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From "Sumadhur Reddy Bolli (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-3564) Make the replication policy pluggable to allow custom replication policies
Date Mon, 25 Jun 2012 23:49:43 GMT
Sumadhur Reddy Bolli created HDFS-3564:
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             Summary: Make the replication policy pluggable to allow custom replication policies
                 Key: HDFS-3564
                 URL: https://issues.apache.org/jira/browse/HDFS-3564
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: name-node
            Reporter: Sumadhur Reddy Bolli


ReplicationTargetChooser currently determines the placement of replicas in hadoop. Making
the replication policy pluggable would help in having custom replication policies that suit
the environment. 

Eg1: Enabling placing replicas across different datacenters(not just racks)
Eg2: Enabling placing replicas across multiple(more than 2) racks
Eg3: Cloud environments like azure have logical concepts like fault and upgrade domains. Each
fault domain spans multiple upgrade domains and each upgrade domain spans multiple fault domains.
Machines are spread typically evenly across both fault and upgrade domains. Fault domain failures
are typically catastrophic/unplanned failures and data loss possibility is high. An upgrade
domain can be taken down by azure for maintenance periodically. Each time an upgrade domain
is taken down a small percentage of machines in the upgrade domain(typically 1-2%) are replaced
due to disk failures, thus losing data. Assuming the default replication factor 3, any 3 data
nodes going down at the same time would mean potential data loss. So, it is important to have
a policy that spreads replicas across both fault and upgrade domains to ensure practically
no data loss. The problem here is two dimensional and the default policy in hadoop is one-dimensional.
Custom policies to address issues like these can be written if we make the policy pluggable.

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