一、備份namenode的元數據

namenode中的元數據非常重要，如丟失或者損壞，則整個系統無法使用。因此應該經常對元數據進行備份，最好是異地備份。

1、將元數據復制到遠程站點

（1）以下代碼將secondary namenode中的元數據復制到一個時間命名的目錄下，然後通過scp命令遠程發送到其它機器

 #!/bin/bash  
 export dirname=/mnt/tmphadoop/dfs/namesecondary/current/`date +%y%m%d%H`  
 if [ ! -d ${dirname} ]  
 then  
 mkdir  ${dirname}  
 cp /mnt/tmphadoop/dfs/namesecondary/current/*  ${dirname}  
 fi  
 scp -r ${dirname} slave1:/mnt/namenode_backup/  
 rm -r ${dirname}  

（2）配置crontab，定時執行此項工作
0 0,8,14,20 * * * bash /mnt/scripts/namenode_backup_script.sh

2、在遠程站點中啟動一個本地namenode守護進程，嘗試加載這些備份文件，以確定是否已經進行了正確備份。

二、數據備份

對於重要的數據，不能完全依賴HDFS，而是需要進行備份，註意以下幾點
（1）盡量異地備份
（2）如果使用distcp備份至另一個hdfs集群，則不要使用同一版本的hadoop，避免hadoop自身導致數據出錯。

三、文件系統檢查

定期在整個文件系統上運行HDFS的fsck工具，主動查找丟失或者損壞的塊。
建議每天執行一次。

 [jediael@master ~]$ hadoop fsck /  
 ……省略輸出（若有錯誤，則在此外出現，否則只會出現點，一個點表示一個文件）……  
 .........Status: HEALTHY  
  Total size:    14466494870 B  
  Total dirs:    502  
  Total files:   1592 (Files currently being written: 2)  
  Total blocks (validated):      1725 (avg. block size 8386373 B)  
  Minimally replicated blocks:   1725 (100.0 %)  
  Over-replicated blocks:        0 (0.0 %)  
  Under-replicated blocks:       648 (37.565216 %)  
  Mis-replicated blocks:         0 (0.0 %)  
  Default replication factor:    2  
  Average block replication:     2.0  
  Corrupt blocks:                0  
  Missing replicas:              760 (22.028986 %)  
  Number of data-nodes:          2  
  Number of racks:               1  
 FSCK ended at Sun Mar 01 20:17:57 CST 2015 in 608 milliseconds  
   
 The filesystem under path ‘/‘ is HEALTHY  

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（1）若hdfs-site.xml中的dfs.replication設置為3，而實現上只有2個datanode，則在執行fsck時會出現以下錯誤；
/hbase/Mar0109_webpage/59ad1be6884739c29d0624d1d31a56d9/il/43e6cd4dc61b49e2a57adf0c63921c09: Under replicated blk_-4711857142889323098_6221. Target Replicas is 3 but found 2 replica(s).
註意，由於原來的dfs.replication為3，後來下線了一臺datanode，並將dfs.replication改為2，但原來已創建的文件也會記錄dfs.replication為3，從而出現以上錯誤，並導致 Under-replicated blocks: 648 (37.565216 %)。

（2）fsck工具還可以用來檢查一個文件包括哪些塊，以及這些塊分別在哪等

 [jediael@master conf]$ hadoop fsck /hbase/Feb2621_webpage/c23aa183c7cb86af27f15d4c2aee2795/s/30bee5fb620b4cd184412c69f70d24a7 -files -blocks -racks  
2.   
3. FSCK started by jediael from /10.171.29.191 for path /hbase/Feb2621_webpage/c23aa183c7cb86af27f15d4c2aee2795/s/30bee5fb620b4cd184412c69f70d24a7 at Sun Mar 01 20:39:35 CST 2015  
4. /hbase/Feb2621_webpage/c23aa183c7cb86af27f15d4c2aee2795/s/30bee5fb620b4cd184412c69f70d24a7 21507169 bytes, 1 block(s):  Under replicated blk_7117944555454804881_3655. Target Replicas is 3 but found  replica(s).  
 0. blk_7117944555454804881_3655 len=21507169 repl=2 [/default-rack/10.171.94.155:50010, /default-rack/10.251.0.197:50010]  
   
 Status: HEALTHY  
  Total size:    21507169 B  
  Total dirs:    0  
  Total files:   1  
  Total blocks (validated):      1 (avg. block size 21507169 B)  
  Minimally replicated blocks:   1 (100.0 %)  
  Over-replicated blocks:        0 (0.0 %)  
  Under-replicated blocks:       1 (100.0 %)  
  Mis-replicated blocks:         0 (0.0 %)  
  Default replication factor:    2  
  Average block replication:     2.0  
  Corrupt blocks:                0  
  Missing replicas:              1 (50.0 %)  
  Number of data-nodes:          2  
  Number of racks:               1  
 FSCK ended at Sun Mar 01 20:39:35 CST 2015 in 0 milliseconds  
   
  
 The filesystem under path ‘/hbase/Feb2621_webpage/c23aa183c7cb86af27f15d4c2aee2795/s/30bee5fb620b4cd184412c69f70d24a7‘ is HEALTHY  

此命令的用法如下：

[jediael@master ~]$ hadoop fsck -files  
 Usage: DFSck  [-move | -delete | -openforwrite] [-files [-blocks [-locations | -racks]]]  
           start checking from this path  
         -move   move corrupted files to /lost+found  
         -delete delete corrupted files  
         -files  print out files being checked  
         -openforwrite   print out files opened for write  
         -blocks print out block report  
         -locations      print out locations for every block  
         -racks  print out network topology for data-node locations  
                 By default fsck ignores files opened for write, use -openforwrite to report such files. They are usually  tagged CORRUPT or HEALTHY depending on their block allocation status  
 Generic options supported are  
 -conf <configuration file>     specify an application configuration file  
-D <property=value>            use value for given property  
 -fs <local|namenode:port>      specify a namenode  
 -jt <local|jobtracker:port>    specify a job tracker  
 -files <comma separated list of files>    specify comma separated files to be copied to the map reduce cluster  
 -libjars <comma separated list of jars>    specify comma separated jar files to include in the classpath.  
 -archives <comma separated list of archives>    specify comma separated archives to be unarchived on the compute machines.  
   
The general command line syntax is  
bin/hadoop command [genericOptions] [commandOptions]  

詳細解釋請見《hadoop權威指南》P376

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