Hadoop集群日常运维

（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

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#!/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。

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[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

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[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：

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[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

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[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

参考文章 : http://blog.csdn.net/jediael_lu/article/details/44006629