平衡指的是 每個DataNode的利用率 與 集群的利用率 之間相差不超過給定的閾值百分比。此處的平衡 指的是各個DataNode之間的平衡，同一個DataNode之間的各個磁盤是不會平衡的。
1、背景
當我們的hadoop集群運行了一段時間之后，各個DataNode上的數據分布并不一定是均勻分布的。比如說： 我們向現有集群中添加了一個新的DataNode。
DataNode數據不均衡
2、什么是平衡
此處是我自己的一個簡單的理解
所謂的平衡指的是 每個DataNode的利用率 與 集群的利用率 之間相差不超過給定的閾值百分比。此處的平衡 指的是各個DataNode之間的平衡，同一個DataNode之間的各個磁盤是不會平衡的。
2.1 每個DataNode的利用率計算
每個DataNode的利用率計算
DataNode的利用率= dfs已用的空間 / 分配給dfs的空間。
注意： 分配給dfs的空間 不是磁盤的總空間。
2.2 集群的利用率
集群的利用率
集群的利用率= 各datanode dfs已使用的空間 / 各datanode總空間
2.3 平衡
假設平衡的閾值是 5%，集群的利用率是 37.5，那么每個節點的利用率在32.5%到42.5%之間都認為是均衡的。也就是說，極端情況下，DataNode的利用率最大相差10%。
3、hdfs balancer語法
[hadoopdeploy@hadoop01 ~]$ hdfs balancer --help
Usage: hdfs balancer
[-policy <policy>] the balancing policy: datanode or blockpool
[-threshold <threshold>] Percentage of disk capacity
[-exclude [-f <hosts-file> | <comma-separated list of hosts>]] Excludes the specified datanodes.
[-include [-f <hosts-file> | <comma-separated list of hosts>]] Includes only the specified datanodes.
[-source [-f <hosts-file> | <comma-separated list of hosts>]] Pick only the specified datanodes as source nodes.
[-blockpools <comma-separated list of blockpool ids>] The balancer will only run on blockpools included in this list.
[-idleiterations <idleiterations>] Number of consecutive idle iterations (-1 for Infinite) before exit.
[-runDuringUpgrade] Whether to run the balancer during an ongoing HDFS upgrade.This is usually not desired since it will not affect used space on over-utilized machines.
[-asService] Run as a long running service.
Generic options supported are:
-conf <configuration file> specify an application configuration file
-D <property=value> define a value for a given property
-fs <file:///|hdfs://namenode:port> specify default filesystem URL to use, overrides 'fs.defaultFS' property from configurations.
-jt <local|resourcemanager:port> specify a ResourceManager
-files <file1,...> specify a comma-separated list of files to be copied to the map reduce cluster
-libjars <jar1,...> specify a comma-separated list of jar files to be included in the classpath
-archives <archive1,...> specify a comma-separated list of archives to be unarchived on the compute machines
The general command line syntax is:
command [genericOptions] [commandOptions]
參數 | 描述 |
threshold | 磁盤容量的百分比。默認值為10%，表示上下浮動10%。 |
policy | 平衡策略。
datanode（默認）：當每一個DataNode是平衡的時候，集群就是平衡的。
blockpool：當每一個DataNode中的blockpool是平衡的，集群就是平衡的。 |
exclude | 不參與平衡的DataNode節點 |
include | 參與平衡的DataNode節點 |
source | 僅選取指定的數據節點作為源節點 |
blockpools | Balancer僅在指定的blockpools中運行 |
idleiterations | 退出前的連續空閑迭代次數（-1表示無限） |
-runDuringUpgrade | 是否在正在進行的HDFS升級過程中運行平衡器。通常不需要這樣做，因為這不會影響過度使用的計算機上的已用空間。 |
-asService | 作為長期運行的服務運行 |
4、運行一個簡單的balance案例
4.1 設置平衡數據傳輸帶寬
[hadoopdeploy@hadoop01 ~]$ hdfs dfsadmin -setBalancerBandwidth 10485760
Balancer bandwidth is set to 10485760
