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From Arun Ramakrishnan <aramakrish...@languageweaver.com>
Subject rebalancing replication help
Date Thu, 08 Jul 2010 01:18:56 GMT
Looks like there is not much activity in the hdfs-user list. So, am reposting it in the general

Hi guys.
  I have a few related questions. I am going to layout the steps I have taken. Please comment
on what I can do better.

  I was trying to to add 5 nodes to my existing 10 node cluster and also increase the replication
factor from 2 to 3.
I thought I don't have to run the balancer cause it would most likely put the new replicas
into the new nodes.

There are about 500k blocks.
I wanted to get it all stabilized(replication and balancing) within 24 hours. Its more than
24 hours now and fsck reports 30% under replication. Is there a way to force hdfs to use balance/replicate
more aggressively.

It would be great if someone explained what/when things happen to blocks in the context of

1)      Rebalancing

2)      -setrep

3)      Restarting cluster with a higher/lower replication factor.

A few questions and a few issues here.

1)      When you restart the cluster with a higher than previous replication value. Does it
also apply to existing blocks or only to new blocks being created ?

2)      Does the balancer take into account under replication of blocks or does it blindly
start moving existing blocks to reach threshold ?

A very specific problem .  I am having this strange problem where the -setrep hangs on one
particular block for hours. Is this because its corrupt ?. But, fsck said its healthy.


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