本文主要参考 http://hadoop.apache.org/docs/r2.8.0/hadoop-project-dist/hadoop-common/RackAwareness.html
hadoop组件是机栈敏感(译注rack,机栈，可以简单理解为节点的摆放）。
例如，HDFS块的分布会利用机栈敏感来做容错，方式是把复制放在不同的机栈。这样即便网络切换导致故障或者其它断开导致的故障，也会有一些数据是可用的。
译注：在重要的系统中，还有完全有必要考虑这个的，网络设备也会出现故障，毕竟这些网路设备的负载也很繁重的。
haoop主守护程序可以获得集群从属节点（数据节点）的机栈id，方式是激活一个完毕脚本或者是配置文件制定的java类。无论使用哪种方式，它们的输出都必须符合java的org.apache.hadoop.net.DNSToSwitchMapping接口。
这个接口要求一一对应，拓扑信息（机器摆放信息）必须形如'/myrack/myhost',其中‘/'是拓扑分隔符，’myrack'是rack的识别符，'myhost'是主机名称。假定一个rack有24个子网，那么其中一个就可以使用'/192.168.100.0/192.168.100.5'.
如果要使用java类做拓扑映射，那么类的名称必须是通过配置文件的net.topology.node.switch.mapping.impl来设定。例如,networkTopology.java,已经包含在hadoop发布程序中，管理员可以配置。
使用java类的好处是，当一个新的数据节点加入的时候，hadoop不需要调用外部进程（这样可以更高效一些）。
如果使用外部脚本，那么必须使用参数net.topology.script.file.name来配置。不同于java类，完毕拓扑脚本并没有包含在hadoop发布程序中，必须由管理员提供。当hadoop调用这些脚本的时候，会发送多个ip地址给ARGV.需要发送给脚本的ip地址个数，是
由net.topology.script.number.args控制的，默认值是100.如果net.topology.script.number.args设置为1，那么数据节点或者几点管理器每提交一个ip地址，脚本就要被调用一次。
如果net.topology.script.file.name 或者 net.topology.node.switch.mapping.impl的值没有设置，那么rack id '/default-rack'就任意ip的返回值。然而，这样的结果看起来一点也不理想，它可能会导致块同步问题（译注:大家一个rack，写入那里无所谓，所以可能性能和容错都会存在一些问题）。
译注：以上几个参数都是在core-site.xml中配置的
原文，给出了两个例子，一个是python,一个是bash
------------------------
-- python
------------------------
#!/usr/bin/python
# this script makes assumptions about the physical environment.
# 1) each rack is its own layer 3 network with a /24 subnet, which
# could be typical where each rack has its own
# switch with uplinks to a central core router.
#
# +-----------+
# |core router|
# +-----------+
# /
# +-----------+ +-----------+
# |rack switch| |rack switch|
# +-----------+ +-----------+
# | data node | | data node |
# +-----------+ +-----------+
# | data node | | data node |
# +-----------+ +-----------+
#
# 2) topology script gets list of IP's as input, calculates network address, and prints '/network_address/ip'.
import netaddr
import sys
sys.argv.pop(0) # discard name of topology script from argv list as we just want IP addresses
netmask = '255.255.255.0' # set netmask to what's being used in your environment. The example uses a /24
for ip in sys.argv: # loop over list of datanode IP's
address = '{0}/{1}'.format(ip, netmask) # format address string so it looks like 'ip/netmask' to make netaddr work
try:
network_address = netaddr.IPNetwork(address).network # calculate and print network address
print "/{0}".format(network_address)
except:
print "/rack-unknown" # print catch-all value if unable to calculate network address
------------------------
-- bash
------------------------
#!/bin/bash
# Here's a bash example to show just how simple these scripts can be
# Assuming we have flat network with everything on a single switch, we can fake a rack topology.
# This could occur in a lab environment where we have limited nodes,like 2-8 physical machines on a unmanaged switch.
# This may also apply to multiple virtual machines running on the same physical hardware.
# The number of machines isn't important, but that we are trying to fake a network topology when there isn't one.
#
# +----------+ +--------+
# |jobtracker| |datanode|
# +----------+ +--------+
# /
# +--------+ +--------+ +--------+
# |datanode|--| switch |--|datanode|
# +--------+ +--------+ +--------+
# /
# +--------+ +--------+
# |datanode| |namenode|
# +--------+ +--------+
#
# With this network topology, we are treating each host as a rack. This is being done by taking the last octet
# in the datanode's IP and prepending it with the word '/rack-'. The advantage for doing this is so HDFS
# can create its 'off-rack' block copy.
# 1) 'echo $@' will echo all ARGV values to xargs.
# 2) 'xargs' will enforce that we print a single argv value per line
# 3) 'awk' will split fields on dots and append the last field to the string '/rack-'. If awk
# fails to split on four dots, it will still print '/rack-' last field value
echo $@ | xargs -n 1 | awk -F '.' '{print "/rack-"$NF}'
总结：
网络拓扑如何设计，无论在哪个集群中，都是需要密切关注的