[hadoopdeploy@hadoop01 ~]$
當我們的集群負載需要調低這個值，當我們的集群負載較低時，可以適當調高這個值。
4.2 執行banalce
[hadoopdeploy@hadoop01 ~]$ hdfs balancer -policy datanode -threshold 5
2023-03-26 14:10:09,785 INFO balancer.Balancer: Using a threshold of 5.0
2023-03-26 14:10:09,786 INFO balancer.Balancer: namenodes = [hdfs://hadoop01:8020]
2023-03-26 14:10:09,786 INFO balancer.Balancer: parameters = Balancer.BalancerParameters [BalancingPolicy.Node, threshold = 5.0, max idle iteration = 5, #excluded nodes = 0, #included nodes = 0, #source nodes = 0, #blockpools = 0, run during upgrade = false]
2023-03-26 14:10:09,786 INFO balancer.Balancer: included nodes = []
2023-03-26 14:10:09,786 INFO balancer.Balancer: excluded nodes = []
2023-03-26 14:10:09,786 INFO balancer.Balancer: source nodes = []
Time Stamp Iteration# Bytes Already Moved Bytes Left To Move Bytes Being Moved NameNode
2023-03-26 14:10:09,787 INFO balancer.NameNodeConnector: getBlocks calls for hdfs://hadoop01:8020 will be rate-limited to 20 per second
2023-03-26 14:10:10,392 INFO balancer.Balancer: dfs.namenode.get-blocks.max-qps = 20 (default=20)
2023-03-26 14:10:10,392 INFO balancer.Balancer: dfs.balancer.movedWinWidth = 5400000 (default=5400000)
2023-03-26 14:10:10,392 INFO balancer.Balancer: dfs.balancer.moverThreads = 1000 (default=1000)
2023-03-26 14:10:10,392 INFO balancer.Balancer: dfs.balancer.dispatcherThreads = 200 (default=200)
2023-03-26 14:10:10,392 INFO balancer.Balancer: dfs.balancer.getBlocks.size = 2147483648 (default=2147483648)
2023-03-26 14:10:10,392 INFO balancer.Balancer: dfs.balancer.getBlocks.min-block-size = 10485760 (default=10485760)
2023-03-26 14:10:10,392 INFO balancer.Balancer: dfs.datanode.balance.max.concurrent.moves = 100 (default=100)
2023-03-26 14:10:10,392 INFO balancer.Balancer: dfs.datanode.balance.bandwidthPerSec = 104857600 (default=104857600)
2023-03-26 14:10:10,395 INFO balancer.Balancer: dfs.balancer.max-size-to-move = 10737418240 (default=10737418240)
2023-03-26 14:10:10,395 INFO balancer.Balancer: dfs.blocksize = 134217728 (default=134217728)
2023-03-26 14:10:10,401 INFO net.NetworkTopology: Adding a new node: /default-rack/192.168.121.141:9866
2023-03-26 14:10:10,401 INFO net.NetworkTopology: Adding a new node: /default-rack/192.168.121.140:9866
2023-03-26 14:10:10,401 INFO net.NetworkTopology: Adding a new node: /default-rack/192.168.121.142:9866
2023-03-26 14:10:10,402 INFO balancer.Balancer: 0 over-utilized: []
2023-03-26 14:10:10,402 INFO balancer.Balancer: 0 underutilized: []
2023-3-26 14:10:10 0 0 B 0 B 0 B 0 hdfs://hadoop01:8020
The cluster is balanced. Exiting...
2023-3-26 14:10:10 Balancing took 810.0 milliseconds
[hadoopdeploy@hadoop01 ~]$
5、參考文檔
1、https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/HdfsUserGuide.html#Balancer
2、https://help.aliyun.com/document_detail/449686.html?
