cloudera manager监控页面HDFS大部分机器出现类似告警"存在隐患 : DataNode 有 xxxxxx 个块。 警告阈值：500,000 块。"，cm给出的建议：

这是 DataNode 运行状况检查，用于检查 DataNode 是否含有过多的块。如果 DataNode 含有过多的块，可能影响 DataNode 的性能。具有大量块数的 DataNode 将需要较大的 java 堆并且可能遇到较长时间的垃圾回收暂停。另外，大量块数可能表明存在许多小文件。不会为处理许多小文件而优化 HDFS，跨许多小文件进行操作时处理时间可能受影响。

如果只有部分 DataNode 有大量块，运行 HDFS 重新平衡命令可以通过移动 DataNode 之间的数据解决该问题。如果 HDFS 重新平衡命令将群集报告为平衡，没有修复块不平衡，则问题与存在的许多小文件有关。参阅 HDFS 文档了解解决该问题的最佳做法。如果许多小文件不是您的使用案例的关注点，则考虑禁用该运行状况测试。如果所有 DataNode 都有大量块数且该问题与小文件无关，则应添加更多 DataNode。

思路：确认hdfs集群中是否确实存在大量小文件，根据实际需要对小文件进行合并，对于历史数据及时清理归档。

排查过程
获取fsimage信息

Usage: hdfs dfsadmin [-fetchImage ]

$ hdfs dfsadmin -fetchImage /data
20/10/27 17:48:04 INFO namenode.TransferFsImage: Opening connection to http://namenode1:50070/imagetransfer?getimage=1&txid=latest
20/10/27 17:48:04 INFO namenode.TransferFsImage: Image Transfer timeout configured to 60000 milliseconds
20/10/27 17:48:05 INFO namenode.TransferFsImage: Transfer took 1.14s at 357022.87 KB/s
$ ll -h /data/fsimage_0000000000930647029
-rw-r----- 1 app app 397M 10月 27 17:48 /data/fsimage_0000000000930647029
格式化fsimage为可读文本
参照官网给出的《Offline Image Viewer Guide》，读取fsimage，格式化为文本，具体操作如下：

$ hdfs oiv -i /data/fsimage_0000000000930647029 -o /data/fsimage.csv -p Delimited -delimiter “,”
20/10/27 19:35:45 INFO offlineImageViewer.PBImageTextWriter: Loading string table
20/10/27 19:35:45 INFO offlineImageViewer.FSImageHandler: Loading 20 strings
20/10/27 19:35:45 INFO offlineImageViewer.PBImageTextWriter: Loading inode references
20/10/27 19:35:45 INFO offlineImageViewer.FSImageHandler: Loading inode references
20/10/27 19:35:45 INFO offlineImageViewer.FSImageHandler: Loaded 0 inode references
20/10/27 19:35:45 INFO offlineImageViewer.PBImageTextWriter: Loading directories
20/10/27 19:35:45 INFO offlineImageViewer.PBImageTextWriter: Loading directories in INode section.
20/10/27 19:35:52 INFO offlineImageViewer.PBImageTextWriter: Found 224875 directories in INode section.
20/10/27 19:35:52 INFO offlineImageViewer.PBImageTextWriter: Finished loading directories in 6598ms
20/10/27 19:35:52 INFO offlineImageViewer.PBImageTextWriter: Loading INode directory section.
20/10/27 19:35:54 INFO offlineImageViewer.PBImageTextWriter: Scanned 214127 INode directories to build namespace.
20/10/27 19:35:54 INFO offlineImageViewer.PBImageTextWriter: Finished loading INode directory section in 1784ms
20/10/27 19:35:54 INFO offlineImageViewer.PBImageTextWriter: Found 3697297 INodes in the INode section
20/10/27 19:36:55 INFO offlineImageViewer.PBImageTextWriter: Outputted 3697297 INodes.
$ head /data/fsimage.csv
Path,Replication,ModificationTime,AccessTime,PreferredBlockSize,BlocksCount,FileSize,NSQUOTA,DSQUOTA,Permission,UserName,GroupName
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=16/000151_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,438472,0,0,-rwxrwxrwt,user1,hive
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=12/000151_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,437489,0,0,-rwxrwxrwt,user1,hive
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=11/000151_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,437340,0,0,-rwxrwxrwt,user1,hive
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=09/000151_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,435482,0,0,-rwxrwxrwt,user1,hive
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=05/000151_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,420584,0,0,-rwxrwxrwt,user1,hive
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=07/000151_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,432046,0,0,-rwxrwxrwt,user1,hive
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=19/000151_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,436986,0,0,-rwxrwxrwt,user1,hive
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=14/000151_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,436246,0,0,-rwxrwxrwt,user1,hive
/user/hive/warehouse/test.db/table1/partitionday=20200914/partitionhour=10/000193_0.lzo,3,2020-09-15 04:15,2020-09-15 04:15,134217728,1,443284,0,0,-rwxrwxrwt,user1,hive
$ sed -i -e “1d” /data/fsimage.csv # 删除fsimage.csv的首行表头
建立存储fsimage的表
CREATE TABLE fsimage_info_csv(
path string,
replication int,
modificationtime string,
accesstime string,
preferredblocksize bigint,
blockscount int,
filesize bigint,
nsquota string,
dsquota string,
permission string,
username string,
groupname string)
ROW FORMAT SERDE
‘org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe’
WITH SERDEPROPERTIES (
‘field.delim’=’,’,
‘serialization.format’=’,’)
STORED AS INPUTFORMAT
‘org.apache.hadoop.mapred.TextInputFormat’
OUTPUTFORMAT
‘org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat’
LOCATION
‘hdfs://nameservice1/user/hive/warehouse/fsimage_info_csv’;
加载数据到hive表
$ hdfs dfs -put /data/fsimage.csv /user/hive/warehouse/fsimage_info_csv/
查看文件大小分布
根据fsimage文件查看一下文件大小的分布，如下：

