最近工作需要，看了HDFS读写数据块这部分。不过可能跟网上大部分帖子不一样，本文主要写了${dfs.data.dir}的选择策略，也就是block在DataNode上的放置策略。我主要是从我们工作需要的角度来读这部分代码的。
1 hdfs-site.xml 2 <property> 3 <name>dfs.data.dir</name> 4 <value>/mnt/datadir1/data,/mnt/datadir2/data,/mnt/datadir3/data</value> 5 </property>
所谓${dfs.data.dir}的选择策略，就是当DataNode配置有多个${dfs.data.dir}目录时(如上面的配置)，该选择哪个目录来存放block。一般多个硬盘分别挂载到不同的${dfs.data.dir}下，所以存储block是要决定block该放到哪个磁盘上。
创建文件总共有两步：
1、在写block之前，需要与NameNode通信来生成文件(INodeFile、INodeFileUnderConstruction)。首先在DFSClient端的create()方法中发起写请求，然后通过RPC由NameNode最终调用FSNameSystem的startFileInternal()方法来创建文件。
1 private void startFileInternal(String src,
2 PermissionStatus permissions,
3 String holder,
4 String clientMachine,
5 boolean overwrite,
6 boolean append,
7 boolean createParent,
8 short replication,
9 long blockSize
10 ) throws IOException {
11 if (NameNode.stateChangeLog.isDebugEnabled()) {
12 NameNode.stateChangeLog.debug("DIR* startFile: src=" + src
13 + ", holder=" + holder
14 + ", clientMachine=" + clientMachine
15 + ", createParent=" + createParent
16 + ", replication=" + replication
17 + ", overwrite=" + overwrite
18 + ", append=" + append);
19 }
20
21 FSPermissionChecker pc = getPermissionChecker();
22 synchronized (this) {
23 if (isInSafeMode())
24 throw new SafeModeException("Cannot create " + src, safeMode);
25 if (!DFSUtil.isValidName(src)) {
26 throw new IOException("Invalid name: " + src);
27 }
28
29 // Verify that the destination does not exist as a directory already.
30 boolean pathExists = dir.exists(src);
31 if (pathExists && dir.isDir(src)) {
32 throw new IOException("Cannot create "+ src + "; already exists as a directory");
33 }
34
35 if (isPermissionEnabled) {
36 if (append || (overwrite && pathExists)) {
37 checkPathAccess(pc, src, FsAction.WRITE);
38 } else {
39 checkAncestorAccess(pc, src, FsAction.WRITE);
40 }
41 }
42
43 if (!createParent) {
44 verifyParentDir(src);
45 }
46
47 try {
48 INode myFile = dir.getFileINode(src); //根据路径寻找该文件
49 recoverLeaseInternal(myFile, src, holder, clientMachine, false);
50
51 try {
52 verifyReplication(src, replication, clientMachine);
53 } catch (IOException e) {
54 throw new IOException("failed to create " + e.getMessage());
55 }
56 if (append) {//若是追加操作
57 if (myFile == null) {
58 throw new FileNotFoundException("failed to append to non-existent "
59 + src + " on client " + clientMachine);
60 } else if (myFile.isDirectory()) {
61 throw new IOException("failed to append to directory " + src
62 + " on client " + clientMachine);
63 }
64 } else if (!dir.isValidToCreate(src)) {
65 if (overwrite) {//允许覆盖原来的文件
66 delete(src, true);
67 } else {
68 throw new IOException("failed to create file " + src
69 + " on client " + clientMachine
70 + " either because the filename is invalid or the file exists");
71 }
72 }
73
74 DatanodeDescriptor clientNode = host2DataNodeMap
75 .getDatanodeByHost(clientMachine);
76
77 if (append) {
78 //
79 // Replace current node with a INodeUnderConstruction.
80 // Recreate in-memory lease record.
81 //
82 INodeFile node = (INodeFile) myFile;
83 INodeFileUnderConstruction cons = new INodeFileUnderConstruction(
84 node.getLocalNameBytes(), node.getReplication(),
85 node.getModificationTime(), node.getPreferredBlockSize(),
86 node.getBlocks(), node.getPermissionStatus(), holder,
87 clientMachine, clientNode);
88 dir.replaceNode(src, node, cons);
89 leaseManager.addLease(cons.clientName, src);
90
91 } else {
92 // Now we can add the name to the filesystem. This file has no
93 // blocks associated with it.
