src/java/org/apache/hadoop/hdfs/BlockReader.java - hadoop-hdfs - Git at Google

 /**
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements.  See the NOTICE file
  * distributed with this work for additional information
  * regarding copyright ownership.  The ASF licenses this file
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
  *
  *     http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 package org.apache.hadoop.hdfs;

 import static org.apache.hadoop.hdfs.protocol.DataTransferProtocol.Status.CHECKSUM_OK;
 import static org.apache.hadoop.hdfs.protocol.DataTransferProtocol.Status.ERROR_ACCESS_TOKEN;
 import static org.apache.hadoop.hdfs.protocol.DataTransferProtocol.Status.SUCCESS;

 import java.io.BufferedInputStream;
 import java.io.BufferedOutputStream;
 import java.io.DataInputStream;
 import java.io.DataOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
 import java.net.InetSocketAddress;
 import java.net.Socket;
 import java.nio.ByteBuffer;

 import org.apache.hadoop.classification.InterfaceAudience;
 import org.apache.hadoop.fs.FSInputChecker;
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.hdfs.protocol.DataTransferProtocol;
 import org.apache.hadoop.hdfs.protocol.ExtendedBlock;
 import org.apache.hadoop.hdfs.protocol.DataTransferProtocol.PacketHeader;
 import org.apache.hadoop.hdfs.security.token.block.BlockTokenIdentifier;
 import org.apache.hadoop.hdfs.security.token.block.InvalidBlockTokenException;
 import org.apache.hadoop.hdfs.server.common.HdfsConstants;
 import org.apache.hadoop.io.IOUtils;
 import org.apache.hadoop.net.NetUtils;
 import org.apache.hadoop.security.token.Token;
 import org.apache.hadoop.util.DataChecksum;

 /** This is a wrapper around connection to datanode
  * and understands checksum, offset etc.
  *
  * Terminology:
  * <dl>
  * <dt>block</dt>
  *   <dd>The hdfs block, typically large (~64MB).
  *   </dd>
  * <dt>chunk</dt>
  *   <dd>A block is divided into chunks, each comes with a checksum.
  *       We want transfers to be chunk-aligned, to be able to
  *       verify checksums.
  *   </dd>
  * <dt>packet</dt>
  *   <dd>A grouping of chunks used for transport. It contains a
  *       header, followed by checksum data, followed by real data.
  *   </dd>
  * </dl>
  * Please see DataNode for the RPC specification.
  */
 @InterfaceAudience.Private
 public class BlockReader extends FSInputChecker {

   Socket dnSock; //for now just sending the status code (e.g. checksumOk) after the read.
   private DataInputStream in;
   private DataChecksum checksum;

   /** offset in block of the last chunk received */
   private long lastChunkOffset = -1;
   private long lastChunkLen = -1;
   private long lastSeqNo = -1;

   /** offset in block where reader wants to actually read */
   private long startOffset;

   /** offset in block of of first chunk - may be less than startOffset
       if startOffset is not chunk-aligned */
   private final long firstChunkOffset;

   private int bytesPerChecksum;
   private int checksumSize;

   /**
    * The total number of bytes we need to transfer from the DN.
    * This is the amount that the user has requested plus some padding
    * at the beginning so that the read can begin on a chunk boundary.
    */
   private final long bytesNeededToFinish;

   private boolean eos = false;
   private boolean sentStatusCode = false;

   byte[] skipBuf = null;
   ByteBuffer checksumBytes = null;
   /** Amount of unread data in the current received packet */
   int dataLeft = 0;

   /* FSInputChecker interface */

   /* same interface as inputStream java.io.InputStream#read()
    * used by DFSInputStream#read()
    * This violates one rule when there is a checksum error:
    * "Read should not modify user buffer before successful read"
    * because it first reads the data to user buffer and then checks
    * the checksum.
    */
   @Override
   public synchronized int read(byte[] buf, int off, int len)
                                throws IOException {

     // This has to be set here, *before* the skip, since we can
     // hit EOS during the skip, in the case that our entire read
     // is smaller than the checksum chunk.
     boolean eosBefore = eos;

     //for the first read, skip the extra bytes at the front.
     if (lastChunkLen < 0 && startOffset > firstChunkOffset && len > 0) {
       // Skip these bytes. But don't call this.skip()!
       int toSkip = (int)(startOffset - firstChunkOffset);
       if ( skipBuf == null ) {
         skipBuf = new byte[bytesPerChecksum];
       }
       if ( super.read(skipBuf, 0, toSkip) != toSkip ) {
         // should never happen
         throw new IOException("Could not skip required number of bytes");
       }
     }

     int nRead = super.read(buf, off, len);

