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From Shekhar Sharma <shekhar2...@gmail.com>
Subject Re: Uploading a file to HDFS
Date Thu, 26 Sep 2013 10:45:17 GMT
Its not the namenode that does the reading or breaking of the file..
When you run the command hadoop fs -put <input> <output>.....
Here "hadoop" is a script file which is default client for hadoop..and
when the client contacts the namenode for writing, then NN creates a
block id and ask 3 dN to host the block ( replication factor to 3) and
this information is sent to client.

client caches 64KB of data on its own side and then pushes the data to
the DN and then this data gets pushed through pipeline..and this
process gets repeated till 64MB data is written and if the client
wants to to write more then he will again contact NN via heart beat
signal and this process continuess...

Check how does writing happens in HDFS?

Som Shekhar Sharma

On Thu, Sep 26, 2013 at 3:41 PM, Karim Awara <karim.awara@kaust.edu.sa> wrote:
> Hi,
> I have a couple of questions about the process of uploading a large file (>
> 10GB) to HDFS.
> To make sure my understanding is correct, assuming I have a cluster of N
> machines.
> What happens in the following:
> Case 1:
>                 assuming i want to uppload a file (input.txt) of size K GBs
> that resides on the local disk of machine 1 (which happens to be the
> namenode only). if I am running the command  -put input.txt {some hdfs dir}
> from the namenode (assuming it does not play the datanode role), then will
> the namenode read the first 64MB in a temporary pipe and then transfers it
> to one of the cluster datanodes once finished?  Or the namenode does not do
> any reading of the file, but rather asks a certain datanode to read the 64MB
> window from the file remotely?
> Case 2:
>              assume machine 1 is the namenode, but i run the -put command
> from machine 3 (which is a datanode). who will start reading the file?
> --
> Best Regards,
> Karim Ahmed Awara
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