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From Dennis Kubes <ku...@apache.org>
Subject Re: Loading data into HDFS
Date Fri, 03 Aug 2007 06:57:04 GMT
You can copy data from any node, so if you can do it from multiple nodes 
your performance would be better (although be sure not to overlap 
files).  The master node is updated once a the block is copied it 
replication number of times.  So if default replication is 3 then the 3 
replicates must be active before the master is updated and the data 
"appears" int the dfs.

How long the updates take to happen is a function of your server load 
and network speed and file size.  Generally it is fast.

So the process is the data is loaded into the dfs, replicates are 
created, and the master node is updated.  In terms of consistency, if 
the data node crashes before the data is loaded then the data won't 
appear in the dfs.  If the name node crashes before it is updated but 
all replicates are active, the data would appear once the name node has 
been fixed and updated through block reports.  If a single node crashes 
that has a replicate once the namenode has been updated then the data 
will be replicated from one of the other 2 replicates to another 3 
system if available.

Dennis Kubes

Venkates .P.B. wrote:
> Am I missing something very fundamental ? Can someone comment on these
> queries ?
> 
> Thanks,
> Venkates P B
> 
> On 8/1/07, Venkates .P.B. <venkates.pb@gmail.com> wrote:
>>
>> Few queries regarding the way data is loaded into HDFS.
>>
>> -Is it a common practice to load the data into HDFS only through the
>> master node ? We are able to copy only around 35 logs (64K each) per minute
>> in a 2 slave configuration.
>>
>> -We are concerned about time it would take to update filenames and block
>> maps in the master node when data is loaded from few/all the slave nodes.
>> Can anyone let me know how long generally it takes for this update to
>> happen.
>>
>> And one more question, what if the node crashes soon after the data is
>> copied into one it. How is data consistency maintained here ?
>>
>> Thanks in advance,
>> Venkates P B
>>
> 

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