格式化HDFS需要运行命令：bin/Hadoop namenode –format
于是打印出如下的日志：
10/11/20 19:52:21 INFO namenode.NameNode: STARTUP_MSG:
/************************************************************
STARTUP_MSG: Starting NameNode
STARTUP_MSG: host = namenode/192.168.1.104
STARTUP_MSG: args = [-format]
STARTUP_MSG: version = 0.19.2
STARTUP_MSG: build = https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.19 -r 789657; compiled by 'root' on Tue Jun 30 12:40:50 EDT 2009
************************************************************/
10/11/20 19:52:21 INFO namenode.FSNamesystem: fsOwner=admin,sambashare
10/11/20 19:52:21 INFO namenode.FSNamesystem: supergroup=supergroup
10/11/20 19:52:21 INFO namenode.FSNamesystem: isPermissionEnabled=true
10/11/20 19:52:21 INFO common.Storage: Image file of size 97 saved in 0 seconds.
10/11/20 19:52:21 INFO common.Storage: Storage directory /data/hadoopdir/tmp/dfs/name has been successfully formatted.
10/11/20 19:52:21 INFO namenode.NameNode: SHUTDOWN_MSG:
/************************************************************
SHUTDOWN_MSG: Shutting down NameNode at namenode/192.168.1.104
************************************************************/
这个时候在NameNode的/data/hadoopdir/tmp下面出现如下的文件树形结构：
+- dfs+- name
+--- current
+---- edits
+---- fsimage
+---- fstime
+---- VERSION
+---image
+---- fsimage
这个时候，DataNode的/data/hadoopdir/tmp中还是空的。
二、启动Hadoop启动Hadoop需要调用命令bin/start-all.sh，输出的日志如下：
starting namenode, logging to logs/hadoop-namenode-namenode.out
192.168.1.106: starting datanode, logging to logs/hadoop-datanode-datanode02.out
192.168.1.105: starting datanode, logging to logs/hadoop-datanode-datanode01.out
192.168.1.107: starting datanode, logging to logs/hadoop-datanode-datanode03.out
192.168.1.104: starting secondarynamenode, logging to logs/hadoop-secondarynamenode-namenode.out
starting jobtracker, logging to logs/hadoop-jobtracker-namenode.out
192.168.1.106: starting tasktracker, logging to logs/hadoop-tasktracker-datanode02.out
192.168.1.105: starting tasktracker, logging to logs/hadoop-tasktracker-datanode01.out
192.168.1.107: starting tasktracker, logging to logs/hadoop-tasktracker-datanode03.out
从日志中我们可以看出，此脚本启动了NameNode, 三个DataNode，SecondaryName，JobTracker以及三个TaskTracker.
下面我们分别从NameNode和三个DataNode中运行jps -l，看看到底运行了那些java程序：
在NameNode中：
22214 org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode
22107 org.apache.hadoop.hdfs.server.namenode.NameNode
22271 org.apache.hadoop.mapred.JobTracker
在datanode01中：
12580 org.apache.hadoop.mapred.TaskTracker
12531 org.apache.hadoop.hdfs.server.datanode.DataNode
在datanode02中：
10548 org.apache.hadoop.hdfs.server.datanode.DataNode
在datanode03中：
12593 org.apache.hadoop.hdfs.server.datanode.DataNode
12644 org.apache.hadoop.mapred.TaskTracker
同我们上面的配置完全符合。
当启动了Hadoop以后，/data/hadoopdir/tmp目录也发生了改变，通过ls -R我们可以看到。
对于NameNode：
在name文件夹中，多了in_use.lock文件，说明NameNode已经启动了
多了nameseondary文件夹，用于存放SecondaryNameNode的数据
.:
dfs
./dfs:
name namesecondary
./dfs/name:
current image in_use.lock
./dfs/name/current:
edits fsimage fstime VERSION
./dfs/name/image:
fsimage
./dfs/namesecondary:
current image in_use.lock
./dfs/namesecondary/current:
edits fsimage fstime VERSION
./dfs/namesecondary/image:
fsimage
对于DataNode:
多了dfs和mapred两个文件夹
dfs文件夹用于存放HDFS的block数据的
mapred用于存放Map-Reduce Task任务执行所需要的数据的。
.:
dfs mapred
./dfs:
data
./dfs/data:
current detach in_use.lock storage tmp
./dfs/data/current:
dncp_block_verification.log.curr VERSION
./dfs/data/detach:
./dfs/data/tmp:
./mapred:
local
./mapred/local:
当然随着Hadoop的启动，logs文件夹下也多个很多的日志：
在NameNode上，日志有：
NameNode的日志：
hadoop-namenode-namenode.log此为log4j的输出日志
hadoop-namenode-namenode.out此为stdout和stderr的输出日志
SecondaryNameNode的日志：
hadoop-secondarynamenode-namenode.log此为log4j的输出日志
hadoop-secondarynamenode-namenode.out此为stdout和stderr的输出日志
JobTracker的日志：
hadoop-jobtracker-namenode.log此为log4j的输出日志
hadoop-jobtracker-namenode.out此为stdout和stderr的输出日志
在DataNode上的日志有(以datanode01为例子)：
DataNode的日志
hadoop-datanode-datanode01.log此为log4j的输出日志
hadoop-datanode-datanode01.out此为stdout和stderr的输出日志
TaskTracker的日志
hadoop-tasktracker-datanode01.log此为log4j的输出日志
hadoop-tasktracker-datanode01.out此为stdout和stderr的输出日志
下面我们详细查看这些日志中的有重要意义的信息：
在hadoop-namenode-namenode.log文件中，我们可以看到NameNode启动的过程：
Namenode up at: namenode/192.168.1.104:9000
//文件的数量
Number of files = 0
Number of files under construction = 0
//加载fsimage和edits文件形成FSNamesystem
Image file of size 97 loaded in 0 seconds.
