[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
[jediael@master log]$ start-balancer.sh
starting balancer, logging to /var/log/hadoop/hadoop-jediael-balancer-master.out
查看日志如下：
[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）:50075/blockScannerReport
列出总体的检测情况
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）: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