Hadoop基础-镜像文件（fsimage）和编辑日志（edits）
作者：尹正杰
版权声明：原创作品，谢绝转载！否则将追究法律责任。
一.查看日志镜像文件（如：fsimage_0000000000000000767）内容
1>.镜像文件的作用
通过查看上面的XML文件，可以明显的知道镜像文件是存放的是目录结构(你也可以理解是一个树形结构)，文件属性等信息，说到这就不说不提一下镜像文件的md5校验文件了，这个校验文件是为了判断镜像文件是否被修改。fsimage文件是namenode中关于元数据的镜像，一般称为检查点。它是在NameNode启动时对整个文件系统的快照 。
2>.用"hdfs oiv"命令下载镜像文件格式为XML,操作如下：
[yinzhengjie@s101 ~]$ ll
total
drwxrwxr-x. yinzhengjie yinzhengjie May : hadoop
drwxrwxr-x. yinzhengjie yinzhengjie May : shell
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ ls -lh /home/yinzhengjie/hadoop/dfs/name/current/ | grep fsimage
-rw-rw-r--. yinzhengjie yinzhengjie .2K May : fsimage_0000000000000000767
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000000767.md5
-rw-rw-r--. yinzhengjie yinzhengjie .4K May : fsimage_0000000000000000932
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000000932.md5
[yinzhengjie@s101 ~]$ hdfs oiv -i ./hadoop/dfs/name/current/fsimage_0000000000000000767 -o yinzhengjie.xml -p XML
[yinzhengjie@s101 ~]$ ll
total
drwxrwxr-x. yinzhengjie yinzhengjie May : hadoop
drwxrwxr-x. yinzhengjie yinzhengjie May : shell
-rw-rw-r--. yinzhengjie yinzhengjie May : yinzhengjie.xml
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ sz yinzhengjie.xml
rz
zmodem trl+C ȡ % KB KB/s :: Errors [yinzhengjie@s101 ~]$
<?xml version="1.0"?>
<fsimage><NameSection>
<genstampV1>1000</genstampV1><genstampV2>1019</genstampV2><genstampV1Limit>0</genstampV1Limit><lastAllocatedBlockId>1073741839</lastAllocatedBlockId><txid>767</txid></NameSection>
<INodeSection><lastInodeId>16414</lastInodeId><inode><id>16385</id><type>DIRECTORY</type><name></name><mtime>1527331031268</mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>9223372036854775807</nsquota><dsquota>-1</dsquota></inode>
<inode><id>16387</id><type>FILE</type><name>xrsync.sh</name><replication>3</replication><mtime>1527308253459</mtime><atime>1527330550802</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission><blocks><block><id>1073741826</id><genstamp>1002</genstamp><numBytes>700</numBytes></block>
</blocks>
</inode>
<inode><id>16389</id><type>FILE</type><name>hadoop-2.7.3.tar.gz</name><replication>3</replication><mtime>1527310784699</mtime><atime>1527310775186</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission><blocks><block><id>1073741827</id><genstamp>1003</genstamp><numBytes>134217728</numBytes></block>
<block><id>1073741828</id><genstamp>1004</genstamp><numBytes>79874467</numBytes></block>
</blocks>
</inode>
<inode><id>16402</id><type>DIRECTORY</type><name>shell</name><mtime>1527331084147</mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>-1</nsquota><dsquota>-1</dsquota></inode>
<inode><id>16403</id><type>DIRECTORY</type><name>awk</name><mtime>1527332686407</mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>-1</nsquota><dsquota>-1</dsquota></inode>
<inode><id>16404</id><type>DIRECTORY</type><name>sed</name><mtime>1527332624472</mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>-1</nsquota><dsquota>-1</dsquota></inode>
<inode><id>16405</id><type>DIRECTORY</type><name>grep</name><mtime>1527332592029</mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>-1</nsquota><dsquota>-1</dsquota></inode>
