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From Felix GV <fe...@mate1inc.com>
Subject Why do some blocks refuse to replicate...?
Date Thu, 28 Mar 2013 20:00:30 GMT
Hello,

I've been running a virtualized CDH 4.2 cluster. I now want to migrate all
my data to another (this time physical) set of slaves and then stop using
the virtualized slaves.

I added the new physical slaves in the cluster, and marked all the old
virtualized slaves as decommissioned using the dfs.hosts.exclude setting in
hdfs-site.xml.

Almost all of the data replicated successfully to the new slaves, but when
I bring down the old slaves, some blocks start showing up as missing or
corrupt (according to the NN UI as well as fsck*). If I restart the old
slaves, then there are no missing blocks reported by fsck.

I've tried shutting down the old slaves two by two, and for some of them I
saw no problem, but then at some point I found two slaves which, when shut
down, resulted in a couple of blocks being under-replicated (1 out of 3
replicas found). For example, fsck would report stuff like this:

/user/hive/warehouse/ads_destinations_hosts/part-m-00012:  Under replicated
BP-1207449144-10.10.10.21-1356639087818:blk_6150201737015349469_121244.
Target Replicas is 3 but found 1 replica(s).

The system then stayed in that state apparently forever. It never actually
fixed the fact some blocks were under-replicated. Does that mean there's
something wrong with some of the old datanodes...? Why do they keep block
for themselves (even thought they're decommissioned) instead of replicating
those blocks to the new (non-decommissioned) datanodes?

How do I force replication of under-replicated blocks?

*Actually, the NN UI and fsck report slightly different things. The NN UI
always seems to report 60 under-replicated blocks, whereas fsck only
reports those 60 under-replicated blocks when I shut down some of the old
datanodes... When the old nodes are up, fsck reports 0 under-replicated
blocks... This is very confusing!

Any help would be appreciated! Please don't hesitate to ask if I should
provide some of my logs, settings, or the output of some commands...!

Thanks :) !

--
Felix

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