...
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```
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So if the replication type is xyz
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```
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x: number of replica in other data centers
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y: number of replica in other racks in the same data center
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z: number of replica in other servers in the same rack
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```
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x,y,z each can be 0, 1, or 2. So there are 27 possible replication types, and can be easily extended. Each replication type will physically create x+y+z+1 copies of volume data files.
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## Example Topology Configuration File
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The WeedFS master server tries to read the topology configuration file. It default to /etc/weedfs/weedfs.conf if not specified. The topology setting to configure data center and racks file format is as this.
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```
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<Configuration>
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<Topology>
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<DataCenter name="dc1">
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<Rack name="rack1">
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<Ip>192.168.1.1</Ip>
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</Rack>
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</DataCenter>
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<DataCenter name="dc2">
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<Rack name="rack1">
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<Ip>192.168.1.2</Ip>
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</Rack>
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<Rack name="rack2">
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<Ip>192.168.1.3</Ip>
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<Ip>192.168.1.4</Ip>
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</Rack>
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</DataCenter>
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</Topology>
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</Configuration>
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```
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## Configure Topology via Volume Server
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Instead of the weed.conf file, volume servers can start with a specific data center name.
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```
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weed volume -dir=/tmp/1 -port=8080 -dataCenter=dc1
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weed volume -dir=/tmp/2 -port=8081 -dataCenter=dc2
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```
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### Allocate File Key on specific data center
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When requesting a file key, an optional "dataCenter" parameter can limit the assigned volume to the specific data center. For example, this specify
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```
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http://localhost:9333/dir/assign?dataCenter=dc1
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```
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