<inode><id>16406</id><type>FILE</type><name>yinzhengjie.sh</name><replication>3</replication><mtime>1527331084161</mtime><atime>1527331084147</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id>16409</id><type>FILE</type><name>1.txt</name><replication>3</replication><mtime>1527332587208</mtime><atime>1527332587194</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id>16410</id><type>FILE</type><name>2.txt</name><replication>3</replication><mtime>1527332592042</mtime><atime>1527332592029</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id>16411</id><type>FILE</type><name>zabbix.sql</name><replication>3</replication><mtime>1527332604168</mtime><atime>1527332604154</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id>16412</id><type>FILE</type><name>nagios.sh</name><replication>3</replication><mtime>1527332624486</mtime><atime>1527332624472</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id>16413</id><type>FILE</type><name>keepalive.sh</name><replication>3</replication><mtime>1527332677350</mtime><atime>1527332677335</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id>16414</id><type>FILE</type><name>nginx.conf</name><replication>3</replication><mtime>1527332686421</mtime><atime>1527332686407</atime><perferredBlockSize>134217728</perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
</INodeSection>
<INodeReferenceSection></INodeReferenceSection><SnapshotSection><snapshotCounter>0</snapshotCounter></SnapshotSection>
<INodeDirectorySection><directory><parent>16385</parent><inode>16389</inode><inode>16402</inode><inode>16387</inode></directory>
<directory><parent>16402</parent><inode>16403</inode><inode>16405</inode><inode>16404</inode><inode>16406</inode></directory>
<directory><parent>16403</parent><inode>16413</inode><inode>16414</inode></directory>
<directory><parent>16404</parent><inode>16412</inode><inode>16411</inode></directory>
<directory><parent>16405</parent><inode>16409</inode><inode>16410</inode></directory>
</INodeDirectorySection>
<FileUnderConstructionSection></FileUnderConstructionSection>
<SnapshotDiffSection><diff><inodeid>16385</inodeid></diff></SnapshotDiffSection>
<SecretManagerSection><currentId>0</currentId><tokenSequenceNumber>0</tokenSequenceNumber></SecretManagerSection><CacheManagerSection><nextDirectiveId>1</nextDirectiveId></CacheManagerSection>
</fsimage
yinzhengjie.xml 文件内容（推荐看这个版本，用的是XML格式）
[yinzhengjie@s101 ~]$ cat yinzhengjie.xml
<?xml version="1.0"?>
<fsimage><NameSection>
<genstampV1></genstampV1><genstampV2></genstampV2><genstampV1Limit></genstampV1Limit><lastAllocatedBlockId></lastAllocatedBlockId><txid></txid></NameSection>
<INodeSection><lastInodeId></lastInodeId><inode><id></id><type>DIRECTORY</type><name></name><mtime></mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota></nsquota><dsquota>-</dsquota></inode>
<inode><id></id><type>FILE</type><name>xrsync.sh</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission><blocks><block><id></id><genstamp></genstamp><numBytes></numBytes></block>
</blocks>
</inode>
<inode><id></id><type>FILE</type><name>hadoop-2.7..tar.gz</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission><blocks><block><id></id><genstamp></genstamp><numBytes></numBytes></block>
<block><id></id><genstamp></genstamp><numBytes></numBytes></block>
</blocks>
</inode>
<inode><id></id><type>DIRECTORY</type><name>shell</name><mtime></mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>-</nsquota><dsquota>-</dsquota></inode>
<inode><id></id><type>DIRECTORY</type><name>awk</name><mtime></mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>-</nsquota><dsquota>-</dsquota></inode>
<inode><id></id><type>DIRECTORY</type><name>sed</name><mtime></mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>-</nsquota><dsquota>-</dsquota></inode>
