前几天报错如下：
Error: org.apache.hadoop.ipc.RemoteException(java.io.IOException): File /user/xxx/part-r-00002 could only be replicated to 0 nodes instead of minReplication (=1). There are 11 datanode(s) running and no node(s) are excluded in this operation.
at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.chooseTarget(BlockManager.java:1327)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getAdditionalBlock(FSNamesystem.java:2278)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.addBlock(NameNodeRpcServer.java:480)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.addBlock(ClientNamenodeProtocolServerSideTranslatorPB.java:297)
at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java:44080)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:453)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:1002)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1695)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1691)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:396)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1408)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:1689)阅读一下代码了解hdfs选择块的策略，可能理解有误，如果有错误请指出，代码来自cdh-4.2.1版本
在 FSNamesystem.getAdditionalBlock()里，有个blockManager.chooseTarget(src, replication, clientNode, excludedNodes, blockSize);
这个方法用来决定在那个节点写数据
代码文件：hadoop-2.0.0-cdh4.2.1\src\hadoop-hdfs-project\hadoop-hdfs\src\main\java\org\apache\hadoop\hdfs\server\blockmanagement\BlockPlacementPolicyDefault.java
chooseTarget方法调用了几次转换，看最终调用的方法
/* choose <i>numOfReplicas</i> from all data nodes */
private DatanodeDescriptor chooseTarget(int numOfReplicas,
DatanodeDescriptor writer,
HashMap<Node, Node> excludedNodes,
long blocksize,
int maxNodesPerRack,
List<DatanodeDescriptor> results) {
if (numOfReplicas == 0 ¦¦ clusterMap.getNumOfLeaves()==0) {
return writer;
}
int totalReplicasExpected = numOfReplicas; // 总共需要的副本数
// results是当前已分配的节点
int numOfResults = results.size();
boolean newBlock = (numOfResults==0);
if (writer == null && !newBlock) {
writer = results.get(0);
}
try {
// 如果还没分配过，先选择本地节点
if (numOfResults == 0) {
writer = chooseLocalNode(writer, excludedNodes,
blocksize, maxNodesPerRack, results);
if (--numOfReplicas == 0) {
return writer;
}
}
// 如果之前已分配一个或零个，在其他机架的一台机器选择
if (numOfResults <= 1) {
chooseRemoteRack(1, results.get(0), excludedNodes,
blocksize, maxNodesPerRack, results);
if (--numOfReplicas == 0) {
return writer;
}
}
// 如果前面还没分配完，已有小于或等于两个副本
if (numOfResults <= 2) {
// 如果前两个在同一个机架上，选择一个其他机架的
if (clusterMap.isOnSameRack(results.get(0), results.get(1))) {
chooseRemoteRack(1, results.get(0), excludedNodes,
blocksize, maxNodesPerRack, results);
} else if (newBlock){
chooseLocalRack(results.get(1), excludedNodes, blocksize,
maxNodesPerRack, results);
} else {
chooseLocalRack(writer, excludedNodes, blocksize,
maxNodesPerRack, results);
}
if (--numOfReplicas == 0) {
return writer;
}
}
// 如果还需要副本，就随机选择
chooseRandom(numOfReplicas, NodeBase.ROOT, excludedNodes,
blocksize, maxNodesPerRack, results);
} catch (NotEnoughReplicasException e) {
LOG.warn("Not able to place enough replicas, still in need of "
+ numOfReplicas + " to reach " + totalReplicasExpected + "\n"
+ e.getMessage());
}
return writer;
}看一下 chooseLocalNode
/* choose <i>localMachine</i> as the target.
* if <i>localMachine</i> is not available,
* choose a node on the same rack
* @return the chosen node
*/
private DatanodeDescriptor chooseLocalNode(
DatanodeDescriptor localMachine,
HashMap<Node, Node> excludedNodes,
long blocksize,
int maxNodesPerRack,
List<DatanodeDescriptor> results)
throws NotEnoughReplicasException {
// if no local machine, randomly choose one node 如果没有本地机器，随机选择一个
if (localMachine == null)
return chooseRandom(NodeBase.ROOT, excludedNodes,
blocksize, maxNodesPerRack, results);
if (preferLocalNode) { // 这个值写死为true
// otherwise try local machine first
Node oldNode = excludedNodes.put(localMachine, localMachine); // 把当前节点先加入排除列表
if (oldNode == null) { // was not in the excluded list 如果返回null表示之前没有在排除列表里
if (isGoodTarget(localMachine, blocksize,
maxNodesPerRack, false, results)) {//判断是否好的目标，后面再看这个方法
results.add(localMachine);
return localMachine;
}
}
}
// try a node on local rack
return chooseLocalRack(localMachine, excludedNodes,
blocksize, maxNodesPerRack, results);// 如果本机不适合，选择同机架的其他机器
}继续看chooseLocalRack
/* choose one node from the rack that <i>localMachine</i> is on.
