HDFS-13156. HDFS Block Placement Policy - Client Local Rack. Contributed by Ayush Saxena.
(cherry picked from commit de44e1064f051248934ceffdd98a3cc13653d886) Project: http://git-wip-us.apache.org/repos/asf/hadoop/repo Commit: http://git-wip-us.apache.org/repos/asf/hadoop/commit/d838d39a Tree: http://git-wip-us.apache.org/repos/asf/hadoop/tree/d838d39a Diff: http://git-wip-us.apache.org/repos/asf/hadoop/diff/d838d39a Branch: refs/heads/branch-3.2 Commit: d838d39a2cb13bfd1ded50fb91f44168b66ed99e Parents: 62d329c Author: Vinayakumar B <vinayakum...@apache.org> Authored: Fri Oct 12 17:27:23 2018 +0530 Committer: Vinayakumar B <vinayakum...@apache.org> Committed: Fri Oct 12 17:28:48 2018 +0530 ---------------------------------------------------------------------- hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) ---------------------------------------------------------------------- http://git-wip-us.apache.org/repos/asf/hadoop/blob/d838d39a/hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md ---------------------------------------------------------------------- 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 471a27f..a0121e9 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, HDFSâs 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. +For the common case, when the replication factor is three, HDFSâs placement policy is to put one replica on the local machine if the writer is on a datanode, otherwise on a random datanode in the same rack as that of the writer, 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 wi thout 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 --------------------------------------------------------------------- To unsubscribe, e-mail: common-commits-unsubscr...@hadoop.apache.org For additional commands, e-mail: common-commits-h...@hadoop.apache.org