<inode><id></id><type>DIRECTORY</type><name>grep</name><mtime></mtime><permission>yinzhengjie:supergroup:rwxr-xr-x</permission><nsquota>-</nsquota><dsquota>-</dsquota></inode>
<inode><id></id><type>FILE</type><name>yinzhengjie.sh</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id></id><type>FILE</type><name>.txt</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id></id><type>FILE</type><name>.txt</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id></id><type>FILE</type><name>zabbix.sql</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id></id><type>FILE</type><name>nagios.sh</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id></id><type>FILE</type><name>keepalive.sh</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
<inode><id></id><type>FILE</type><name>nginx.conf</name><replication></replication><mtime></mtime><atime></atime><perferredBlockSize></perferredBlockSize><permission>yinzhengjie:supergroup:rw-r--r--</permission></inode>
</INodeSection>
<INodeReferenceSection></INodeReferenceSection><SnapshotSection><snapshotCounter></snapshotCounter></SnapshotSection>
<INodeDirectorySection><directory><parent></parent><inode></inode><inode></inode><inode></inode></directory>
<directory><parent></parent><inode></inode><inode></inode><inode></inode><inode></inode></directory>
<directory><parent></parent><inode></inode><inode></inode></directory>
<directory><parent></parent><inode></inode><inode></inode></directory>
<directory><parent></parent><inode></inode><inode></inode></directory>
</INodeDirectorySection>
<FileUnderConstructionSection></FileUnderConstructionSection>
<SnapshotDiffSection><diff><inodeid></inodeid></diff></SnapshotDiffSection>
<SecretManagerSection><currentId></currentId><tokenSequenceNumber></tokenSequenceNumber></SecretManagerSection><CacheManagerSection><nextDirectiveId></nextDirectiveId></CacheManagerSection>
</fsimage>
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ cat yinzhengjie.xml
二.查看编辑日志文件内容
1.编辑日志的作用
顾名思义，编辑日志当然是记录对文件或者目录的修改信息啦，比如删除目录，修改文件等信息都会被该文件记录。编辑日志一般命名规则为：“edits_*”(下面你会看到类似的文件)， 它是在NameNode启动后，记录对文件系统的改动序列。
2>.使用oev命令查询hadoop的编辑日志文件，操作如下：
[yinzhengjie@s101 ~]$ ls -lh /home/yinzhengjie/hadoop/dfs/name/current/ | grep edits | tail -
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001001-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001003-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001005-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001007-
-rw-rw-r--. yinzhengjie yinzhengjie 1.0M May : edits_inprogress_0000000000000001009
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ ll
total
drwxrwxr-x. yinzhengjie yinzhengjie May : hadoop
drwxrwxr-x. yinzhengjie yinzhengjie May : shell
-rw-rw-r--. yinzhengjie yinzhengjie May : yinzhengjie.xml
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ hdfs oev -i ./hadoop/dfs/name/current/edits_0000000000000001007- -o edits.xml -p XML
[yinzhengjie@s101 ~]$ ll
total
-rw-rw-r--. yinzhengjie yinzhengjie May : edits.xml
drwxrwxr-x. yinzhengjie yinzhengjie May : hadoop
drwxrwxr-x. yinzhengjie yinzhengjie May : shell
-rw-rw-r--. yinzhengjie yinzhengjie May : yinzhengjie.xml
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ cat edits.xml
<?xml version="1.0" encoding="UTF-8"?>
<EDITS>
<EDITS_VERSION>-</EDITS_VERSION>
<RECORD>
<OPCODE>OP_START_LOG_SEGMENT</OPCODE>
<DATA>
<TXID></TXID>
</DATA>
</RECORD>
<RECORD>
<OPCODE>OP_END_LOG_SEGMENT</OPCODE>
<DATA>
<TXID></TXID>
</DATA>
</RECORD>
</EDITS>
[yinzhengjie@s101 ~]$
3>.查看正在使用的编辑日志文件