Edits file /data/hadoopdir/tmp/dfs/name/current/edits of size 4 edits # 0 loaded in 0 seconds.
Image file of size 97 saved in 0 seconds.
Finished loading FSImage in 12812 msecs
//统计block的数量和状态
Total number of blocks = 0
Number of invalid blocks = 0
Number of under-replicated blocks = 0
Number of over-replicated blocks = 0
//离开safe mode
Leaving safe mode after 12 secs.
//注册DataNode
Adding a new node: /default-rack/192.168.1.106:50010
Adding a new node: /default-rack/192.168.1.105:50010
Adding a new node: /default-rack/192.168.1.107:50010
在hadoop-secondarynamenode-namenode.log文件中，我们可以看到SecondaryNameNode的启动过程：
Secondary Web-server up at: 0.0.0.0:50090
//进行Checkpoint的周期
Checkpoint Period :3600 secs (60 min)
Log Size Trigger :67108864 bytes (65536 KB)
//进行一次checkpoint，从NameNode下载fsimage和edits
Downloaded file fsimage size 97 bytes.
Downloaded file edits size 370 bytes.
//加载edit文件，进行合并，将合并后的fsimage保存，我们可以看到fsimage变大了
Edits file /data/hadoopdir/tmp/dfs/namesecondary/current/edits of size 370 edits # 6 loaded in 0 seconds.
Image file of size 540 saved in 0 seconds.
//此次checkpoint结束
Checkpoint done. New Image Size: 540
在hadoop-jobtracker-namenode.log文件中，我们可以看到JobTracker的启动过程：
JobTracker up at: 9001
JobTracker webserver: 50030
//清除HDFS中的/data/hadoopdir/tmp/mapred/system文件夹，是用于Map-Reduce运行过程中保存数据的
Cleaning up the system directory
//不断的从TaskTracker收到heartbeat，第一次是注册TaskTracker
Got heartbeat from: tracker_datanode01:localhost/127.0.0.1:58297
Adding a new node: /default-rack/datanode01
Got heartbeat from: tracker_datanode03:localhost/127.0.0.1:37546
Adding a new node: /default-rack/datanode03
在hadoop-datanode-datanode01.log中，可以看到DataNode的启动过程：
//格式化DataNode存放block的文件夹
Storage directory /data/hadoopdir/tmp/dfs/data is not formatted.
Formatting ...
//启动DataNode
Opened info server at 50010
Balancing bandwith is 1048576 bytes/s
Initializing JVM Metrics with processName=DataNode, sessionId=null
//向NameNode注册此DataNode
dnRegistration = DatanodeRegistration(datanode01:50010, storageID=, infoPort=50075, ipcPort=50020)
New storage id DS-1042573498-192.168.1.105-50010-1290313555129 is assigned to data-node 192.168.1.105:5001
DatanodeRegistration(192.168.1.105:50010, storageID=DS-1042573498-192.168.1.105-50010-1290313555129, infoPort=50075, ipcPort=50020)In DataNode.run, data = FSDataset{dirpath='/data/hadoopdir/tmp/dfs/data/current'}
//启动block scanner
Starting Periodic block scanner.
在hadoop-tasktracker-datanode01.log中，可以看到TaskTracker的启动过程：
//启动TaskTracker
Initializing JVM Metrics with processName=TaskTracker, sessionId=
TaskTracker up at: localhost/127.0.0.1:58297
Starting tracker tracker_datanode01:localhost/127.0.0.1:58297
//向JobTracker发送heartbeat
Got heartbeatResponse from JobTracker with responseId: 0 and 0 actions
一个特殊的log文件是hadoop-tasktracker-datanode02.log中，因为我们设置的最大Map Task数目和最大Reduce Task数据为0，而报了一个Exception，Can not start task tracker because java.lang.IllegalArgumentException，从而使得datanode02上的TaskTracker没有启动起来。
当Hadoop启动起来以后，在HDFS中也创建了一些文件夹/data/hadoopdir/tmp/mapred/system，用来保存Map-Reduce运行时候的共享资源。