hadoop日常运维

@(HADOOP)[hadoop]

（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]thenmkdir{dirname} ] then mkdir {dirname}
cp /mnt/tmphadoop/dfs/namesecondary/current/* dirnamefiscp−r{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: HEALTHYTotal size:    14466494870 BTotal dirs:    502Total 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:    2Average block replication:     2.0Corrupt blocks:                0Missing replicas:              760 (22.028986 %)Number of data-nodes:          2Number of racks:               1
FSCK ended at Sun Mar 01 20:17:57 CST 2015 in 608 millisecondsThe 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 -racksFSCK 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: HEALTHYTotal size:    21507169 BTotal dirs:    0Total files:   1Total 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:    2Average block replication:     2.0Corrupt blocks:                0Missing replicas:              1 (50.0 %)Number of data-nodes:          2Number of racks:               1
FSCK ended at Sun Mar 01 20:39:35 CST 2015 in 0 millisecondsThe 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 locationsBy 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）http://datanote: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）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