     // if eos was set in the previous read, send a status code to the DN
     if (eos && !eosBefore && nRead >= 0) {
       if (needChecksum()) {
         sendReadResult(dnSock, CHECKSUM_OK);
       } else {
         sendReadResult(dnSock, SUCCESS);
       }
     }
     return nRead;
   }

   @Override
   public synchronized long skip(long n) throws IOException {
     /* How can we make sure we don't throw a ChecksumException, at least
      * in majority of the cases?. This one throws. */
     if ( skipBuf == null ) {
       skipBuf = new byte[bytesPerChecksum];
     }

     long nSkipped = 0;
     while ( nSkipped < n ) {
       int toSkip = (int)Math.min(n-nSkipped, skipBuf.length);
       int ret = read(skipBuf, 0, toSkip);
       if ( ret <= 0 ) {
         return nSkipped;
       }
       nSkipped += ret;
     }
     return nSkipped;
   }

   @Override
   public int read() throws IOException {
     throw new IOException("read() is not expected to be invoked. " +
                           "Use read(buf, off, len) instead.");
   }

   @Override
   public boolean seekToNewSource(long targetPos) throws IOException {
     /* Checksum errors are handled outside the BlockReader.
      * DFSInputStream does not always call 'seekToNewSource'. In the
      * case of pread(), it just tries a different replica without seeking.
      */
     return false;
   }

   @Override
   public void seek(long pos) throws IOException {
     throw new IOException("Seek() is not supported in BlockInputChecker");
   }

   @Override
   protected long getChunkPosition(long pos) {
     throw new RuntimeException("getChunkPosition() is not supported, " +
                                "since seek is not required");
   }

   /**
    * Makes sure that checksumBytes has enough capacity
    * and limit is set to the number of checksum bytes needed
    * to be read.
    */
   private void adjustChecksumBytes(int dataLen) {
     int requiredSize =
       ((dataLen + bytesPerChecksum - 1)/bytesPerChecksum)*checksumSize;
     if (checksumBytes == null || requiredSize > checksumBytes.capacity()) {
       checksumBytes =  ByteBuffer.wrap(new byte[requiredSize]);
     } else {
       checksumBytes.clear();
     }
     checksumBytes.limit(requiredSize);
   }

   @Override
   protected synchronized int readChunk(long pos, byte[] buf, int offset,
                                        int len, byte[] checksumBuf)
                                        throws IOException {
     // Read one chunk.
     if (eos) {
       // Already hit EOF
       return -1;
     }

     // Read one DATA_CHUNK.
     long chunkOffset = lastChunkOffset;
     if ( lastChunkLen > 0 ) {
       chunkOffset += lastChunkLen;
     }

     // pos is relative to the start of the first chunk of the read.
     // chunkOffset is relative to the start of the block.
     // This makes sure that the read passed from FSInputChecker is the
     // for the same chunk we expect to be reading from the DN.
     if ( (pos + firstChunkOffset) != chunkOffset ) {
       throw new IOException("Mismatch in pos : " + pos + " + " +
                             firstChunkOffset + " != " + chunkOffset);
     }

     // Read next packet if the previous packet has been read completely.
     if (dataLeft <= 0) {
       //Read packet headers.
       PacketHeader header = new PacketHeader();
       header.readFields(in);

       if (LOG.isDebugEnabled()) {
         LOG.debug("DFSClient readChunk got header " + header);
       }

       // Sanity check the lengths
       if (!header.sanityCheck(lastSeqNo)) {
            throw new IOException("BlockReader: error in packet header " +
                                  header);
       }

       lastSeqNo = header.getSeqno();
       dataLeft = header.getDataLen();
       adjustChecksumBytes(header.getDataLen());
       if (header.getDataLen() > 0) {
         IOUtils.readFully(in, checksumBytes.array(), 0,
                           checksumBytes.limit());
       }
     }

     // Sanity checks
     assert len >= bytesPerChecksum;
     assert checksum != null;
     assert checksumSize == 0 || (checksumBuf.length % checksumSize == 0);


     int checksumsToRead, bytesToRead;

     if (checksumSize > 0) {

       // How many chunks left in our packet - this is a ceiling
       // since we may have a partial chunk at the end of the file
       int chunksLeft = (dataLeft - 1) / bytesPerChecksum + 1;

