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From Jonathan Disher <jdis...@parad.net>
Subject Re: HDFS network bottleneck - namenode?
Date Tue, 10 May 2011 21:22:06 GMT
Okay, thanks.  I -hoped- it was this way.

Sadly, all my files are small (the largest are around 40MB).  But oh well!

-j

On May 10, 2011, at 10:46 AM, Matthew Foley wrote:

> Will's right, meta-data transactions go through the Namenode, but all the content data

> read/write activity is directly between Clients and Datanodes, and replication activity
is 
> Datanode-to-Datanode.  No bottlenecks, as long as your Namenode has enough ram to 
> hold the namespace in memory, and enough cores to handle a modestly high transaction
> rate.
> 
> And if the individual data files are large (Hadoop-scale "large", that is :-) ), you
can even 
> decrease the meta-data/data ratio by increasing the block size from the default 32MB

> to 64MB or even 128MB.
> 
> --Matt
> 
> 
> On May 10, 2011, at 6:03 AM, Will Maier wrote:
> 
> Hi Jonathan-
> 
> On Tue, May 10, 2011 at 05:50:03AM -0700, Jonathan Disher wrote:
>> I will preface this with a couple statements: a) it's almost 6am, and I've
>> been up all night b) I'm drugged up from an allergic reaction, so I may not be
>> firing on all 64 bits.
>> 
>> Do I correctly understand the HDFS architecture in that the namenode is a
>> network bottleneck into the system? I.e., it doesn't really matter how many
>> ethernet interfaces I roll into my data nodes, I will always be limited in
>> how much traffic I can drive to the HDFS pool by the network capacity of the
>> namenode?
> 
> No. This diagram should help:
> 
>   http://hadoop.apache.org/hdfs/docs/current/hdfs_design.html#NameNode+and+DataNodes
> 
> The Namenode is a single point of failure, not (under most imaginable
> conditions) a bottleneck.
> 
>> I am trying to move a -lot- of data, and i'd like to not throttle the namenode
>> (especially in the old cluster, where I cannot just bond up more interfaces).
>> If there's a way to spread the inbound network (for block writes) traffic I'd
>> love to hear it.
> 
> During our (highly distributed) migration, we were writing into HDFS at up to 5 GB/s.
> The more datanodes and writers you have, the faster your aggregate throughput.
> 
> -- 
> 
> Will Maier - UW High Energy Physics
> cel: 608.438.6162
> tel: 608.263.9692
> web: http://www.hep.wisc.edu/~wcmaier/


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