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From "Boyu Zhang" <boyuzhan...@gmail.com>
Subject RE: Questions on How the Namenode Assign Blocks to Datanodes
Date Fri, 24 Jul 2009 14:07:57 GMT
Dear Steve,

Thank you for your reply. I did worried about my email got lost, but I will
wait for an answer longer next time, thank you for reminding me : )

I understand that if you have data replica = 3, the namenode will assign the
blocks that way. However, I still have a question, if the data replica = 1,
I just use it for testing to see how HDFS works, what is the policy to
decide which datanode gets which block? Thank you so much!


Boyu Zhang

Ph. D. Student
Computer and Information Sciences Department
University of Delaware

(210) 274-2104
bzhang@udel.edu
http://www.eecis.udel.edu/~bzhang

-----Original Message-----
From: Steve Loughran [mailto:stevel@apache.org] 
Sent: Friday, July 24, 2009 7:09 AM
To: common-user@hadoop.apache.org
Subject: Re: Questions on How the Namenode Assign Blocks to Datanodes

1. dont panic if nobody replies to your message in an hour and resend. 
Hadoop developers/users are in many different timezones, and people 
often only look at this at odd times in the day. Its best to wait 24 
hours before worrying if your email got lost

2. The namenode decides, usually two blocks to one rack, another block 
to a different rack. This is to save on datacentre backbone bandwidth, 
but isolate you from the loss of an entire rack (not so unusual once 
your rack is on shared DC power/PSUs).

3. There has been discussion on having plug-in policy here, but it would 
need to work with the load balancer, the code that balances blocks 
across machines in the background.



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