NameNode 与 SecondaryNameNode 的工作机制

一、NameNode、Fsimage 、Edits 和 SecondaryNameNode 概述

NameNode：在内存中储存 HDFS 文件的元数据信息（目录）

如果节点故障或断电，存在内存中的数据会丢失，显然只在内存中保存是不可靠的
实际在磁盘当中也有保存：Fsimage 和 Edits，一个 NameNode 节点在重启后会根据这磁盘上的这两个文件来恢复到之前的状态

Fsimage（镜像文件） 和 Edits（编辑日志）：记录内存中的元数据

如果每次对 HDFS 的操作都实时的把内存中的元数据信息往磁盘上传输，这样显然效率不够高，也不稳定
这时就出现了 Edits 文件，用来记录每次对 HDFS 的操作，这样在磁盘上每次就只用做很小改动（只进行追加操作）
当  Edits 文件达到了一定大小或过了一定的时间，就需要把 Edits 文件转化 Fsimage 文件，然后清空 Edits
这样的 Fsimage 文件不会和内存中的元数据实时同步，需要加上 Edits 文件才相等

SecondaryNameNode：负责 Edits 转化成 Fsimage

SecondaryNameNode 不是 NameNode 的备份
SecondaryNameNode 会定时定量的把集群中的 Edits 文件转化为 Fsimage 文件，来保证 NameNode 中数据的可靠性

二、NameNode 工作机制

磁盘上的元数据文件

三、查看 Edits 与 Fsimage

Edits 和 Fsimage 并非明文存储，需要转换后才能查看，使用 hdfs 命令进行转换

# 部分命令参数
Usage: hdfs [--config confdir] [--loglevel loglevel] COMMAND
       where COMMAND is one of:

# 转换 fsimage 文件
  oiv                  apply the offline fsimage viewer to an fsimage
# 转换旧版 fsimage 文件
  oiv_legacy           apply the offline fsimage viewer to an legacy fsimage
# 转换 edits 文件
  oev                  apply the offline edits viewer to an edits file

1.转换 Fsimage 文件

hdfs oiv：查看参数

部分参数说明

Usage: bin/hdfs oiv [OPTIONS] -i INPUTFILE -o OUTPUTFILE

-i 要转换的文件
-o 转换后文件路径
-p 转换格式 (XML|FileDistribution|ReverseXML|Web|Delimited)