       // How many chunks we can fit in databuffer
       //  - note this is a floor since we always read full chunks
       int chunksCanFit = Math.min(len / bytesPerChecksum,
                                   checksumBuf.length / checksumSize);

       // How many chunks should we read
       checksumsToRead = Math.min(chunksLeft, chunksCanFit);
       // How many bytes should we actually read
       bytesToRead = Math.min(
         checksumsToRead * bytesPerChecksum, // full chunks
         dataLeft); // in case we have a partial
     } else {
       // no checksum
       bytesToRead = Math.min(dataLeft, len);
       checksumsToRead = 0;
     }

     if ( bytesToRead > 0 ) {
       // Assert we have enough space
       assert bytesToRead <= len;
       assert checksumBytes.remaining() >= checksumSize * checksumsToRead;
       assert checksumBuf.length >= checksumSize * checksumsToRead;
       IOUtils.readFully(in, buf, offset, bytesToRead);
       checksumBytes.get(checksumBuf, 0, checksumSize * checksumsToRead);
     }

     dataLeft -= bytesToRead;
     assert dataLeft >= 0;

     lastChunkOffset = chunkOffset;
     lastChunkLen = bytesToRead;

     // If there's no data left in the current packet after satisfying
     // this read, and we have satisfied the client read, we expect
     // an empty packet header from the DN to signify this.
     // Note that pos + bytesToRead may in fact be greater since the
     // DN finishes off the entire last chunk.
     if (dataLeft == 0 &&
         pos + bytesToRead >= bytesNeededToFinish) {

       // Read header
       int packetLen = in.readInt();
       long offsetInBlock = in.readLong();
       long seqno = in.readLong();
       boolean lastPacketInBlock = in.readBoolean();
       int dataLen = in.readInt();

       if (!lastPacketInBlock ||
           dataLen != 0) {
         throw new IOException("Expected empty end-of-read packet! Header: " +
                               "(packetLen : " + packetLen +
                               ", offsetInBlock : " + offsetInBlock +
                               ", seqno : " + seqno +
                               ", lastInBlock : " + lastPacketInBlock +
                               ", dataLen : " + dataLen);
       }

       eos = true;
     }

     if ( bytesToRead == 0 ) {
       return -1;
     }

     return bytesToRead;
   }

   private BlockReader(String file, String bpid, long blockId,
       DataInputStream in, DataChecksum checksum, boolean verifyChecksum,
       long startOffset, long firstChunkOffset, long bytesToRead, Socket dnSock) {
     // Path is used only for printing block and file information in debug
     super(new Path("/blk_" + blockId + ":" + bpid + ":of:"+ file)/*too non path-like?*/,
           1, verifyChecksum,
           checksum.getChecksumSize() > 0? checksum : null,
           checksum.getBytesPerChecksum(),
           checksum.getChecksumSize());

     this.dnSock = dnSock;
     this.in = in;
     this.checksum = checksum;
     this.startOffset = Math.max( startOffset, 0 );

     // The total number of bytes that we need to transfer from the DN is
     // the amount that the user wants (bytesToRead), plus the padding at
     // the beginning in order to chunk-align. Note that the DN may elect
     // to send more than this amount if the read starts/ends mid-chunk.
     this.bytesNeededToFinish = bytesToRead + (startOffset - firstChunkOffset);

     this.firstChunkOffset = firstChunkOffset;
     lastChunkOffset = firstChunkOffset;
     lastChunkLen = -1;

     bytesPerChecksum = this.checksum.getBytesPerChecksum();
     checksumSize = this.checksum.getChecksumSize();
   }

   public static BlockReader newBlockReader(Socket sock, String file,
       ExtendedBlock block, Token<BlockTokenIdentifier> blockToken,
       long startOffset, long len, int bufferSize) throws IOException {
     return newBlockReader(sock, file, block, blockToken, startOffset, len, bufferSize,
         true);
   }

   /** Java Doc required */
   public static BlockReader newBlockReader( Socket sock, String file,
                                      ExtendedBlock block,
                                      Token<BlockTokenIdentifier> blockToken,
                                      long startOffset, long len,
                                      int bufferSize, boolean verifyChecksum)
                                      throws IOException {
     return newBlockReader(sock, file, block, blockToken, startOffset,
                           len, bufferSize, verifyChecksum, "");
   }

