Exploring the HDFS Default Value Behaviour

In this post I’ll explore how the HDFS default values really work, which I found to be quite surprising and non-intuitive, so there s a good lesson here.

In my case, I have my local virtual Hadoop cluster, and I have a different client outside the cluster. I’ll just put a file from the external client into the cluster in two configurations.

All the nodes of my Hadoop (1.2.1) cluster have the same hdfs-site.xml file with the same (non-default) value for dfs.block.size (renamed to dfs.blocksize in Hadoop 2.x) of 134217728, which is 128MB. In my external node, I also have the hadoop executables with a minimal hdfs-site.xml.

First, I have set dfs.block.size to 268435456 (256MB) in my client hdfs-site.xml and copied a 400MB file to HDFS:

 ./hadoop fs -copyFromLocal /sw/400MB.file /user/ofir

Checking its block size from the NameNode:

./hadoop fsck /user/ofir/400MB.file -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /user/ofir/400MB.file at Thu Jan 30 22:21:29 UTC 2014
/user/ofir/400MB.file 419430400 bytes, 2 block(s):  OK
0. blk_-5656069114314652598_27957 len=268435456 repl=3 [/rack03/10.0.1.133:50010, /rack03/10.0.1.132:50010, /rack02/10.0.1.122:50010]
1. blk_3668240125470951962_27957 len=150994944 repl=3 [/rack03/10.0.1.133:50010, /rack03/10.0.1.132:50010, /rack01/10.0.1.113:50010]

Status: HEALTHY
 Total size:    419430400 B
 Total dirs:    0
 Total files:    1
 Total blocks (validated):    2 (avg. block size 209715200 B)
 Minimally replicated blocks:    2 (100.0 %)
 Over-replicated blocks:    0 (0.0 %)
 Under-replicated blocks:    0 (0.0 %)
 Mis-replicated blocks:        0 (0.0 %)
 Default replication factor:    1
 Average block replication:    3.0
 Corrupt blocks:        0
 Missing replicas:        0 (0.0 %)
 Number of data-nodes:        6
 Number of racks:        3
FSCK ended at Thu Jan 30 22:21:29 UTC 2014 in 1 milliseconds

So far, looks good – the first block is 256MB.

Now, let’s remove the reference for dfs.block.size from my hdfs-site.xml file on the client. I expected that, since we don’t specify a value in our client, it should not ask for any specific block size, so the actual block size will be the value set in the NameNode and DataNodes – 128MB. However, this is what I got:

./hadoop fs -copyFromLocal /sw/400MB.file /user/ofir/400MB.file2

Checking from the NameNode:

# ./hadoop fsck /user/ofir/400MB.file2 -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /user/ofir/400MB.file2 at Thu Jan 30 22:27:52 UTC 2014
/user/ofir/400MB.file2 419430400 bytes, 7 block(s):  OK
0. blk_1787803769905799654_27959 len=67108864 repl=3 [/rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack01/10.0.1.113:50010]
1. blk_3875160333629322317_27959 len=67108864 repl=3 [/rack01/10.0.1.112:50010, /rack01/10.0.1.113:50010, /rack03/10.0.1.132:50010]
2. blk_-3214658865520475261_27959 len=67108864 repl=3 [/rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack02/10.0.1.122:50010]
3. blk_-3454099016147726286_27959 len=67108864 repl=3 [/rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack01/10.0.1.113:50010]
4. blk_-4422811025086071415_27959 len=67108864 repl=3 [/rack01/10.0.1.112:50010, /rack01/10.0.1.113:50010, /rack02/10.0.1.122:50010]
5. blk_8134056524051282216_27959 len=67108864 repl=3 [/rack02/10.0.1.123:50010, /rack02/10.0.1.122:50010, /rack03/10.0.1.132:50010]
6. blk_2811491227269477239_27959 len=16777216 repl=3 [/rack02/10.0.1.122:50010, /rack02/10.0.1.123:50010, /rack03/10.0.1.133:50010]

