Hadoop学习笔记之（二）：实验Hadoop的文件块复制删除操作感受强大的容灾性

博客园 - Elaine Shi

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Elaine Shi · 2013-05-18 · via 博客园 - Elaine Shi

首先来了解一下HDFS的一些基本特性

HDFS设计基础与目标

硬件错误是常态。因此需要冗余
流式数据访问。即数据批量读取而非随机读写，Hadoop擅长做的是数据分析而不是事务处理
大规模数据集
简单一致性模型。为了降低系统复杂度，对文件采用一次性写多次读的逻辑设计，即是文件一经写入，关闭，就再也不能修改
程序采用“数据就近”原则分配节点执行

HDFS体系结构

NameNode
DataNode
事务日志
映像文件
SecondaryNameNode

Namenode

管理文件系统的命名空间
记录每个文件数据块在各个Datanode上的位置和副本信息
协调客户端对文件的访问
记录命名空间内的改动或空间本身属性的改动
Namenode使用事务日志记录HDFS元数据的变化。使用映像文件存储文件系统的命名空间，包括文件映射，文件属性等

Datanode

负责所在物理节点的存储管理
一次写入，多次读取（不修改）
文件由数据块组成，典型的块大小是64MB
数据块尽量散布道各个节点

读取数据流程

客户端要访问HDFS中的一个文件
首先从namenode获得组成这个文件的数据块位置列表
根据列表知道存储数据块的datanode
访问datanode获取数据
Namenode并不参与数据实际传输

HDFS的可靠性

冗余副本策略
机架策略
心跳机制
安全模式
使用文件块的校验和 Checksum来检查文件的完整性
回收站
元数据保护
快照机制

我分别试验了冗余副本策略/心跳机制/安全模式/回收站。下面实验是关于冗余副本策略的。

环境：

Namenode/Master/jobtracker: h1/192.168.221.130
SecondaryNameNode: h1s/192.168.221.131
四个Datanode: h2~h4 (IP段:142~144)

为以防文件太小只有一个文件块(block/blk)，我们准备一个稍微大一点的（600M）的文件，使之能分散分布到几个datanode，再停掉其中一个看有没有问题。
先来put一个文件（为了方便起见，建议将hadoop/bin追加到$Path变量后
:hadoop fs –put ~/Documents/IMMAUSWX201304
结束后，我们想查看一下文件块的情况，可以去网页上看，也可以在namenode上使用fsck命令来检查一下，关于fsck命令
:bin/hadoop fsck /user/hadoop_admin/in/bigfile -files -blocks -locations > ~/hadoopfiles/log1.txt
下面打印结果说明个600M文件被划分为9个64M的blocks，并且被分散到我当前所有datanode上（共4个），看起来比较平均，

/user/hadoop_admin/in/bigfile/USWX201304 597639882 bytes, 9 block(s): OK
0. blk_-4541681964616523124_1011 len=67108864 repl=3 [192.168.221.131:50010, 192.168.221.142:50010, 192.168.221.144:50010]
1. blk_4347039731705448097_1011 len=67108864 repl=3 [192.168.221.143:50010, 192.168.221.131:50010, 192.168.221.144:50010]
2. blk_-4962604929782655181_1011 len=67108864 repl=3 [192.168.221.142:50010, 192.168.221.143:50010, 192.168.221.144:50010]
3. blk_2055128947154747381_1011 len=67108864 repl=3 [192.168.221.143:50010, 192.168.221.142:50010, 192.168.221.144:50010]
4. blk_-2280734543774885595_1011 len=67108864 repl=3 [192.168.221.131:50010, 192.168.221.142:50010, 192.168.221.144:50010]
5. blk_6802612391555920071_1011 len=67108864 repl=3 [192.168.221.143:50010, 192.168.221.142:50010, 192.168.221.144:50010]
6. blk_1890624110923458654_1011 len=67108864 repl=3 [192.168.221.143:50010, 192.168.221.142:50010, 192.168.221.144:50010]
7. blk_226084029380457017_1011 len=67108864 repl=3 [192.168.221.143:50010, 192.168.221.131:50010, 192.168.221.144:50010]
8. blk_-1230960090596945446_1011 len=60768970 repl=3 [192.168.221.142:50010, 192.168.221.143:50010, 192.168.221.144:50010]

