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Posted to common-commits@hadoop.apache.org by aa...@apache.org on 2017/05/16 15:53:28 UTC
hadoop git commit: HDFS-11833. HDFS architecture documentation
descibes outdated placement policy. Contributed by Chen Liang.
Repository: hadoop
Updated Branches:
refs/heads/trunk 489f85933 -> 1d1c52b42
HDFS-11833. HDFS architecture documentation descibes outdated placement policy. Contributed by Chen Liang.
Project: http://git-wip-us.apache.org/repos/asf/hadoop/repo
Commit: http://git-wip-us.apache.org/repos/asf/hadoop/commit/1d1c52b4
Tree: http://git-wip-us.apache.org/repos/asf/hadoop/tree/1d1c52b4
Diff: http://git-wip-us.apache.org/repos/asf/hadoop/diff/1d1c52b4
Branch: refs/heads/trunk
Commit: 1d1c52b42feae5a4271ef4b771d0d8de43e83c15
Parents: 489f859
Author: Akira Ajisaka <aa...@apache.org>
Authored: Tue May 16 11:52:33 2017 -0400
Committer: Akira Ajisaka <aa...@apache.org>
Committed: Tue May 16 11:52:55 2017 -0400
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hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
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http://git-wip-us.apache.org/repos/asf/hadoop/blob/1d1c52b4/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md
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diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md b/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md
index bda53a9..4bf1897 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md
@@ -102,7 +102,7 @@ Large HDFS instances run on a cluster of computers that commonly spread across m
The NameNode determines the rack id each DataNode belongs to via the process outlined in [Hadoop Rack Awareness](../hadoop-common/RackAwareness.html).
A simple but non-optimal policy is to place replicas on unique racks. This prevents losing data when an entire rack fails and allows use of bandwidth from multiple racks when reading data. This policy evenly distributes replicas in the cluster which makes it easy to balance load on component failure. However, this policy increases the cost of writes because a write needs to transfer blocks to multiple racks.
-For the common case, when the replication factor is three, HDFS’s placement policy is to put one replica on one node in the local rack, another on a different node in the local rack, and the last on a different node in a different rack. This policy cuts the inter-rack write traffic which generally improves write performance. The chance of rack failure is far less than that of node failure; this policy does not impact data reliability and availability guarantees. However, it does reduce the aggregate network bandwidth used when reading data since a block is placed in only two unique racks rather than three. With this policy, the replicas of a file do not evenly distribute across the racks. One third of replicas are on one node, two thirds of replicas are on one rack, and the other third are evenly distributed across the remaining racks. This policy improves write performance without compromising data reliability or read performance.
+For the common case, when the replication factor is three, HDFS’s placement policy is to put one replica on the local machine if the writer is on a datanode, otherwise on a random datanode, another replica on a node in a different (remote) rack, and the last on a different node in the same remote rack. This policy cuts the inter-rack write traffic which generally improves write performance. The chance of rack failure is far less than that of node failure; this policy does not impact data reliability and availability guarantees. However, it does reduce the aggregate network bandwidth used when reading data since a block is placed in only two unique racks rather than three. With this policy, the replicas of a file do not evenly distribute across the racks. One third of replicas are on one node, two thirds of replicas are on one rack, and the other third are evenly distributed across the remaining racks. This policy improves write performance without compromising data reliability or
read performance.
If the replication factor is greater than 3,
the placement of the 4th and following replicas are determined randomly
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