Status: HEALTHY
 Total size:    419430400 B
 Total dirs:    0
 Total files:    1
 Total blocks (validated):    7 (avg. block size 59918628 B)
 Minimally replicated blocks:    7 (100.0 %)
 Over-replicated blocks:    0 (0.0 %)
 Under-replicated blocks:    0 (0.0 %)
 Mis-replicated blocks:        0 (0.0 %)
 Default replication factor:    1
 Average block replication:    3.0
 Corrupt blocks:        0
 Missing replicas:        0 (0.0 %)
 Number of data-nodes:        6
 Number of racks:        3
FSCK ended at Thu Jan 30 22:27:52 UTC 2014 in 2 milliseconds

The filesystem under path '/user/ofir/400MB.file2' is HEALTHY

Well, the file definitely has a block size of 64MB, not 128MB! This is actually the default value for dfs.block.size in Hadoop 1.x.

It seems that the value of dfs.block.size is dictated directly by the client, regarding of the cluster setting. If a value is not specified, the client just picks the default value. This finding is not  specific to this parameter – for example, the same thing happens with dfs.replication and others….

So, if you ever wondered why you must have full copy of the Hadoop config files around instead of just pointing to the NameNode and JobTracker, now you know…
Now double check your client setup!

Corrections and comments are welcomed!

P.S. Just as I was publishing it, I found a shorter way to check the block size of a file, that also works from any client:

# ./hadoop fs -stat %o /user/ofir/400MB.file
268435456
# ./hadoop fs -stat %o  /user/ofir/400MB.file2
67108864
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Exploring HDFS Block Placement Policy

In this post I’ll cover how to see where each data block is actually placed. I’m using my fully-distributed Hadoop cluster on my laptop for exploration and the network topology definitions from my previous post – you’ll need a multi-node cluster to reproduce it.

Background

When a file is written to HDFS, it is split up into big chucks called data blocks, whose size is controlled by the parameter dfs.block.size in the config file hdfs-site.xml (in my case – left as the default which is 64MB). Each block is stored on one or more nodes, controlled by the parameter dfs.replication in the same file (in most of this post – set to 3, which is the default). Each copy of a block is called a replica.

Regarding my setup – I have nine nodes spread over three racks. Each rack has one admin node and two worker nodes (running DataNode and TaskTracker), using Apache Hadoop 1.2.1. Please note that in my configuration, there is a simple mapping between host name, IP address and rack number -host name is hadoop[rack number][node number] and IP address is 10.0.1.1[rack number][node number].

Where should the NameNode place each replica?  Here is the theory:

The default block placement policy is as follows:

  • Place the first replica somewhere – either a random node (if the HDFS client is outside the Hadoop/DataNode cluster) or on the local node (if the HDFS client is running on a node inside the cluster).
  • Place the second replica in a different rack.
  • Place the third replica in the same rack as the second replica
  • If there are more replicas – spread them across the rest of the racks.

That placement algorithm is a decent default compromise – it provides protection against a rack failure while somewhat minimizing inter-rack traffic.

So, let’s put some files on HDFS and see if it behaves like the theory in my setup – try it on yours to validate.

Putting a file into HDFS from outside the Hadoop cluster

As my block size is 64MB,  I created a file (called /sw/big) on my laptop for testing. The file is about 125MB – so it consumes two HDFS blocks. It is not inside one of the data node containers but outside the Hadoop cluster. I then copied it to HDFS twice:

# /sw/hadoop-1.2.1/bin/hadoop fs -copyFromLocal /sw/big /big1
# /sw/hadoop-1.2.1/bin/hadoop fs -copyFromLocal /sw/big /big2

In order to see the actual block placement, of a specific file, I ran hadoop fsck filename -files -blocks -racks from the NameNode server and looked at the top of the report:

# bin/hadoop fsck /big1 -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /big1 at Mon Jan 06 11:20:01 UTC 2014
/big1 130633102 bytes, 2 block(s):  OK
0. blk_3712902403633386081_1008 len=67108864 repl=3 [/rack03/10.0.1.133:50010, /rack03/10.0.1.132:50010, /rack02/10.0.1.123:50010]
1. blk_381038406874109076_1008 len=63524238 repl=3 [/rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack01/10.0.1.113:50010]