* if no such node is available, choose one node from the rack where
* a second replica is on.
* if still no such node is available, choose a random node
* in the cluster.
* @return the chosen node
*/
private DatanodeDescriptor chooseLocalRack(
DatanodeDescriptor localMachine,
HashMap<Node, Node> excludedNodes,
long blocksize,
int maxNodesPerRack,
List<DatanodeDescriptor> results)
throws NotEnoughReplicasException {
// no local machine, so choose a random machine
if (localMachine == null) {
return chooseRandom(NodeBase.ROOT, excludedNodes,
blocksize, maxNodesPerRack, results);
}
// choose one from the local rack 选择同一个机架的
try {
return chooseRandom(
localMachine.getNetworkLocation(),
excludedNodes, blocksize, maxNodesPerRack, results);
} catch (NotEnoughReplicasException e1) {
// find the second replica 从已有副本里面找同一个机架的
DatanodeDescriptor newLocal=null;
for(Iterator<DatanodeDescriptor> iter=results.iterator();
iter.hasNext();) {
DatanodeDescriptor nextNode = iter.next();
if (nextNode != localMachine) {
newLocal = nextNode;
break;
}
}
if (newLocal != null) {
try {
return chooseRandom(
newLocal.getNetworkLocation(),
excludedNodes, blocksize, maxNodesPerRack, results);
} catch(NotEnoughReplicasException e2) {
//otherwise randomly choose one from the network 如果还是找不到，随机选择
return chooseRandom(NodeBase.ROOT, excludedNodes,
blocksize, maxNodesPerRack, results);
}
} else {
//otherwise randomly choose one from the network
return chooseRandom(NodeBase.ROOT, excludedNodes,
blocksize, maxNodesPerRack, results);
}
}
}继续chooseRemoteRack
/* choose <i>numOfReplicas</i> nodes from the racks
* that <i>localMachine</i> is NOT on.
* if not enough nodes are available, choose the remaining ones
* from the local rack
*/
private void chooseRemoteRack(int numOfReplicas,
DatanodeDescriptor localMachine,
HashMap<Node, Node> excludedNodes,
long blocksize,
int maxReplicasPerRack,
List<DatanodeDescriptor> results)
throws NotEnoughReplicasException {
int oldNumOfReplicas = results.size();
// randomly choose one node from remote racks 选择其他机架的节点，注意这里的"~"是表示排除本机架，如果找不到就只有在本机架找
try {
chooseRandom(numOfReplicas, "~"+localMachine.getNetworkLocation(),
excludedNodes, blocksize, maxReplicasPerRack, results);
} catch (NotEnoughReplicasException e) {
chooseRandom(numOfReplicas-(results.size()-oldNumOfReplicas),
localMachine.getNetworkLocation(), excludedNodes, blocksize,
maxReplicasPerRack, results);
}
}
最后一个 chooseRandom
/* Randomly choose <i>numOfReplicas</i> targets from <i>nodes</i>.
*/
private void chooseRandom(int numOfReplicas,
String nodes,
HashMap<Node, Node> excludedNodes,
long blocksize,
int maxNodesPerRack,
List<DatanodeDescriptor> results)
throws NotEnoughReplicasException {
int numOfAvailableNodes =
clusterMap.countNumOfAvailableNodes(nodes, excludedNodes.keySet()); // 当前可用节点数
StringBuilder builder = null;
if (LOG.isDebugEnabled()) {
builder = threadLocalBuilder.get();
builder.setLength(0);
builder.append("[");
}
boolean badTarget = false;
while(numOfReplicas > 0 && numOfAvailableNodes > 0) { // 循环直到所有可用副本数为0
DatanodeDescriptor chosenNode =
(DatanodeDescriptor)(clusterMap.chooseRandom(nodes));
Node oldNode = excludedNodes.put(chosenNode, chosenNode);
if (oldNode == null) {
numOfAvailableNodes--;
if (isGoodTarget(chosenNode, blocksize, maxNodesPerRack, results)) {
numOfReplicas--;
results.add(chosenNode);
} else {
badTarget = true;
}
}
}
if (numOfReplicas>0) {
String detail = enableDebugLogging;
if (LOG.isDebugEnabled()) {
if (badTarget && builder != null) {
detail = builder.append("]").toString();
builder.setLength(0);
} else detail = "";
}
throw new NotEnoughReplicasException(detail);
}
}最后来看如何判断节点是否可用，isGoodTarget
/* judge if a node is a good target.
