为了让存储目录的子目录存储的数据块达到负载均衡，FSDir使用了lastChildIdx来循环的选择子目录存储数据块。
[java]
public File addBlock(Block b, File src) throws IOException { //First try without creating subdirectories File file = addBlock(b, src, false, false); return (file != null) ? file : addBlock(b, src, true, true); } private File addBlock(Block b, File src, boolean createOk, boolean resetIdx) throws IOException { //直接将数据块保存到当前主目录下 if (numBlocks < maxBlocksPerDir) { File dest = new File(dir, b.getBlockName()); File metaData = getMetaFile( src, b ); File newmeta = getMetaFile(dest, b); if ( ! metaData.renameTo( newmeta ) || ! src.renameTo( dest ) ) { throw new IOException( "could not move files for " + b + " from tmp to " + dest.getAbsolutePath() ); } if (DataNode.LOG.isDebugEnabled()) { DataNode.LOG.debug("addBlock: Moved " + metaData + " to " + newmeta); DataNode.LOG.debug("addBlock: Moved " + src + " to " + dest); } //当前主目录存储的数据块加1 numBlocks += 1; return dest; } if (lastChildIdx < 0 && resetIdx) { //reset so that all children will be checked lastChildIdx = random.nextInt(children.length); } if (lastChildIdx >= 0 && children != null) { //Check if any child-tree has room for a block. for (int i=0; i < children.length; i++) { int idx = (lastChildIdx + i)%children.length; File file = children[idx].addBlock(b, src, false, resetIdx); if (file != null) { lastChildIdx = idx; return file; } } lastChildIdx = -1; } if (!createOk) { return null; } //在当前主目录下创建若干个子目录来存储数据块 <pre name="code" class="java"> if (children == null || children.length == 0) { children = new FSDir[maxBlocksPerDir]; for (int idx = 0; idx < maxBlocksPerDir; idx++) { children[idx] = new FSDir(new File(dir, DataStorage.BLOCK_SUBDIR_PREFIX+idx)); } } //now pick a child randomly for creating a new set of subdirs. lastChildIdx = random.nextInt(children.length); return children[ lastChildIdx ].addBlock(b, src, true, false); }2.从数据块对应的元数据文件名中解析文件的时间戳