Status: HEALTHY
 Total size:    130633102 B
 Total dirs:    0
 Total files:    1
 Total blocks (validated):    2 (avg. block size 65316551 B)
 Minimally replicated blocks:    2 (100.0 %)
 Over-replicated blocks:    0 (0.0 %)
 Under-replicated blocks:    0 (0.0 %)
 Mis-replicated blocks:        0 (0.0 %)
 Default replication factor:    3
 Average block replication:    3.0
 Corrupt blocks:        0
 Missing replicas:        0 (0.0 %)
 Number of data-nodes:        6
 Number of racks:        3
FSCK ended at Mon Jan 06 11:20:01 UTC 2014 in 3 milliseconds

The filesystem under path '/big1' is HEALTHY

You can see near the end that the NameNode correctly reports it know of six data nodes and three racks. If we look specifically at the block report:

0. blk_37129... len=67108864 repl=3 [/rack03/10.0.1.133:50010, /rack03/10.0.1.132:50010, /rack02/10.0.1.123:50010]
1. blk_38103... len=63524238 repl=3 [/rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack01/10.0.1.113:50010]

We can see:

  • Block 0 of file big1 is on hadoop23 (rack 2), hadoop32 (rack 3), hadoop33 (rack 3)
  • Block 1 of file big1 is on hadoop13 (rack1), hadoop32 (rack 3), hadoop33 (rack 3)

Running the same report on big2:

# bin/hadoop fsck /big2 -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /big2 at Mon Jan 06 11:49:36 UTC 2014
/big2 130633102 bytes, 2 block(s):  OK
0. blk_-2514176538290345389_1009 len=67108864 repl=3 [/rack02/10.0.1.122:50010, /rack02/10.0.1.123:50010, /rack03/10.0.1.133:50010]
1. blk_316929026766197453_1009 len=63524238 repl=3 [/rack01/10.0.1.113:50010, /rack01/10.0.1.112:50010, /rack03/10.0.1.133:50010]
...

We can see that:

  • Block 0 of file big2 is on hadoop33 (rack 3), hadoop22 (rack 2), hadoop23 (rack 2)
  • Block 1 of file big2 is on hadoop33 (rack 3), hadoop12 (rack 1), hadoop13 (rack 1)

Conclusion – we can see that when the HDFS client is outside the Hadoop cluster, for each block, there is a single replica on a random rack and two other replicas on a different, random rack, as expected.

Putting a file into HDFS from within the Hadoop cluster

OK, let’s repeat the exercise, now copying the big file from a local path on one of the data nodes (I used hadoop22) to HDFS:

# /usr/local/hadoop-1.2.1/bin/hadoop fs -copyFromLocal big /big3
# /usr/local/hadoop-1.2.1/bin/hadoop fs -copyFromLocal big /big4

We expect to see the first replica of all blocks to be local – on node hadoop22.
Running the report on big3:

# bin/hadoop fsck /big3 -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /big3 at Mon Jan 06 12:10:03 UTC 2014
/big3 130633102 bytes, 2 block(s):  OK
0. blk_-509446789413926742_1013 len=67108864 repl=3 [/rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack02/10.0.1.122:50010]
1. blk_1461305132201468085_1013 len=63524238 repl=3 [/rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack02/10.0.1.122:50010]
...

We can see that:

  • Block 0 of file big3 is on hadoop22 (rack 2), hadoop33 (rack 3), hadoop32 (rack 3)
  • Block 1 of file big3 is on hadoop22 (rack 2), hadoop33 (rack 3), hadoop32 (rack 3)

And for big4:

# bin/hadoop fsck /big4 -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /big4 at Mon Jan 06 12:12:52 UTC 2014
/big4 130633102 bytes, 2 block(s):  OK
0. blk_-8460635609179877275_1014 len=67108864 repl=3 [/rack03/10.0.1.133:50010, /rack03/10.0.1.132:50010, /rack02/10.0.1.122:50010]
1. blk_8256749577534695387_1014 len=63524238 repl=3 [/rack01/10.0.1.112:50010, /rack02/10.0.1.122:50010, /rack01/10.0.1.113:50010]
  • Block 0 of file big4 is on hadoop22 (rack 2), hadoop32 (rack 3), hadoop33 (rack 3)
  • Block 1 of file big4 is on hadoop22 (rack 2), hadoop12 (rack 1), hadoop13 (rack 1)

Conclusion – we can see that when the HDFS client is inside the Hadoop cluster for each block, there is a single replica on the local node (hadoop22 in this case) and two other replicas on a different,random rack, as expected.

