（一）备份namenode的元数据
namenode中的元数据非常重要，如丢失或者损坏，则整个系统无法使用。因此应该经常对元数据进行备份，最好是异地备份。
1、将元数据复制到远程站点
（1）以下代码将secondary namenode中的元数据复制到一个时间命名的目录下，然后通过scp命令远程发送到其它机器
#!/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 ${dirname}
（2）配置crontab，定时执行此项工作
0 0,8,14,20 * * * bash /mnt/scripts/namenode_backup_script.sh
2、在远程站点中启动一个本地namenode守护进程，尝试加载这些备份文件，以确定是否已经进行了正确备份。
（二）数据备份
对于重要的数据，不能完全依赖HDFS，而是需要进行备份，注意以下几点
（1）尽量异地备份
（2）如果使用distcp备份至另一个hdfs集群，则不要使用同一版本的hadoop，避免hadoop自身导致数据出错。
（三）文件系统检查
定期在整个文件系统上运行HDFS的fsck工具，主动查找丢失或者损坏的块。
建议每天执行一次。
[jediael@master ~]$ hadoop fsck /
……省略输出（若有错误，则在此外出现，否则只会出现点，一个点表示一个文件）……
.........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
（1）若hdfs-site.xml中的dfs.replication设置为3，而实现上只有2个datanode，则在执行fsck时会出现以下错误；
/hbase/Mar0109_webpage/59ad1be6884739c29d0624d1d31a56d9/il/43e6cd4dc61b49e2a57adf0c63921c09: Under replicated blk_-4711857142889323098_6221. Target Replicas is 3 but found 2 replica(s).
注意，由于原来的dfs.replication为3，后来下线了一台datanode，并将dfs.replication改为2，但原来已创建的文件也会记录dfs.replication为3，从而出现以上错误，并导致 Under-replicated blocks: 648 (37.565216 %)。
（2）fsck工具还可以用来检查一个文件包括哪些块，以及这些块分别在哪等
[jediael@master conf]$ hadoop fsck /hbase/Feb2621_webpage/c23aa183c7cb86af27f15d4c2aee2795/s/30bee5fb620b4cd184412c69f70d24a7 -files -blocks -racks
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
/hbase/Feb2621_webpage/c23aa183c7cb86af27f15d4c2aee2795/s/30bee5fb620b4cd184412c69f70d24a7 21507169 bytes, 1 block(s): Under replicated blk_7117944555454804881_3655. Target Replicas is 3 but found 2 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 <path> [-move | -delete | -openforwrite] [-files [-blocks [-locations | -racks]]]
<path> 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]
详细解释请见《hadoop权威指南》P376
（四）均衡器
随时时间推移，各个datanode上的块分布来越来越不均衡，这将降低MR的本地性，导致部分datanode相对更加繁忙。
均衡器是一个hadoop守护进程，它将块从忙碌的DN移动相对空闲的DN，同时坚持块复本放置策略，将复本分散到不同的机器、机架。
建议定期执行均衡器，如每天或者每周。
(1)通过以下命令运行均衡器
[plain] view plaincopy
[jediael@master log]$ start-balancer.sh
starting balancer, logging to /var/log/hadoop/hadoop-jediael-balancer-master.out
查看日志如下：
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[jediael@master hadoop]$ pwd
/var/log/hadoop
[jediael@master hadoop]$ ls
hadoop-jediael-balancer-master.log hadoop-jediael-balancer-master.out
[jediael@master hadoop]$ cat hadoop-jediael-balancer-master.log
2015-03-01 21:08:08,027 INFO org.apache.hadoop.net.NetworkTopology: Adding a new node: /default-rack/10.251.0.197:50010
2015-03-01 21:08:08,028 INFO org.apache.hadoop.net.NetworkTopology: Adding a new node: /default-rack/10.171.94.155:50010
2015-03-01 21:08:08,028 INFO org.apache.hadoop.hdfs.server.balancer.Balancer: 0 over utilized nodes:
2015-03-01 21:08:08,028 INFO org.apache.hadoop.hdfs.server.balancer.Balancer: 0 under utilized nodes:
（2）均衡器将每个DN的使用率与整个集群的使用率接近，这个“接近”是通过-threashold参数指定的，默认是10%。
（3）不同节点之间复制数据的带宽是受限的，默认是1MB/s，可以通过hdfs-site.xml文件中的dfs.balance.bandwithPerSec属性指定（单位是字节）。
（五）datanode块扫描器
每个datanode均会运行一个块扫描器，定期检测本节点上的所有块，若发现存在错误（如检验和错误），则通知namenode，然后由namenode发起数据重新创建复本或者修复。
扫描周期由dfs.datanode.scan.period.hours指定，默认为三周（504小时）。
通过地址以下地址查看扫描信息：
（1）http://datanote:50075/blockScannerReport
列出总体的检测情况
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Total Blocks : 1919
Verified in last hour : 4
Verified in last day : 170
Verified in last week : 535
Verified in last four weeks : 535
Verified in SCAN_PERIOD : 535
Not yet verified : 1384
Verified since restart : 559
Scans since restart : 91
Scan errors since restart : 0
Transient scan errors : 0
Current scan rate limit KBps : 1024
Progress this period : 113%
Time left in cur period : 97.14%
（2）http://123.56.92.95:50075/blockScannerReport?listblocks
列出所有的块及最新验证状态
blk_8482244195562050998_3796 : status : ok type : none scan time : 0 not yet verified
blk_3985450615149803606_7952 : status : ok type : none scan time : 0 not yet verified
尚未验证的情况如上。各字段意义可参考权威指南P379