* return true if <i>node</i> has enough space,
* does not have too much load, and the rack does not have too many nodes
*/
private boolean isGoodTarget(DatanodeDescriptor node,
long blockSize, int maxTargetPerLoc,
List<DatanodeDescriptor> results) {
return isGoodTarget(node, blockSize, maxTargetPerLoc,
this.considerLoad, results);
}
private boolean isGoodTarget(DatanodeDescriptor node,
long blockSize, int maxTargetPerLoc,
boolean considerLoad,
List<DatanodeDescriptor> results) {
// check if the node is (being) decommissed 判断是否退役了
if (node.isDecommissionInProgress() ¦¦ node.isDecommissioned()) {
if(LOG.isDebugEnabled()) {
threadLocalBuilder.get().append(node.toString()).append(": ")
.append("Node ").append(NodeBase.getPath(node))
.append(" is not chosen because the node is (being) decommissioned ");
}
return false;
}
long remaining = node.getRemaining() -
(node.getBlocksScheduled() * blockSize);
// check the remaining capacity of the target machine 判断剩余的磁盘空间， 块大小 * 5 > 剩余大小 表示不可用
if (blockSize* HdfsConstants.MIN_BLOCKS_FOR_WRITE>remaining) {
if(LOG.isDebugEnabled()) {
threadLocalBuilder.get().append(node.toString()).append(": ")
.append("Node ").append(NodeBase.getPath(node))
.append(" is not chosen because the node does not have enough space ");
}
return false;
}
// check the communication traffic of the target machine 判断各个datanode的负载情况，这里的负载不是系统负载，而是根据xceiver线程数，这个线程好像是负责读写的。
if (considerLoad) { // 默认这个为true，配置项 dfs.namenode.replication.considerLoad
double avgLoad = 0;
int size = clusterMap.getNumOfLeaves(); // 所有datanode数
if (size != 0 && stats != null) {
avgLoad = (double)stats.getTotalLoad()/size; // 获取总的xceiver线程数除以所有datanode数，算出平均负载
}
if (node.getXceiverCount() > (2.0 * avgLoad)) { // 如果当前线程数 大于 平均值的两倍，则不可使用
if(LOG.isDebugEnabled()) {
threadLocalBuilder.get().append(node.toString()).append(": ")
.append("Node ").append(NodeBase.getPath(node))
.append(" is not chosen because the node is too busy ");
}
return false;
}
}
// check if the target rack has chosen too many nodes 检查是否有太多节点在当前机架上
String rackname = node.getNetworkLocation();
int counter=1;
for(Iterator<DatanodeDescriptor> iter = results.iterator();
iter.hasNext();) {
Node result = iter.next();
if (rackname.equals(result.getNetworkLocation())) {
counter++;
}
}
//maxNodesPerRack的算法，没看懂，可能是每个机架的平均数+2，不知道为什么定一个这样的值 int maxNodesPerRack = (totalNumOfReplicas-1)/clusterMap.getNumOfRacks()+2;
if (counter>maxTargetPerLoc) {
if(LOG.isDebugEnabled()) {
threadLocalBuilder.get().append(node.toString()).append(": ")
.append("Node ").append(NodeBase.getPath(node))
.append(" is not chosen because the rack has too many chosen nodes ");
}
return false;
}
return true;
}我们当前集群状态，总共11台datanode，没有退役机器。其中7台机器磁盘已满或者基本满，剩下4台机器可写。没有划分机架，所以整个集群认为只有一个机架。所以应该不会触发“同一个机架不能有太多个节点的问题”（具体原因看上面代码，这里表述可能不是很准确）
怀疑检查负载的逻辑，7台满的机器datanode负载可能很低，但因为磁盘满不可以使用，导致剩下的4台机器线程数超过平均值的两倍。
当前只能加入日志，等下次出错后检查。