转换文件

hdfs oiv -p XML -i ./fsimage_0000000000000000326 -o ./fsimage.xml

转换后的文件，记录了文件信息

<?xml version="1.0"?>
<fsimage>
    <version>
        <layoutVersion>-63</layoutVersion>
        <onDiskVersion>1</onDiskVersion>
        <oivRevision>Unknown</oivRevision>
    </version>
    <NameSection>
        <namespaceId>1803101069</namespaceId>
        <genstampV1>1000</genstampV1>
        <genstampV2>1015</genstampV2>
        <genstampV1Limit>0</genstampV1Limit>
        <lastAllocatedBlockId>1073741839</lastAllocatedBlockId>
        <txid>326</txid>
    </NameSection>
    <INodeSection>
        <lastInodeId>16400</lastInodeId>
        <numInodes>11</numInodes>
        <inode>
            <id>16385</id>
            <type>DIRECTORY</type>
            <name></name>
            <mtime>1555218314075</mtime>
            <permission>root:supergroup:0755</permission>
            <nsquota>9223372036854775807</nsquota>
            <dsquota>-1</dsquota>
        </inode>
        <inode>
            <id>16388</id>
            <type>FILE</type>
            <name>javafx-src.zip</name>
            <replication>2</replication>
            <mtime>1554996304962</mtime>
            <atime>15550 84059617</atime>
            <preferredBlockSize>134217728</preferredBlockSize>
            <permission>root:supergroup:0644</permission>
            <blocks>
                <block>
                    <id>107 3741829</id>
                    <genstamp>1005</genstamp>
                    <numBytes>5211154</numBytes>
                </block>
            </blocks>
            <storagePolicyId>0</storagePolicyId>
        </inode>
        <inode>
            <id>16389</id>
            <type>DIRECTORY</type>
            <name>0529</name>
            <mtime>1554999968472</mtime>
            <permission>root:supergroup:0755</permission>
            <nsquota>-1</nsquota>
            <dsquota>-1</dsquota>
        </inode>
        <inode>
            <id>16390</id>
            <type>DIRECTORY</type>
            <name>mkmk</name>
            <mtime>1555000275046</mtime>
            <permission>root:supergroup:0755</permission>
            <nsquota>-1</nsquota>
            <dsquota>-1</dsquota>
        </inode>
        <inode>
            <id>16391</id>
            <type>DIRECTORY</type>
            <name>10088</name>
            <mtime>1555000763405</mtime>
            <permission>root:supergroup:0755</permission>
            <nsquota>-1</nsquota>
            <dsquota>-1</dsquota>
        </inode>
        <inode>
            <id>16394</id>
            <type>FILE</type>
            <name>Writage-1.12.msi</name>
            <replication>3</replication>
            <mtime>1555086090236</mtime>
            <atime>155 5086085824</atime>
            <preferredBlockSize>134217728</preferredBlockSize>
            <permission>root:supergroup:0644</permission>
            <blocks>
                <block>
                    <id>1 073741832</id>
                    <genstamp>1008</genstamp>
                    <numBytes>12771328</numBytes>
                </block>
            </blocks>
            <storagePolicyId>0</storagePolicyId>
        </inode>
        <inode>
            <id>16395</id>
            <type>FILE</type>
            <name>hadoop-2.9.2-win10-64.tar.gz</name>
            <replication>3</replication>
            <mtime>1555086717860</mtime>
            <atime>1555086612935</atime>
            <preferredBlockSize>134217728</preferredBlockSize>
            <permission>root:supergroup:0644</permission>
            <blocks>
                <block>
                    <id>1073741833</id>
                    <genstamp>1009</genstamp>
                    <numBytes>134217728</numBytes>
                </block>
                <block>
                    <id>1073741834</id>
