long getDfsUsed() throws IOException {
   return dfsUsage.getUsed();
 }
 File addBlock(Block b, File f) throws IOException {
   File blockFile = dataDir.addBlock(b, f);
   File metaFile = getMetaFile(blockFile, b);
   dfsUsage.incDfsUsed(b.getNumBytes() + metaFile.length());
   return blockFile;
 }
 void decDfsUsed(long value) {
   dfsUsage.decDfsUsed(value);
 }