Status: HEALTHY
Total size:    597639882 B
Total dirs:    0
Total files:   1
Total blocks (validated):      9 (avg. block size 66404431 B)
Minimally replicated blocks:   9 (100.0 %)
Over-replicated blocks:        0 (0.0 %)
Under-replicated blocks:       0 (0.0 %)
Mis-replicated blocks:         0 (0.0 %)
Default replication factor:    3
Average block replication:     3.0
Corrupt blocks:                0
Missing replicas:              0 (0.0 %)
Number of data-nodes:          4
Number of racks:               1

h1s,h2,h3,h4四个DD全部参与，跑去h2 (142),h3(143) stop datanode, 从h4上面get，发现居然能够get回，而且初步来看,size正确，看一下上图中黄底和绿底都DEAD了，每个blk都有源可以取回，所以GET后数据仍然是完整的，从这点看hadoop确实是强大啊，load balancing也做得很不错，数据看上去很坚强，容错性做得不错

再检查一下，我本来想测试safemode的，结果隔一会一刷，本来有几个blk只有1个livenode的，现在又被全部复制为确保每个有2个了！

hadoop_admin@h1:~/hadoop-0.20.2$ hadoop fsck /user/hadoop_admin/in/bigfile -files -blocks -locations
/user/hadoop_admin/in/bigfile/USWX201304 597639882 bytes, 9 block(s):
Under replicated blk_-4541681964616523124_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_4347039731705448097_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_-4962604929782655181_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_2055128947154747381_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_-2280734543774885595_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_6802612391555920071_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_1890624110923458654_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_226084029380457017_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_-1230960090596945446_1011. Target Replicas is 3 but found 2 replica(s).
0. blk_-4541681964616523124_1011 len=67108864 repl=2 [192.168.221.131:50010, 192.168.221.144:50010]
1. blk_4347039731705448097_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
2. blk_-4962604929782655181_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
3. blk_2055128947154747381_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
4. blk_-2280734543774885595_1011 len=67108864 repl=2 [192.168.221.131:50010, 192.168.221.144:50010]
5. blk_6802612391555920071_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
6. blk_1890624110923458654_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
7. blk_226084029380457017_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
8. blk_-1230960090596945446_1011 len=60768970 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]

我决定再关一个datanode，结果等了好半天也没见namenode发现它死了，这是因为心跳机制，datanode每隔3秒会向namenode发送heartbeat指令表明它的存活，但如果namenode很长时间（5~10分钟看设置）没有收到heartbeat即认为这个NODE死掉了，就会做出BLOCK的复制操作，以保证有足够的replica来保证数据有足够的容灾/错性，现在再打印看看，发现因为只有一个live datanode，所以现在每个blk都有且只有一份

hadoop_admin@h1:~$ hadoop fsck /user/hadoop_admin/in/bigfile -files -blocks -locations
/user/hadoop_admin/in/bigfile/USWX201304 597639882 bytes, 9 block(s): Under replicated blk_-4541681964616523124_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_4347039731705448097_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_-4962604929782655181_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_2055128947154747381_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_-2280734543774885595_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_6802612391555920071_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_1890624110923458654_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_226084029380457017_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_-1230960090596945446_1011. Target Replicas is 3 but found 1 replica(s).

我现在把其中一个BLK从这个仅存的Datanode中移走使之corrupt，我想实验，重启一个DATANODE后，会不会复员
hadoop_admin@h4:/hadoop_run/data/current$ mv blk_4347039731705448097_1011* ~/Documents/
然后为了不必要等8分钟DN发block report，我手动修改了h4的dfs.blockreport.intervalMsec值为30000，stop datanode,再start (另外，你应该把hadoop/bin也加入到Path变量后面，这样你可以不带全路径执行hadoop命令，结果，检测它已被损坏
hadoop_admin@h1:~$ hadoop fsck /user/hadoop_admin/in/bigfile -files -blocks -locations

/user/hadoop_admin/in/bigfile/USWX201304 597639882 bytes, 9 block(s): Under replicated blk_-4541681964616523124_1011. Target Replicas is 3 but found 1 replica(s).