哪个文件是正在使用的编辑日志文件呢？估计你已经看出来了，就是带有“edits_inprogress_*”字样的文件。
[yinzhengjie@s101 ~]$ ll
total
drwxrwxr-x. yinzhengjie yinzhengjie May : hadoop
drwxrwxr-x. yinzhengjie yinzhengjie May : shell
[yinzhengjie@s101 ~]$ ls -lh /home/yinzhengjie/hadoop/dfs/name/current/ | grep edits | tail -
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001053-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001055-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001057-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001059-
-rw-rw-r--. yinzhengjie yinzhengjie 1.0M May : edits_inprogress_0000000000000001072
[yinzhengjie@s101 ~]$ hdfs oev -i ./hadoop/dfs/name/current/edits_inprogress_0000000000000001072 -o yinzhengjie.xml -p XML
[yinzhengjie@s101 ~]$ ll
total
drwxrwxr-x. yinzhengjie yinzhengjie May : hadoop
drwxrwxr-x. yinzhengjie yinzhengjie May : shell
-rw-rw-r--. yinzhengjie yinzhengjie May : yinzhengjie.xml
[yinzhengjie@s101 ~]$ more yinzhengjie.xml
<?xml version="1.0" encoding="UTF-8"?>
<EDITS>
<EDITS_VERSION>-</EDITS_VERSION>
<RECORD>
<OPCODE>OP_START_LOG_SEGMENT</OPCODE>
<DATA>
<TXID></TXID>
</DATA>
</RECORD>
</EDITS>
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ hdfs oev -i ./hadoop/dfs/name/current/edits_inprogress_0000000000000001072 -o yinzhengjie.xml -p XML
三.手动对编辑日志进行滚动
1>.用hdfs dfsadmin 命令进行日志滚动操作
[yinzhengjie@s101 ~]$ ls -lh /home/yinzhengjie/hadoop/dfs/name/current/ | grep edits | tail -
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001090-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001092-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001094-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001096-
-rw-rw-r--. yinzhengjie yinzhengjie 1.0M May : edits_inprogress_0000000000000001098
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ hdfs dfsadmin -rollEdits
Successfully rolled edit logs.
New segment starts at txid
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ ls -lh /home/yinzhengjie/hadoop/dfs/name/current/ | grep edits | tail -
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001092-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001094-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001096-
-rw-rw-r--. yinzhengjie yinzhengjie May : edits_0000000000000001098-
-rw-rw-r--. yinzhengjie yinzhengjie 1.0M May : edits_inprogress_0000000000000001100
[yinzhengjie@s101 ~]$
2>.启动hdfs时，编辑日志滚动
重启hdfs服务器是，镜像文件编辑日子进行融合，会自动滚动编辑日志。其实我们可以通过webUI来查看这个过程。
四.保存名称空间(也就是手动保存镜像文件)
1>.查看hdfs当前的模式,默安全模式默认是关闭的
[yinzhengjie@s101 ~]$ hdfs dfsadmin -safemode get
Safe mode is OFF
[yinzhengjie@s101 ~]$
2>.开启安全模式
开启安全模式我们只需要执行：“hdfs dfsadmin -safemode enter” 即可。
[yinzhengjie@s101 ~]$ hdfs dfsadmin -safemode get
Safe mode is OFF
[yinzhengjie@s101 ~]$ hdfs dfsadmin -safemode enter
Safe mode is ON
[yinzhengjie@s101 ~]$ hdfs dfsadmin -safemode get
Safe mode is ON
[yinzhengjie@s101 ~]$
NameNode进入安全模式后，无法进行写的操作，但是读取的操作依然是可以正常进行的哟，此时如下：
[yinzhengjie@s101 ~]$ ll
total
drwxrwxr-x. yinzhengjie yinzhengjie May : hadoop
drwxrwxr-x. yinzhengjie yinzhengjie May : shell
-rw-rw-r--. yinzhengjie yinzhengjie May : yinzhengjie.xml
[yinzhengjie@s101 ~]$ hdfs dfs -ls /
Found items
-rw-r--r-- yinzhengjie supergroup -- : /hadoop-2.7..tar.gz
-rw-r--r-- yinzhengjie supergroup -- : /jdk-8u131-linux-x64.tar.gz
-rw-r--r-- yinzhengjie supergroup -- : /xrsync.sh
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ hdfs dfs -put yinzhengjie.xml /
put: Cannot create file/yinzhengjie.xml._COPYING_. Name node is in safe mode.
