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From Ravi Prakash <ravihad...@gmail.com>
Subject Re: Confusion between dfs.replication and dfs.namenode.replication.min options in hdfs-site.xml
Date Thu, 02 Feb 2017 22:06:17 GMT
Hi Andrey!

Your assumption is absolutely correct. dfs.namenode.replication.min is what
you should set to 2 in your case. You should also look at
dfs.client.block.write.replace-datanode-on-failure.enable and


On Wed, Feb 1, 2017 at 1:37 PM, Andrey Elenskiy <andrey.elenskiy@arista.com>

> Hello,
> I use hadoop 2.7.3 non-HA setup with hbase 1.2.3 on top of it.
> I'm trying to understand these options in hdfs-site.xml:
> dfs.replication
> 3 Default block replication. The actual number of replications can be
> specified when the file is created. The default is used if replication is
> not specified in create time.
> dfs.namenode.replication.min
> 1 Minimal block replication.
> What I'm trying to do is to make sure that on write we always end up with
> 2 replicas minimum. In other words, a write should fail if we don't end up
> with 2 replicas of each block.
> As I understand, on write, hadoop creates a write pipeline of datanodes
> where each datanode writes to the next one. Here's a diagram from Cloudera:
> [image: Inline image 1]
> Is it correct to say that the dfs.namenode.replication.min option
> controls how many datanodes in the pipeline must have COMPLETEd the block
> in order to consider a write successful and then acks to the client about
> success? And dfs.replication option means that we eventually want to have
> this many replicas of each block, but it doesn't need to be done at the
> write time but could be done asynchronously later by the Namenode?
> So, essentially, if I want a guarantee that I have one back up of each
> block at all times, I need to set to dfs.namenode.replication.min=2. And,
> if I want to make sure that I won't go into safemode on startup too
> often, I should set dfs.replication = 3 to tolerate one replica loss.

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