Just for fun – create a file with more than three replicas

We can override Hadoop parameters when invoking the shell using -D option. Let’s use it to create a file with four replicas from node hadoop22:

# /usr/local/hadoop-1.2.1/bin/hadoop fs -D dfs.replication=4 -copyFromLocal ~/big /big5

Running the report on big5:

# bin/hadoop fsck /big5 -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /big5 at Mon Jan 06 12:36:28 UTC 2014
/big5 130633102 bytes, 2 block(s):  OK
0. blk_-4060957873241541489_1015 len=67108864 repl=4 [/rack01/10.0.1.112:50010, /rack03/10.0.1.133:50010, /rack03/10.0.1.132:50010, /rack02/10.0.1.122:50010]
1. blk_8361194277567439305_1015 len=63524238 repl=4 [/rack03/10.0.1.133:50010, /rack01/10.0.1.113:50010, /rack01/10.0.1.112:50010, /rack02/10.0.1.122:50010]
...

We can see that:

  • Block 0 of file big3 is on hadoop22 (rack 2), hadoop33 (rack 3), hadoop32 (rack3), hadooop12 (rack1)
  • Block 1 of file big3 is on hadoop22 (rack 2), hadoop13 (rack 1), hadoop12 (rack1), hadooop33 (rack3)

So far, works as expected! That’s a nice surprise…

Finally, let’s try to break something 🙂

What will happen when we’ll try to specify more replicas than data nodes?

While it is “stupid”, it will likely happen in pseudo-distributed config every time you try dfs.replication > 1, as you have only one DataNode.

# /usr/local/hadoop-1.2.1/bin/hadoop fs -D dfs.replication=7 -copyFromLocal ~/big /big7

We do have only six data nodes. Running the report on big7:

# bin/hadoop fsck /big7 -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /big7 at Mon Jan 06 12:40:05 UTC 2014
/big7 130633102 bytes, 2 block(s):  Under replicated blk_1565930173693194428_1016. Target Replicas is 7 but found 6 replica(s).
 Under replicated blk_-2825553771191528109_1016. Target Replicas is 7 but found 6 replica(s).
0. blk_1565930173693194428_1016 len=67108864 repl=6 [/rack01/10.0.1.112:50010, /rack01/10.0.1.113:50010, /rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack02/10.0.1.123:50010, /rack02/10.0.1.122:50010]
1. blk_-2825553771191528109_1016 len=63524238 repl=6 [/rack01/10.0.1.112:50010, /rack01/10.0.1.113:50010, /rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack02/10.0.1.123:50010, /rack02/10.0.1.122:50010]

Status: HEALTHY
 Total size:    130633102 B
 Total dirs:    0
 Total files:    1
 Total blocks (validated):    2 (avg. block size 65316551 B)
 Minimally replicated blocks:    2 (100.0 %)
 Over-replicated blocks:    0 (0.0 %)
 Under-replicated blocks:    2 (100.0 %)
 Mis-replicated blocks:        0 (0.0 %)
 Default replication factor:    3
 Average block replication:    6.0
 Corrupt blocks:        0
 Missing replicas:        2 (16.666666 %)
 Number of data-nodes:        6
 Number of racks:        3
FSCK ended at Mon Jan 06 12:40:05 UTC 2014 in 1 milliseconds

We can see that each data node has a single copy of each block, and fsck notifies us that the two blocks are under-replicated. Nice!

Changing the replication factor of an existing file.

