HDFS中的File由Block组成，一个File包含一个或多个Block，当创建File时会创建一个Block，然后根据配置的副本数量（默认是3）申请3个Datanode来存放这个Block；
通过hdfs fsck命令可以查看一个文件具体的Block、Datanode、Rack信息，例如：
hdfs fsck /tmp/test.sql -files -blocks -locations -racks
Connecting to namenode via http://name_node:50070
FSCK started by hadoop (auth:SIMPLE) from /client for path /tmp/test.sql at Thu Dec 13 15:44:12 CST 2018
/tmp/test.sql 16 bytes, 1 block(s): OK
0. BP-436366437-name_node-1493982655699:blk_1449692331_378721485 len=16 repl=3 [/DEFAULT/server111:50010, /DEFAULT/server121:50010, /DEFAULT/server43:50010]Status: HEALTHY
Total size: 16 B
Total dirs: 0
Total files: 1
Total symlinks: 0
Total blocks (validated): 1 (avg. block size 16 B)
Minimally replicated blocks: 1 (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: 193
Number of racks: 1
FSCK ended at Thu Dec 13 15:44:12 CST 2018 in 1 milliseconds
The filesystem under path '/tmp/test.sql' is HEALTHY
那3个Datanode是如何选择出来的？有一个优先级：
1 当前机架（相对hdfs client而言）
2 远程机架（相对hdfs client而言）
3 另一机架
4 全部随机
然后每个机架能选择几个Datanode（即maxNodesPerRack）有一个计算公式，详见代码
org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer
private int findNewDatanode(final DatanodeInfo[] original
) throws IOException {
if (nodes.length != original.length + 1) {
throw new IOException(
new StringBuilder()
.append("Failed to replace a bad datanode on the existing pipeline ") .append("due to no more good datanodes being available to try. ") .append("(Nodes: current=").append(Arrays.asList(nodes)) .append(", original=").append(Arrays.asList(original)).append("). ") .append("The current failed datanode replacement policy is ") .append(dfsClient.dtpReplaceDatanodeOnFailure).append(", and ") .append("a client may configure this via '") .append(DFSConfigKeys.DFS_CLIENT_WRITE_REPLACE_DATANODE_ON_FAILURE_POLICY_KEY) .append("' in its configuration.") .toString()); }
注释：当没有找到新的datanode时会报异常，报错如下：
Caused by: java.io.IOException: Failed to replace a bad datanode on the existing pipeline due to no more good datanodes being available to try. (Nodes: current=[server82:50010], original=[server.82:50010]).
The current failed datanode replacement policy is ALWAYS, and a client may configure this via 'dfs.client.block.write.replace-datanode-on-failure.policy' in its configuration.
private void addDatanode2ExistingPipeline() throws IOException {
...
final DatanodeInfo[] original = nodes;
final LocatedBlock lb = dfsClient.namenode.getAdditionalDatanode(
src, fileId, block, nodes, storageIDs,
failed.toArray(new DatanodeInfo[failed.size()]),
1, dfsClient.clientName);
setPipeline(lb);
//find the new datanode
final int d = findNewDatanode(original);
注释：会调用getAdditionalDatanode方法来获取1个新的datanode，此处略去很多调用堆栈
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault
private DatanodeStorageInfo[] chooseTarget(int numOfReplicas,
Node writer,
List<DatanodeStorageInfo> chosenStorage,
boolean returnChosenNodes,
Set<Node> excludedNodes,
long blocksize,
final BlockStoragePolicy storagePolicy) { ... int[] result = getMaxNodesPerRack(chosenStorage.size(), numOfReplicas); numOfReplicas = result[0]; int maxNodesPerRack = result[1]; ... final Node localNode = chooseTarget(numOfReplicas, writer, excludedNodes, blocksize, maxNodesPerRack, results, avoidStaleNodes, storagePolicy, EnumSet.noneOf(StorageType.class), results.isEmpty());
注释：此处maxNodesPerRack表示每个机架最多只能分配几个datanode
private Node chooseTarget(int numOfReplicas,
Node writer,
final Set<Node> excludedNodes,
final long blocksize,
final int maxNodesPerRack,
final List<DatanodeStorageInfo> results,
