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https://issues.apache.org/jira/browse/HDFS-14786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17521952#comment-17521952
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Mingliang Liu commented on HDFS-14786:
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[~panlijie] I did not have chance to work on this so I did not assign to me.
Feel free to pick it up if you are interested. It became more interesting when
EKS supports placement group. My previous projects later explore S3 as long
term storage tier for that highly critical use case (HBase/Phoenix).
> 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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