94 //
95 checkFsObjectLimit();
96
97 // increment global generation stamp
98 long genstamp = nextGenerationStamp();
99 INodeFileUnderConstruction newNode = dir.addFile(src, permissions,
100 replication, blockSize, holder, clientMachine, clientNode,
101 genstamp);
102 if (newNode == null) {
103 throw new IOException("DIR* startFile: Unable to add to namespace");
104 }
105 leaseManager.addLease(newNode.clientName, src);
106 if (NameNode.stateChangeLog.isDebugEnabled()) {
107 NameNode.stateChangeLog.debug("DIR* startFile: "
108 +"add "+src+" to namespace for "+holder);
109 }
110 }
111 } catch (IOException ie) {
112 NameNode.stateChangeLog.warn("DIR* startFile: "
113 +ie.getMessage());
114 throw ie;
115 }
116 }
117 }
startFileInternal()
该方法的主要内容如下：
1）首先做一些检查(安全模式、权限、该路径是否已经以文件夹形式存在等)
2）若不是追加操作：
生成generation stamp(针对每个文件生成一个)；并构造INodeFileUnderConstruction对象(preferredBlockSize)；将这个文件添加到filesystem；添加租约(即有时间限制的写锁)；
若是追加操作：
将src下的INodeFile替换成INodeFileUnderConstruction；添加租约；
2、在NameNode端生成文件之后，client向NameNode申请block，并将其写入到DataNode。在上面的工作完成后，就启动DataStreamer线程来向DataNode中写入block。整个流程如下：
1）一些前期检查
2）向NameNode申请block(与NameNode有一次通信)
a. 根据副本放置策略，选择N个DataNode作为block的放置位置；
b. 随机生成一个不重复的blockID；
c. 把该block添加到对应的文件；
3）将目标DN组织成pipeline，并向第一个DN发送Packet
选择其中几个比较重要的方法分析下：
1 /**
2 * The client would like to obtain an additional block for the indicated
3 * filename (which is being written-to). Return an array that consists
4 * of the block, plus a set of machines. The first on this list should
5 * be where the client writes data. Subsequent items in the list must
6 * be provided in the connection to the first datanode.
7 *
8 * Make sure the previous blocks have been reported by datanodes and
9 * are replicated. Will return an empty 2-elt array if we want the
10 * client to "try again later".
11 */
12 //向NameNode申请block
13 public LocatedBlock getAdditionalBlock(String src,
14 String clientName,
15 HashMap<Node, Node> excludedNodes
16 ) throws IOException {
17 long fileLength, blockSize;
18 int replication;
19 DatanodeDescriptor clientNode = null;
20 Block newBlock = null;
21
22 NameNode.stateChangeLog.debug("BLOCK* getAdditionalBlock: "
23 +src+" for "+clientName);
24
25 synchronized (this) {
26 if (isInSafeMode()) {//check safemode first for failing-fast
27 throw new SafeModeException("Cannot add block to " + src, safeMode);
28 }
29 // have we exceeded the configured limit of fs objects.
30 checkFsObjectLimit();
31
32 INodeFileUnderConstruction pendingFile = checkLease(src, clientName);
33
34 //
35 // If we fail this, bad things happen!
36 //
37 if (!checkFileProgress(pendingFile, false)) {
38 throw new NotReplicatedYetException("Not replicated yet:" + src);
39 }
40 fileLength = pendingFile.computeContentSummary().getLength();
41 blockSize = pendingFile.getPreferredBlockSize();
42 clientNode = pendingFile.getClientNode();
43 replication = (int)pendingFile.getReplication();
44 }
45
46 // choose targets for the new block to be allocated.
47 //选择副本存放的位置
48 DatanodeDescriptor targets[] = replicator.chooseTarget(src,
49 replication,
50 clientNode,
51 excludedNodes,
52 blockSize);
53 if (targets.length < this.minReplication) {
54 throw new IOException("File " + src + " could only be replicated to " +
55 targets.length + " nodes, instead of " +
56 minReplication);
57 }
58
59 // Allocate a new block and record it in the INode.