                    <genstamp>1010</genstamp>
                    <numBytes>134217728</numBytes>
                </block>
                <block>
                    <id>1073741835</id>
                    <genstamp>1011</genstamp>
                    <numBytes>43080138</numBytes>
                </block>
            </blocks>
            <storagePolicyId>0</storagePolicyId>
        </inode>
        <inode>
            <id>16396</id>
            <type>DIRECTORY</type>
            <name>Priority</name>
            <mtime>1555219450567</mtime>
            <permission>root:supergroup:0755</permission>
            <nsquota>-1</nsquota>
            <dsquota>-1</dsquota>
        </inode>
        <inode>
            <id>16398</id>
            <type>FILE</type>
            <name>AAA.msi</name>
            <replication>3</replication>
            <mtime>1555219070073</mtime>
            <atime>155521906501 6</atime>
            <preferredBlockSize>134217728</preferredBlockSize>
            <permission>root:supergroup:0644</permission>
            <blocks>
                <block>
                    <id>1073741837</id>
                    <genstamp>1013</genstamp>
                    <numBytes>12771328</numBytes>
                </block>
            </blocks>
            <storagePolicyId>0</storagePolicyId>
        </inode>
        <inode>
            <id>16399</id>
            <type>FILE</type>
            <name>BBB.msi</name>
            <replication>1</replication>
            <mtime>1555219285187</mtime>
            <atime>155521928012 9</atime>
            <preferredBlockSize>134217728</preferredBlockSize>
            <permission>root:supergroup:0644</permission>
            <blocks>
                <block>
                    <id>1073741838</id>
                    <genstamp>1014</genstamp>
                    <numBytes>12771328</numBytes>
                </block>
            </blocks>
            <storagePolicyId>0</storagePolicyId>
        </inode>
        <inode>
            <id>16400</id>
            <type>FILE</type>
            <name>CCC.msi</name>
            <replication>2</replication>
            <mtime>1555219455139</mtime>
            <atime>155521945056 7</atime>
            <preferredBlockSize>134217728</preferredBlockSize>
            <permission>root:supergroup:0644</permission>
            <blocks>
                <block>
                    <id>1073741839</id>
                    <genstamp>1015</genstamp>
                    <numBytes>12771328</numBytes>
                </block>
            </blocks>
            <storagePolicyId>0</storagePolicyId>
        </inode>
    </INodeSection>
    <INodeReferenceSection></INodeReferenceSection>
    <SnapshotSection>
        <snapshotCounter>0</snapshotCounter>
        <numSnapshots>0</numSnapshots>
    </SnapshotSection>
    <INodeDirectorySection>
        <directory>
            <parent>16385</parent>
            <child>16389</child>
            <child>16391</child>
            <child>16396</child>
            <child>16394</child>
            <child>16395</child>
            <child>16388</child>
            <child>16390</child>
        </directory>
        <directory>
            <parent>16396</parent>
            <child>16398</child>
            <child>16399</child>
            <child>16400</child>
        </directory>
    </INodeDirectorySection>
    <FileUnderConstructionSection></FileUnderConstructionSection>
    <SecretManagerSection>
        <currentId>0</currentId>
        <tokenSequenceNumber>0</tokenSequenceNumber>
        <numDelegationKeys>0</numDelegationKeys>
        <numTokens>0</numTokens>
    </SecretManagerSection>
    <CacheManagerSection>
        <nextDirectiveId>1</nextDirectiveId>
        <numDirectives>0</numDirectives>
        <numPools>0</numPools>
    </CacheManagerSection>
</fsimage>