final boolean avoidStaleNodes, final BlockStoragePolicy storagePolicy, final EnumSet<StorageType> unavailableStorages, final boolean newBlock) { ... if (numOfResults <= 1) { chooseRemoteRack(1, dn0, excludedNodes, blocksize, maxNodesPerRack, results, avoidStaleNodes, storageTypes); if (--numOfReplicas == 0) { return writer; } }
注释：此处会尝试在远程机架（即与已有的datanode不同的机架）获取一个新的datanode
protected void chooseRemoteRack(int numOfReplicas,
DatanodeDescriptor localMachine,
Set<Node> excludedNodes,
long blocksize,
int maxReplicasPerRack,
List<DatanodeStorageInfo> results,
boolean avoidStaleNodes, EnumMap<StorageType, Integer> storageTypes) throws NotEnoughReplicasException { ... chooseRandom(numOfReplicas, "~" + localMachine.getNetworkLocation(), excludedNodes, blocksize, maxReplicasPerRack, results, avoidStaleNodes, storageTypes);
注释：此处会在所有可选的datanode中随机选择一个
protected DatanodeStorageInfo chooseRandom(int numOfReplicas,
String scope,
Set<Node> excludedNodes,
long blocksize,
int maxNodesPerRack,
List<DatanodeStorageInfo> results,
boolean avoidStaleNodes, EnumMap<StorageType, Integer> storageTypes) throws NotEnoughReplicasException { ... int numOfAvailableNodes = clusterMap.countNumOfAvailableNodes( scope, excludedNodes); ... if (numOfReplicas>0) { String detail = enableDebugLogging; if (LOG.isDebugEnabled()) { if (badTarget && builder != null) { detail = builder.toString(); builder.setLength(0); } else { detail = ""; } } throw new NotEnoughReplicasException(detail); }
注释：如果由于一些原因（比如节点磁盘满或者下线），导致numOfAvailableNodes计算结果为0，会抛出NotEnoughReplicasException
其中maxNodesPerRack计算逻辑如下：
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault
/**
* Calculate the maximum number of replicas to allocate per rack. It also
* limits the total number of replicas to the total number of nodes in the
* cluster. Caller should adjust the replica count to the return value.
*
* @param numOfChosen The number of already chosen nodes.
* @param numOfReplicas The number of additional nodes to allocate.
* @return integer array. Index 0: The number of nodes allowed to allocate
* in addition to already chosen nodes.
* Index 1: The maximum allowed number of nodes per rack. This
* is independent of the number of chosen nodes, as it is calculated
* using the target number of replicas.
*/
private int[] getMaxNodesPerRack(int numOfChosen, int numOfReplicas) {
int clusterSize = clusterMap.getNumOfLeaves();
int totalNumOfReplicas = numOfChosen + numOfReplicas;
if (totalNumOfReplicas > clusterSize) {
numOfReplicas -= (totalNumOfReplicas-clusterSize);
totalNumOfReplicas = clusterSize; } // No calculation needed when there is only one rack or picking one node. int numOfRacks = clusterMap.getNumOfRacks(); if (numOfRacks == 1 || totalNumOfReplicas <= 1) { return new int[] {numOfReplicas, totalNumOfReplicas}; } int maxNodesPerRack = (totalNumOfReplicas-1)/numOfRacks + 2; // At this point, there are more than one racks and more than one replicas // to store. Avoid all replicas being in the same rack. // // maxNodesPerRack has the following properties at this stage. // 1) maxNodesPerRack >= 2 // 2) (maxNodesPerRack-1) * numOfRacks > totalNumOfReplicas // when numOfRacks > 1 // // Thus, the following adjustment will still result in a value that forces // multi-rack allocation and gives enough number of total nodes. if (maxNodesPerRack == totalNumOfReplicas) { maxNodesPerRack--; } return new int[] {numOfReplicas, maxNodesPerRack}; }
注释：
int maxNodesPerRack = (totalNumOfReplicas-1)/numOfRacks + 2;
if (maxNodesPerRack == totalNumOfReplicas) {
maxNodesPerRack--;
}