/user/hadoop_admin/in/bigfile/USWX201304: CORRUPT block blk_4347039731705448097
Under replicated blk_-4962604929782655181_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_2055128947154747381_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_-2280734543774885595_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_6802612391555920071_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_1890624110923458654_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_226084029380457017_1011. Target Replicas is 3 but found 1 replica(s).
Under replicated blk_-1230960090596945446_1011. Target Replicas is 3 but found 1 replica(s).
MISSING 1 blocks of total size 67108864 B
0. blk_-4541681964616523124_1011 len=67108864 repl=1 [192.168.221.144:50010]
1. blk_4347039731705448097_1011 len=67108864 MISSING!
2. blk_-4962604929782655181_1011 len=67108864 repl=1 [192.168.221.144:50010]
3. blk_2055128947154747381_1011 len=67108864 repl=1 [192.168.221.144:50010]
4. blk_-2280734543774885595_1011 len=67108864 repl=1 [192.168.221.144:50010]
5. blk_6802612391555920071_1011 len=67108864 repl=1 [192.168.221.144:50010]
6. blk_1890624110923458654_1011 len=67108864 repl=1 [192.168.221.144:50010]
7. blk_226084029380457017_1011 len=67108864 repl=1 [192.168.221.144:50010]
8. blk_-1230960090596945446_1011 len=60768970 repl=1 [192.168.221.144:50010]

Status: CORRUPT
Total size:    597639882 B
Total dirs:    0
Total files:   1
Total blocks (validated):      9 (avg. block size 66404431 B)
   ********************************
   CORRUPT FILES:        1
   MISSING BLOCKS:       1
   MISSING SIZE:         67108864 B
   CORRUPT BLOCKS:       1
   ********************************
Minimally replicated blocks:   8 (88.888885 %)
Over-replicated blocks:        0 (0.0 %)
Under-replicated blocks:       8 (88.888885 %)
Mis-replicated blocks:         0 (0.0 %)
Default replication factor:    3
Average block replication:     0.8888889
Corrupt blocks:                1
Missing replicas:              16 (200.0 %)
Number of data-nodes:          1
Number of racks:               1

The filesystem under path '/user/hadoop_admin/in/bigfile' is CORRUPT

我现在启动一个DATANODE h1s（131），结果很快的在30秒之内，它就被hadoop原地满HP复活了，现在每个blk都有了两份replica
hadoop_admin@h1:~$ hadoop fsck /user/hadoop_admin/in/bigfile -files -blocks -locations
/user/hadoop_admin/in/bigfile/USWX201304 597639882 bytes, 9 block(s): Under replicated blk_-4541681964616523124_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_4347039731705448097_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_-4962604929782655181_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_2055128947154747381_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_-2280734543774885595_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_6802612391555920071_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_1890624110923458654_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_226084029380457017_1011. Target Replicas is 3 but found 2 replica(s).
Under replicated blk_-1230960090596945446_1011. Target Replicas is 3 but found 2 replica(s).
0. blk_-4541681964616523124_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
1. blk_4347039731705448097_1011 len=67108864 repl=2 [192.168.221.131:50010, 192.168.221.144:50010]
2. blk_-4962604929782655181_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
3. blk_2055128947154747381_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
4. blk_-2280734543774885595_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
5. blk_6802612391555920071_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
6. blk_1890624110923458654_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
7. blk_226084029380457017_1011 len=67108864 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]
8. blk_-1230960090596945446_1011 len=60768970 repl=2 [192.168.221.144:50010, 192.168.221.131:50010]

发现这个文件被从131成功复制回了144 (h4)。

结论：HADOOP容灾太坚挺了，我现在坚信不疑了！

另外有一个没有粘出来的提示就是，h4 datanode上有不少重新format遗留下来的badLinkBlock，在重新put同一个文件的时候，hadoop将那些老旧残留的block文件全部都删除了。这说明它是具有删除无效bad block的功能的。

实验简单来讲就是

1. put 一个600M文件，分散3个replica x 9个block 共18个blocks到4个datanode

2. 我关掉了两个datanode，使得大部分的block只在一个datanode上存在，但因为9个很分散，所以文件能正确取回（靠的是checksum来计算文件值）

3. hadoop namenode很迅速的复制了仅有一个replica的block使之成为 3 replica(2) but only found 2

4. 我再关掉一个datanode，结果发现每个datanode被很均衡的分配了block，这样即使只有一个datanode，也因为之前有确保2个replicas的比率，所以依然healthy

5. 我从这个仅存的datanode中删除一个blk，namenode report这个文件corrupt，（我其实一直很希望能进safemode，结果-safemode get一直是OFF）

6. 然后我启动另外一个datanode，30秒不到，这个missing的block被从这个新启动的datanode中迅速“扩展”为2个replicas

容灾性非常可靠，如果使用至少三个rack的话，数据会非常坚挺，对HADOOP信任值 level up!

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博客园 - Elaine Shi