2.转换 Edits 文件

hdfs oev：查看参数

参数与上面类似，-p 有些不同

-p  binary (native binary format that Hadoop uses), xml (default, XML  format), stats (prints statistics about edits file)

转换文件

hdfs oev -p xml -i ./edits_inprogress_0000000000000000329 -o ./edits.xml

转换后的文件，记录 hdfs 的操作记录

<?xml version="1.0" encoding="UTF-8"?>
<EDITS>
    <EDITS_VERSION>-63</EDITS_VERSION>
    <RECORD>
        <OPCODE>OP_START_LOG_SEGMENT</OPCODE>
        <DATA>
            <TXID>16</TXID>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_ADD</OPCODE>
        <DATA>
            <TXID>17</TXID>
            <LENGTH>0</LENGTH>
            <INODEID>16386</INODEID>
            <PATH>/javafx-src.zip._COPYING_</PATH>
            <REPLICATION>3</REPLICATION>
            <MTIME>1554995796290</MTIME>
            <ATIME>1554995796290</ATIME>
            <BLOCKSIZE>134217728</BLOCKSIZE>
            <CLIENT_NAME>DFSClient_NONMAPREDUCE_1941526907_1</CLIENT_NAME>
            <CLIENT_MACHINE>192.168.8.136</CLIENT_MACHINE>
            <OVERWRITE>true</OVERWRITE>
            <PERMISSION_STATUS>
                <USERNAME>root</USERNAME>
                <GROUPNAME>supergroup</GROUPNAME>
                <MODE>420</MODE>
            </PERMISSION_STATUS>
            <RPC_CLIENTID>5b80319d-463f-4591-b090-2d1462563e48</RPC_CLIENTID>
            <RPC_CALLID>3</RPC_CALLID>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_ALLOCATE_BLOCK_ID</OPCODE>
        <DATA>
            <TXID>18</TXID>
            <BLOCK_ID>1073741825</BLOCK_ID>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_SET_GENSTAMP_V2</OPCODE>
        <DATA>
            <TXID>19</TXID>
            <GENSTAMPV2>1001</GENSTAMPV2>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_ADD_BLOCK</OPCODE>
        <DATA>
            <TXID>20</TXID>
            <PATH>/javafx-src.zip._COPYING_</PATH>
            <BLOCK>
                <BLOCK_ID>1073741825</BLOCK_ID>
                <NUM_BYTES>0</NUM_BYTES>
                <GENSTAMP>1001</GENSTAMP>
            </BLOCK>
            <RPC_CLIENTID></RPC_CLIENTID>
            <RPC_CALLID>-2</RPC_CALLID>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_UPDATE_BLOCKS</OPCODE>
        <DATA>
            <TXID>21</TXID>
            <PATH>/javafx-src.zip._COPYING_</PATH>
            <RPC_CLIENTID></RPC_CLIENTID>
            <RPC_CALLID>-2</RPC_CALLID>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_ALLOCATE_BLOCK_ID</OPCODE>
        <DATA>
            <TXID>22</TXID>
            <BLOCK_ID>1073741826</BLOCK_ID>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_SET_GENSTAMP_V2</OPCODE>
        <DATA>
            <TXID>23</TXID>
            <GENSTAMPV2>1002</GENSTAMPV2>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_ADD_BLOCK</OPCODE>
        <DATA>
            <TXID>24</TXID>
            <PATH>/javafx-src.zip._COPYING_</PATH>
            <BLOCK>
                <BLOCK_ID>1073741826</BLOCK_ID>
                <NUM_BYTES>0</NUM_BYTES>
                <GENSTAMP>1002</GENSTAMP>
            </BLOCK>
            <RPC_CLIENTID></RPC_CLIENTID>
            <RPC_CALLID>-2</RPC_CALLID>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_CLOSE</OPCODE>
        <DATA>
            <TXID>25</TXID>
            <LENGTH>0</LENGTH>
            <INODEID>0</INODEID>
            <PATH>/javafx-src.zip._COPYING_</PATH>
            <REPLICATION>3</REPLICATION>
            <MTIME>1554995797095</MTIME>
            <ATIME>1554995796290</ATIME>
            <BLOCKSIZE>134217728</BLOCKSIZE>
            <CLIENT_NAME></CLIENT_NAME>
            <CLIENT_MACHINE></CLIENT_MACHINE>
            <OVERWRITE>false</OVERWRITE>
            <BLOCK>
                <BLOCK_ID>1073741826</BLOCK_ID>
                <NUM_BYTES>5211154</NUM_BYTES>
                <GENSTAMP>1002</GENSTAMP>
            </BLOCK>
            <PERMISSION_STATUS>
                <USERNAME>root</USERNAME>
                <GROUPNAME>supergroup</GROUPNAME>
                <MODE>420</MODE>
            </PERMISSION_STATUS>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_RENAME_OLD</OPCODE>
        <DATA>
            <TXID>26</TXID>
            <LENGTH>0</LENGTH>
            <SRC>/javafx-src.zip._COPYING_</SRC>
            <DST>/javafx-src.zip</DST>
            <TIMESTAMP>1554995797102</TIMESTAMP>
            <RPC_CLIENTID>5b80319d-463f-4591-b090-2d1462563e48</RPC_CLIENTID>
            <RPC_CALLID>11</RPC_CALLID>
        </DATA>
    </RECORD>
    <RECORD>
        <OPCODE>OP_DELETE</OPCODE>
        <DATA>
            <TXID>27</TXID>
            <LENGTH>0</LENGTH>
            <PATH>/javafx-src.zip</PATH>
            <TIMESTAMP>1554995877570</TIMESTAMP>
            <RPC_CLIENTID>847eb009-9524-4f1c-a5a5-5253908ec1b5</RPC_CLIENTID>
            <RPC_CALLID>3</RPC_CALLID>
        </DATA>
    </RECORD>
</EDITS>

四、CheckPoint 时间设置

1.以时间为准

<property>
    <name>dfs.namenode.checkpoint.period</name>
    <value>3600</value>
</property>

SecondaryNameNode 默认每隔一小时执行一次

查看默认配置：http://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/hdfs-default.xml

2.以次数为准

<property>
    <name>dfs.namenode.checkpoint.txns</name>
    <value>10000</value>
    <description>作次数到达一万次就发起合并</description>
</property>
<property>
    <name>dfs.namenode.checkpoint.check.period</name>
    <value>600</value>
    <description>每间隔10分钟检查一次操作次数是否到达</description>
</property>

---

http://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/HdfsEditsViewer.html

http://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/HdfsImageViewer.html

https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-hdfs/HdfsUserGuide.html#Secondary_NameNode