   /**
    * Create a new BlockReader specifically to satisfy a read.
    * This method also sends the OP_READ_BLOCK request.
    *
    * @param sock  An established Socket to the DN. The BlockReader will not close it normally
    * @param file  File location
    * @param block  The block object
    * @param blockToken  The block token for security
    * @param startOffset  The read offset, relative to block head
    * @param len  The number of bytes to read
    * @param bufferSize  The IO buffer size (not the client buffer size)
    * @param verifyChecksum  Whether to verify checksum
    * @param clientName  Client name
    * @return New BlockReader instance, or null on error.
    */
   public static BlockReader newBlockReader( Socket sock, String file,
                                      ExtendedBlock block,
                                      Token<BlockTokenIdentifier> blockToken,
                                      long startOffset, long len,
                                      int bufferSize, boolean verifyChecksum,
                                      String clientName)
                                      throws IOException {
     // in and out will be closed when sock is closed (by the caller)
     DataTransferProtocol.Sender.opReadBlock(
         new DataOutputStream(new BufferedOutputStream(
             NetUtils.getOutputStream(sock,HdfsConstants.WRITE_TIMEOUT))),
         block, startOffset, len, clientName, blockToken);

     //
     // Get bytes in block, set streams
     //

     DataInputStream in = new DataInputStream(
         new BufferedInputStream(NetUtils.getInputStream(sock),
                                 bufferSize));

     DataTransferProtocol.Status status = DataTransferProtocol.Status.read(in);
     if (status != SUCCESS) {
       if (status == ERROR_ACCESS_TOKEN) {
         throw new InvalidBlockTokenException(
             "Got access token error for OP_READ_BLOCK, self="
                 + sock.getLocalSocketAddress() + ", remote="
                 + sock.getRemoteSocketAddress() + ", for file " + file
                 + ", for pool " + block.getBlockPoolId() + " block "
                 + block.getBlockId() + "_" + block.getGenerationStamp());
       } else {
         throw new IOException("Got error for OP_READ_BLOCK, self="
             + sock.getLocalSocketAddress() + ", remote="
             + sock.getRemoteSocketAddress() + ", for file " + file
             + ", for pool " + block.getBlockPoolId() + " block "
             + block.getBlockId() + "_" + block.getGenerationStamp());
       }
     }
     DataChecksum checksum = DataChecksum.newDataChecksum( in );
     //Warning when we get CHECKSUM_NULL?

     // Read the first chunk offset.
     long firstChunkOffset = in.readLong();

     if ( firstChunkOffset < 0 || firstChunkOffset > startOffset ||
         firstChunkOffset >= (startOffset + checksum.getBytesPerChecksum())) {
       throw new IOException("BlockReader: error in first chunk offset (" +
                             firstChunkOffset + ") startOffset is " +
                             startOffset + " for file " + file);
     }

     return new BlockReader(file, block.getBlockPoolId(), block.getBlockId(),
         in, checksum, verifyChecksum, startOffset, firstChunkOffset, len, sock);
   }

   @Override
   public synchronized void close() throws IOException {
     startOffset = -1;
     checksum = null;
     if (dnSock != null) {
       dnSock.close();
     }

     // in will be closed when its Socket is closed.
   }

   /** kind of like readFully(). Only reads as much as possible.
    * And allows use of protected readFully().
    */
   public int readAll(byte[] buf, int offset, int len) throws IOException {
     return readFully(this, buf, offset, len);
   }

   /**
    * Take the socket used to talk to the DN.
    */
   public Socket takeSocket() {
     assert hasSentStatusCode() :
       "BlockReader shouldn't give back sockets mid-read";
     Socket res = dnSock;
     dnSock = null;
     return res;
   }

   /**
    * Whether the BlockReader has reached the end of its input stream
    * and successfully sent a status code back to the datanode.
    */
   public boolean hasSentStatusCode() {
     return sentStatusCode;
   }

   /**
    * When the reader reaches end of the read, it sends a status response
    * (e.g. CHECKSUM_OK) to the DN. Failure to do so could lead to the DN
    * closing our connection (which we will re-open), but won't affect
    * data correctness.
    */
   void sendReadResult(Socket sock, DataTransferProtocol.Status statusCode) {
     assert !sentStatusCode : "already sent status code to " + sock;
     try {
       OutputStream out = NetUtils.getOutputStream(sock, HdfsConstants.WRITE_TIMEOUT);
       statusCode.writeOutputStream(out);
       out.flush();
       sentStatusCode = true;
     } catch (IOException e) {
       // It's ok not to be able to send this. But something is probably wrong.
       LOG.info("Could not send read status (" + statusCode + ") to datanode " +
                sock.getInetAddress() + ": " + e.getMessage());
     }
   }