Since this warning in fsck might be annoying (and depending on your monitoring scripts, sets various alarms…), we should change the replication factor of this file:

# bin/hadoop fs -setrep 6 /big7
Replication 6 set: hdfs://hadoop11:54310/big7

In this case, the NameNode won’t immediate delete the extra replicas:

# bin/hadoop fsck /big7 -files -blocks -racks
FSCK started by root from /10.0.1.111 for path /big7 at Mon Jan 06 12:51:00 UTC 2014
/big7 130633102 bytes, 2 block(s):  OK
0. blk_1565930173693194428_1016 len=67108864 repl=6 [/rack01/10.0.1.112:50010, /rack01/10.0.1.113:50010, /rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack02/10.0.1.123:50010, /rack02/10.0.1.122:50010]
1. blk_-2825553771191528109_1016 len=63524238 repl=6 [/rack01/10.0.1.112:50010, /rack01/10.0.1.113:50010, /rack03/10.0.1.132:50010, /rack03/10.0.1.133:50010, /rack02/10.0.1.123:50010, /rack02/10.0.1.122:50010]

Status: HEALTHY
 Total size:    130633102 B
 Total dirs:    0
 Total files:    1
 Total blocks (validated):    2 (avg. block size 65316551 B)
 Minimally replicated blocks:    2 (100.0 %)
 Over-replicated blocks:    2 (100.0 %)
 Under-replicated blocks:    0 (0.0 %)
 Mis-replicated blocks:        0 (0.0 %)
 Default replication factor:    3
 Average block replication:    6.0
 Corrupt blocks:        0
 Missing replicas:        0 (0.0 %)
 Number of data-nodes:        6
 Number of racks:        3
FSCK ended at Mon Jan 06 12:51:00 UTC 2014 in 1 milliseconds

The filesystem under path '/big7' is HEALTHY

If you are in a hurry, adding -w to the command should trigger immediate replication level change:

# bin/hadoop fs -setrep 5 -w /big7
Replication 5 set: hdfs://hadoop11:54310/big7
Waiting for hdfs://hadoop11:54310/big7 .... done

Exploring The Hadoop Network Topology

In this post I’ll cover how to define and debug a simple Hadoop network topology.

Background

Hadoop is designed to run on large clusters of commodity servers – in many cases spanning many physical racks of servers. A physical rack is in many cases a single point of failure (for example, having typically a single switch for lower cost), so HDFS tries to place block replicas on more than one rack. Also, there is typically more bandwidth within a rack than between the racks, so the software on the cluser (HDFS and MapReduce / YARN) can take it into account. This leads to a question:

How does the NameNode know the network topology?

By default, the NameNode has no idea which node is in which rack. It therefore by default assumes that all nodes are in the same rack, which is likely true for small clusters. It calls this rack “/default-rack“.

So, we have to teach Hadoop our cluster network topology – the way the nodes are grouped into racks. Hadoop supports a pluggable rack topology implementation – controlled by the parameter topology.node.switch.mapping.impl in core-site.xml, which specifies a java class implementation. The default implementation is using a user-provided script, specified in topology.script.file.name in the same config file, a script that gets a list of IP addresses or host names and returns a list of rack names (in Hadoop2: net.topology.script.file.name). The script will get up to topology.script.number.args parameters per invocation, by default up to 100 requests per invocation (in Hadoop2: net.topology.script.number.args).

In my case, I set it to /usr/local/hadoop-1.2.1/conf/topology.script.sh which I copied from the Hadoop wiki. I just made a few changes – I changed the path to my conf directory in the second line, added some logging of the call in the third line and changed the default rack name near the end to /rack01:

#!/bin/bash          
HADOOP_CONF=/usr/local/hadoop-1.2.1/conf
echo `date` input: $@ >> $HADOOP_CONF/topology.log
while [ $# -gt 0 ] ; do
  nodeArg=$1
  exec< ${HADOOP_CONF}/topology.data
  result=""
  while read line ; do
    ar=( $line )
    if [ "${ar[0]}" = "$nodeArg" ] ; then
      result="${ar[1]}"
    fi
  done
  shift
  if [ -z "$result" ] ; then
#    echo -n "/default/rack "
     echo -n "/rack01"
  else
    echo -n "$result "
  fi
done