[yinzhengjie@s101 ~]$
3>.退出安全模式
通过上面的测试案例我们知道，当NameNode进入安全模式时，无法进行删除操作，只要我们退出安全模式就可以正常上传文件啦。
[yinzhengjie@s101 ~]$ hdfs dfs -ls /
Found items
-rw-r--r-- yinzhengjie supergroup -- : /hadoop-2.7..tar.gz
-rw-r--r-- yinzhengjie supergroup -- : /jdk-8u131-linux-x64.tar.gz
-rw-r--r-- yinzhengjie supergroup -- : /xrsync.sh
[yinzhengjie@s101 ~]$ ll
total
drwxrwxr-x. yinzhengjie yinzhengjie May : hadoop
drwxrwxr-x. yinzhengjie yinzhengjie May : shell
-rw-rw-r--. yinzhengjie yinzhengjie May : yinzhengjie.xml
[yinzhengjie@s101 ~]$ hdfs dfs -put yinzhengjie.xml /
put: Cannot create file/yinzhengjie.xml._COPYING_. Name node is in safe mode.
[yinzhengjie@s101 ~]$
[yinzhengjie@s101 ~]$ hdfs dfsadmin -safemode leave
Safe mode is OFF
[yinzhengjie@s101 ~]$ hdfs dfs -put yinzhengjie.xml /
[yinzhengjie@s101 ~]$ hdfs dfs -ls /
Found items
-rw-r--r-- yinzhengjie supergroup -- : /hadoop-2.7..tar.gz
-rw-r--r-- yinzhengjie supergroup -- : /jdk-8u131-linux-x64.tar.gz
-rw-r--r-- yinzhengjie supergroup -- : /xrsync.sh
-rw-r--r-- yinzhengjie supergroup -- : /yinzhengjie.xml
[yinzhengjie@s101 ~]$
4>.等待安全模式的应用
我们先将客户端开启安全模式
然后我们再退出安全模式：
5>.保存名称空间
[yinzhengjie@s101 shell]$ hdfs dfsadmin -safemode get
Safe mode is OFF
[yinzhengjie@s101 shell]$ hdfs dfsadmin -safemode enter
Safe mode is ON
[yinzhengjie@s101 shell]$ hdfs dfsadmin -safemode get
Safe mode is ON
[yinzhengjie@s101 shell]$ ls -lh /home/yinzhengjie/hadoop/dfs/name/current/ | grep fsimage
-rw-rw-r--. yinzhengjie yinzhengjie .4K May : fsimage_0000000000000000932
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000000932.md5
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000001110
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000001110.md5
[yinzhengjie@s101 shell]$
[yinzhengjie@s101 shell]$ hdfs dfsadmin -saveNamespace
Save namespace successful
[yinzhengjie@s101 shell]$
[yinzhengjie@s101 shell]$ ls -lh /home/yinzhengjie/hadoop/dfs/name/current/ | grep fsimage
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000001110
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000001110.md5
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000001200