60 synchronized (this) {
61 if (isInSafeMode()) { //make sure it is not in safemode again.
62 throw new SafeModeException("Cannot add block to " + src, safeMode);
63 }
64 INode[] pathINodes = dir.getExistingPathINodes(src);
65 int inodesLen = pathINodes.length;
66 checkLease(src, clientName, pathINodes[inodesLen-1]);
67 INodeFileUnderConstruction pendingFile = (INodeFileUnderConstruction)
68 pathINodes[inodesLen - 1];
69
70 if (!checkFileProgress(pendingFile, false)) {
71 throw new NotReplicatedYetException("Not replicated yet:" + src);
72 }
73
74 // allocate new block record block locations in INode.
75 //分配block，并随机生成一个不重复的blockID，然后在INode中记录该block
76 newBlock = allocateBlock(src, pathINodes);
77 pendingFile.setTargets(targets);
78
79 for (DatanodeDescriptor dn : targets) {
80 dn.incBlocksScheduled();
81 }
82 dir.persistBlocks(src, pendingFile);
83 }
84 if (persistBlocks) {
85 getEditLog().logSync();
86 }
87
88 // Create next block
89 LocatedBlock b = new LocatedBlock(newBlock, targets, fileLength);
90 if (isAccessTokenEnabled) {
91 b.setBlockToken(accessTokenHandler.generateToken(b.getBlock(),
92 EnumSet.of(BlockTokenSecretManager.AccessMode.WRITE)));
93 }
94 return b;
95 }
getAdditionalBlock
上面的方法还涉及到了块的选择策略，这个留在下一篇再说。下面这个图来总结下上面方法的调用层次：
最后重点说一下block在DataNode上的存储策略。其调度层次如下：
首先说一下其中涉及到的数据结构：
1 class FSVolume { //卷信息,代表${dfs.data.dir}
2 private File currentDir; //存放block，即${dfs.data.dir}/current
3 private FSDir dataDir; //表示currentDir有哪些块文件
4 private File tmpDir; //存放一些临时文件，即${dfs.data.dir}/tmp
5 private File blocksBeingWritten; //放置正在写的block，即${dfs.data.dir}/ blocksBeingWritten
6 private File detachDir; //是否写分离，即${dfs.data.dir}/detach
7 private DF usage;
8 private DU dfsUsage;
9 private long reserved;
1 static class FSVolumeSet { //卷信息集合,代表多个${dfs.data.dir}
2 FSVolume[] volumes = null; //代表多个FSVolume，并将其组织成一个数组
3 int curVolume = 0; //指示当前正在使用哪一个FSVolume
FSVolumeSet 代表多个${dfs.data.dir}目录的集合，它将这些目录组织成一个数组volumes，然后用curVolume来指示当前正在使用的是哪个${dfs.data.dir}目录。${dfs.data.dir}的选择策略如下：
当有多个${dfs.data.dir}时，DataNode顺序地从volumes选择一个FSVolume用来存放block(先放在blocksBeingWritten目录下，写完后再转移到current目录下)；每次写完一个block， curVolume增1。以此实现多个${dfs.data.dir}目录的轮流写。该策略在FSDataSet.FSVolumeSet的getNextVolume()方法中实现
1 synchronized FSVolume getNextVolume(long blockSize) throws IOException {
2
3 if(volumes.length < 1) {
4 throw new DiskOutOfSpaceException("No more available volumes");
5 }
6
7 // since volumes could‘ve been removed because of the failure
8 // make sure we are not out of bounds
9 if(curVolume >= volumes.length) {
10 curVolume = 0;
11 }
12
13 int startVolume = curVolume;
14
15 while (true) {
16 FSVolume volume = volumes[curVolume];
17 curVolume = (curVolume + 1) % volumes.length; //增1
18 if (volume.getAvailable() > blockSize) { return volume; }
19 if (curVolume == startVolume) {
20 throw new DiskOutOfSpaceException("Insufficient space for an additional block");
21 }
22 }
23 }
本文基于hadoop1.2.1
如有错误，还请指正
参考文章：《Hadoop技术内幕 深入理解MapReduce架构设计与实现原理》 董西成
转载请注明出处：
时间： 2024-10-25 20:06:16