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https://issues.apache.org/jira/browse/HDFS-14786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16917655#comment-16917655
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Steve Loughran commented on HDFS-14786:
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I'd talk to Dhruba and the other FB people; they mentioned that they had a
placement policy which actually split their DC up into zones (let's call them
'replication zones, RZs), and only replicated blocks within an RZ. What that
meant was that when you lost a rack, only the RZ suffered replication load -the
other RZs didn't have any extra DN or NN traffic, so the overall QoS was
actually less affected.
Think there's a JIRA where Rodrigo Schmidt mentions this and hinted that they
needed to write this up.
> A new block placement policy tolerating availability zone failure
> -----------------------------------------------------------------
>
> Key: HDFS-14786
> URL: https://issues.apache.org/jira/browse/HDFS-14786
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: block placement
> Reporter: Mingliang Liu
> Priority: Major
>
> {{NetworkTopology}} assumes "/datacenter/rack/host" 3 layer topology. Default
> block placement policies are rack awareness for better fault tolerance. Newer
> block placement policy like {{BlockPlacementPolicyRackFaultTolerant}} tries
> its best to place the replicas to most racks, which further tolerates more
> racks failing. HADOOP-8470 brought {{NetworkTopologyWithNodeGroup}} to add
> another layer under rack, i.e. "/datacenter/rack/host/nodegroup" 4 layer
> topology. With that, replicas within a rack can be placed in different node
> groups for better isolation.
> Existing block placement policies tolerate one rack failure since at least
> two racks are chosen in those cases. Chances are all replicas could be placed
> in the same datacenter, though there are multiple data centers in the same
> cluster topology. In other words, fault of higher layers beyond rack is not
> well tolerated.
> However, more deployments in public cloud are leveraging multiple available
> zones (AZ) for high-availability since the inter-AZ latency seems affordable
> in many cases. In a single AZ, some cloud providers like AWS support
> [partitioned placement
> groups|https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/placement-groups.html#placement-groups-partition]
> which basically are different racks. A simple network topology mapped to
> HDFS is "/availabilityzone/rack/host" 3 layers.
> To achieve high availability tolerating zone failure, this JIRA proposes a
> new data placement policy which tries its best to place replicas in most AZs,
> most racks, and most evenly distributed.
> Examples with 3 replicas, we choose racks as following:
> - 1AZ: fall back to {{BlockPlacementPolicyRackFaultTolerant}} to place among
> most racks
> - 2AZ: randomly choose one rack in one AZ and randomly choose two racks in
> the other AZ
> - 3AZ: randomly choose one rack in every AZ
> - 4AZ: randomly choose three AZs and randomly choose one rack in every AZ
> After racks are picked, hosts are chosen randomly within racks honoring local
> storage, favorite nodes, excluded nodes, storage types etc. Data may become
> imbalance if topology is very uneven in AZs. This seems not a problem as in
> public cloud, infrastructure provisioning is more flexible than 1P.
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