The script basically just parses a text file (topology.data) that holds a mapping from IP address or host name to rack name. Here are the content of my file:

hadoop11        /rack01
hadoop12        /rack01
hadoop13        /rack01
hadoop14        /rack01
haddop15        /rack01
hadoop21        /rack02
hadoop22        /rack02
hadoop23        /rack02
hadoop24        /rack02
hadoop25        /rack02
hadoop31        /rack03
hadoop32        /rack03
hadoop33        /rack03
hadoop34        /rack03
hadoop35        /rack03
10.0.1.111      /rack01
10.0.1.112      /rack01
10.0.1.113      /rack01
10.0.1.114      /rack01
10.0.1.115      /rack01
10.0.1.121      /rack02
10.0.1.122      /rack02
10.0.1.123      /rack02
10.0.1.124      /rack02
10.0.1.125      /rack02
10.0.1.131      /rack03
10.0.1.132      /rack03
10.0.1.133      /rack03
10.0.1.134      /rack03
10.0.1.135      /rack03

A bit long, but pretty straight forward. Please note that in my configuration, there is a simple mapping between host name, IP address and rack number – the host name is hadoop[rack number][node number] and IP address is 10.0.1.1[rack number][node number].

You could of course write any logic into the script. For example, using my naming convention, I could have written a simple script that just takes the second-to-last character and translate it to rack number – that would work on both IP addresses and host names. As another example – I know some companies allocate IP addresses for their Hadoop cluster as x.y.[rack_number].[node_number] – so they can again just parse it directly.

Before we test the script, if you have read my previous post on LXC setup, please note that I made  a minor change – I switched to a three-rack cluster, to make the block placement more interesting.  So, my LXC nodes are:

hadoop11 namenode zookeeper
hadoop12 datanode tasktracker
hadoop13 datanode tasktracker
hadoop21 jobtracker zookeeper
hadoop22 datanode tasktracker
hadoop23 datanode tasktracker
hadoop31 secondarynamenode hiveserver2 zookeeper
hadoop32 datanode tasktracker
hadoop33 datanode tasktracker

OK, now that we covered the script, let’s start the NameNode and see see what gets logged:

# bin/hadoop-daemon.sh --config conf/ start namenode
starting namenode, logging to /usr/local/hadoop-1.2.1/libexec/../logs/hadoop-root-namenode-hadoop11.out
# cat conf/topology.log 
Mon Jan 6 19:04:03 UTC 2014 input: 10.0.1.123 10.0.1.122 10.0.1.113 10.0.1.112 10.0.1.133 10.0.1.132

As the NameNode started, it asked in a single called what is the rack name of all our nodes. This is what the script returns to the NameNode:

# conf/topology.script.sh 10.0.1.123 10.0.1.122 10.0.1.113 10.0.1.112 10.0.1.133 10.0.1.132
/rack02 /rack02 /rack01 /rack01 /rack03 /rack03

Why did the NameNode send all the IP addresses of the DataNodes in a single call? In this case, I have pre-configured another HDFS parameter called dfs.hosts in hdfs-site.xml. This parameter points to a file with a list of all nodes that are allowed to run a data node. So, when the NameNode started, it just asked for the mapping of all known data node servers.

What happens if you don’t use dfs.hosts? To check, I removed this parameter from my hdfs-site.xml file, restarted the NameNode and started all the data nodes. In this case, the NameNode called the topology script once per DataNode (when they first reported their status to the NameNode):

# cat topology.log 
Mon Jan 6 19:04:03 UTC 2014 input: 10.0.1.123 10.0.1.122 10.0.1.113 10.0.1.112 10.0.1.133 10.0.1.132
Mon Jan 6 19:07:53 UTC 2014 input: 10.0.1.112
Mon Jan 6 19:07:53 UTC 2014 input: 10.0.1.123
Mon Jan 6 19:07:53 UTC 2014 input: 10.0.1.113
Mon Jan 6 19:07:53 UTC 2014 input: 10.0.1.133
Mon Jan 6 19:07:54 UTC 2014 input: 10.0.1.132
Mon Jan 6 19:07:54 UTC 2014 input: 10.0.1.122

I hope this post has enough data for you to hack and QA your own basic network topologies. In the next post I hope to investigate block placement – how to see where HDFS is actually putting each copy of each block under various conditions.