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https://issues.apache.org/jira/browse/HDFS-8945?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14712854#comment-14712854
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Masatake Iwasaki commented on HDFS-8945:
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Thanks for the comments, [~andrew.wang]!
bq. For 4+ replicas, since we've already guaranteed multi-rack with the first
3, I thought the 4th+ are just pure random.
{{BlockPlacementPolicyDefault#isGoodDatanode}} checks that the number of
replicas in the same rack is under the limit given by
{{BlockPlacementPolicyDefault#getMaxNodesPerRack}} (which was added by
HDFS-2576).
{code}
int maxNodesPerRack = (totalNumOfReplicas-1)/numOfRacks + 2;
{code}
The limit avoids that the rest of replicas allocated under the same rack.
In addition, experiment using the code of {{TestDefaultBlockPlacementPolicy}}
showed me that setting replication factor to total number of nodes in the
cluster does not always result in replicas located on all nodes.
I changed the number of nodes of mini culster to 9. /RACK0 has 6 nodes, /RACK2
has 2 and /RACK3 has 1.
{code}
final String[] racks = { "/RACK0", "/RACK0", "/RACK2", "/RACK3", "/RACK2",
"/RACK0", "/RACK0", "/RACK0", "/RACK0" };
final String[] hosts = { "/host0", "/host1", "/host2", "/host3", "/host4"
,"/host5", "/host6", "/host7", "/host8" };
{code}
When I added the code to create a file with replication factor 9, I always got
7 replicas located as below because maxNodesPerRack is 4 in this case, though
this is unusual case in which nodes are not evenly distributed among racks.
{noformat}
/RACK0
/RACK0
/RACK0
/RACK0
/RACK2
/RACK2
/RACK3
{noformat}
> Update the description about replica placement in HDFS Architecture
> documentation
> ---------------------------------------------------------------------------------
>
> Key: HDFS-8945
> URL: https://issues.apache.org/jira/browse/HDFS-8945
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: documentation
> Reporter: Masatake Iwasaki
> Assignee: Masatake Iwasaki
> Priority: Minor
> Attachments: HDFS-8945.001.patch
>
>
> The description about replica placement should have
> * Explanation about storage types and storage policies should be added
> * placement policy for replication factor greater than 4
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