$ hdfs oiv -p FileDistribution -i fsimage_0000000000930647029 -o fs_distribution
$ cat fs_distribution
Processed 0 inodes.
Processed 1048576 inodes.
Processed 2097152 inodes.
Processed 3145728 inodes.
Size NumFiles
0 209746
2097152 2360944
4194304 184952
6291456 121774
8388608 37136
// 省略中间部分
10485760 36906
12582912 51616
14680064 19209
16777216 14617
18874368 7655
20971520 5625
23068672 26746
25165824 112429
27262976 10304
29360128 12315
31457280 11966
33554432 15739
35651584 10180
115425148928 1
totalFiles = 3472422
totalDirectories = 224875
totalBlocks = 3401315
totalSpace = 122170845300822
maxFileSize = 115423398874
逐级目录统计文件数量
以查找三级目录下的小文件数量为例，如下：

SELECT
dir_path ,
COUNT(*) AS small_file_num
FROM
( SELECT
relative_size,
dir_path
FROM
( SELECT
(
CASE filesize < 4194304
WHEN TRUE
THEN ‘small’
ELSE ‘large’
END) AS relative_size,
concat(’/’,split(PATH,’/’)[1], ‘/’,split(PATH,’/’)[2], ‘/’
,split(PATH,’/’)[3], ‘/’,split(PATH,’/’)[4], ‘/’, split(
PATH,’/’)[5]) AS dir_path
FROM
DEFAULT.fsimage_info_csv
WHERE
permission LIKE ‘d%’) t1
WHERE
relative_size=‘small’) t2
GROUP BY
dir_path
ORDER BY
small_file_num
相应数据脱敏后输出如下：

/data/load/201905032130 1
//省略中间部分
/user/hive/warehouse/teset.db/table1 2244
/user/hive/warehouse/teset.db/table2 2244
/user/hive/warehouse/teset.db/table3 2244
/user/hive/warehouse/teset.db/table4 2246
/user/hive/warehouse/teset.db/table5 2246
/user/hive/warehouse/teset.db/table6 2248
/user/hive/warehouse/teset.db/table7 2508
/user/hive/warehouse/teset.db/table8 3427
Time taken: 53.929 seconds, Fetched: 32947 row(s)
小文件处理
根据涉及目录，反向找到涉及程序，尝试优化避免小文件的产生
及时合并归档小文件
及时清理历史小文件