-rw-rw-r--. yinzhengjie yinzhengjie May : fsimage_0000000000000001200.md5
[yinzhengjie@s101 shell]$
[yinzhengjie@s101 shell]$ hdfs dfsadmin -safemode leave
Safe mode is OFF
[yinzhengjie@s101 shell]$ hdfs dfsadmin -safemode get
Safe mode is OFF
[yinzhengjie@s101 shell]$
五.hdfs启动过程如下
hdfs启动过程大致分为以下三个步骤：
第一：将以“edits_inprogress_*”文件实例化成为新编辑日志文件;
第二：将镜像文件和新编辑日志文件加载到内存，重现编辑日志的执行过程，生成新的镜像文件检查点文件（.ckpt文件）;
第三：将新的镜像文件检查点文件（.ckpt文件）后缀去掉，变为新镜像文件；
为了方便了理解，我在《Hadoop》权威指南找到一张详细的流程图，如下：
如果你觉得上图描述的不够详细的话，也可以参考我画的草图如下，已经很清楚的描述了NameNode和SecondaryNameNode的工作机制：
六.日志滚动（secondarynamenode，简称2nn）
上面我演示了如何手动实现滚动日志以及保存名称空间，但是手动滚动日志的话需要进入安全模式，一旦进入安全模式用户就无法正常进行写入操作，只能进行读取，且不说运维人员如果频繁的这样操作很麻烦，咱们就说说用户体验度肯定是会下降的。这时候就有了SecondaryNameNode，在说SecondaryNameNode的功能之前，我们想来说一下NameNode和DataNode。
1>.NameNode
NameNode是存放目录，类型，权限等元数据信息的，你可以理解它只是存放真实路径的一个映射！还记得我们年轻的时候看过周星驰的一部电影吗？它是我喜欢的中国喜剧演员，我记得在他主演的一部叫做《鹿鼎记》的电影中，韦小宝拜陈近南为师时，陈近南给韦小宝一本秘籍，韦小宝说：“咦，这么大一本？我看要练个把月”，陈近南接着说：“这一本只不过是绝世武功的目录”。
2>.DataNode
存放真实数据的服务器，用户所有上传的文件都会保存到DataNode的，真实环境中，保存的分量可能不止一份哟！
3>.SecondaryNameNode
我们知道NameNode保存是实时记录元数据信息的，这些数据信息的记录信息会临时保存到内存中，然后将数据记录到编辑日志中（比如：edits_0000000000000001007-0000000000000001008）,当重启NameNode服务时，编辑日志才会合并到镜像文件中，从而得到一个文件系统的最新快照。但是在产品集群中NameNode是很少重启的，这也意味着当NameNode运行了很长时间后，edit logs文件会变得很大。在这种情况下就会出现一些问题：比如：edit logs文件会变的很大，怎么去管理这个文件是一个挑战。NameNode的重启会花费很长时间，因为编辑日志中i很多改动要合并到fsimage文件上。 如果NameNode挂掉了，那我们就丢失了很多改动因为此时的fsimage文件非常旧。
因此为了克服这个问题，我们需要一个易于管理的机制来帮助我们减小edit logs文件的大小和得到一个最新的fsimage文件（这个跟咱们玩的虚拟机的快照功能类似），这样也会减小在NameNode上的压力。这时候SecondaryNameNode就可以帮我们轻松搞定这个问题。SecondaryNameNode默认每个3600s(1h) 创建一次检查点（这个时间周期可以在配置文件修改“dfs.namenode.checkpoint.period“属性实现自定义时间周期检查点），创建检查点的细节如下：
第一：在NameNode中先进行日志的滚动；
第二：将镜像文件和新的编辑（edits）日志文件更新（fetch）到SecondaryNameNode进行融合操作，产生新的检查点文件（*.ckpt）;
第三：将检查点文件（.ckpt）发送到NameNode；
第四：NameNode将其重命名为新的的镜像文件；
那么问题来了：SecondaryNameNode会自动帮我们实现日志滚动，并将生成的文件放在NameNode存放镜像的目录中，当NameNode重启时，是重新生成新的fsimage文件还是直接使用SecondaryNameNode给他最新提供好的呢？答案是：重新生成，他们各干各的不耽误，只不过NameNode重新生成的起点是SecondaryNameNode最近一次融合的结束点开始的！
七.Hadoop默认的webＵＩ访问端口
1>.Namenode的默认访问端口是50070
2>.SecondaryNameNode的默认访问端口是50090
3>.NameNode默认的客户端链接端口是8020