   /**
    * File name to print when accessing a block directly (from servlets)
    * @param s Address of the block location
    * @param poolId Block pool ID of the block
    * @param blockId Block ID of the block
    * @return string that has a file name for debug purposes
    */
   public static String getFileName(final InetSocketAddress s,
       final String poolId, final long blockId) {
     return s.toString() + ":" + poolId + ":" + blockId;
   }
 }
	/**
	* Licensed to the Apache Software Foundation (ASF) under one
	* or more contributor license agreements. See the NOTICE file
	* distributed with this work for additional information
	* regarding copyright ownership. The ASF licenses this file
	* to you under the Apache License, Version 2.0 (the
	* "License"); you may not use this file except in compliance
	* with the License. You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/
	package org.apache.hadoop.hdfs;

	import static org.apache.hadoop.hdfs.protocol.DataTransferProtocol.Status.CHECKSUM_OK;
	import static org.apache.hadoop.hdfs.protocol.DataTransferProtocol.Status.ERROR_ACCESS_TOKEN;
	import static org.apache.hadoop.hdfs.protocol.DataTransferProtocol.Status.SUCCESS;

	import java.io.BufferedInputStream;
	import java.io.BufferedOutputStream;
	import java.io.DataInputStream;
	import java.io.DataOutputStream;
	import java.io.IOException;
	import java.io.OutputStream;
	import java.net.InetSocketAddress;
	import java.net.Socket;
	import java.nio.ByteBuffer;

	import org.apache.hadoop.classification.InterfaceAudience;
	import org.apache.hadoop.fs.FSInputChecker;
	import org.apache.hadoop.fs.Path;
	import org.apache.hadoop.hdfs.protocol.DataTransferProtocol;
	import org.apache.hadoop.hdfs.protocol.ExtendedBlock;
	import org.apache.hadoop.hdfs.protocol.DataTransferProtocol.PacketHeader;
	import org.apache.hadoop.hdfs.security.token.block.BlockTokenIdentifier;
	import org.apache.hadoop.hdfs.security.token.block.InvalidBlockTokenException;
	import org.apache.hadoop.hdfs.server.common.HdfsConstants;
	import org.apache.hadoop.io.IOUtils;
	import org.apache.hadoop.net.NetUtils;
	import org.apache.hadoop.security.token.Token;
	import org.apache.hadoop.util.DataChecksum;

	/** This is a wrapper around connection to datanode
	* and understands checksum, offset etc.
	*
	* Terminology:
	* <dl>
	* <dt>block</dt>
	* <dd>The hdfs block, typically large (~64MB).
	* </dd>
	* <dt>chunk</dt>
	* <dd>A block is divided into chunks, each comes with a checksum.
	* We want transfers to be chunk-aligned, to be able to
	* verify checksums.
	* </dd>
	* <dt>packet</dt>
	* <dd>A grouping of chunks used for transport. It contains a
	* header, followed by checksum data, followed by real data.
	* </dd>
	* </dl>
	* Please see DataNode for the RPC specification.
	*/
	@InterfaceAudience.Private
	public class BlockReader extends FSInputChecker {

	Socket dnSock; //for now just sending the status code (e.g. checksumOk) after the read.
	private DataInputStream in;
	private DataChecksum checksum;

	/** offset in block of the last chunk received */
	private long lastChunkOffset = -1;
	private long lastChunkLen = -1;
	private long lastSeqNo = -1;

	/** offset in block where reader wants to actually read */
	private long startOffset;

	/** offset in block of of first chunk - may be less than startOffset
	if startOffset is not chunk-aligned */
	private final long firstChunkOffset;

	private int bytesPerChecksum;
	private int checksumSize;

	/**
	* The total number of bytes we need to transfer from the DN.
	* This is the amount that the user has requested plus some padding
	* at the beginning so that the read can begin on a chunk boundary.
	*/
	private final long bytesNeededToFinish;

	private boolean eos = false;
	private boolean sentStatusCode = false;

	byte[] skipBuf = null;
	ByteBuffer checksumBytes = null;
	/** Amount of unread data in the current received packet */
	int dataLeft = 0;

	/* FSInputChecker interface */

	/* same interface as inputStream java.io.InputStream#read()
	* used by DFSInputStream#read()
	* This violates one rule when there is a checksum error:
	* "Read should not modify user buffer before successful read"
	* because it first reads the data to user buffer and then checks
	* the checksum.
	*/
	@Override
	public synchronized int read(byte[] buf, int off, int len)
	throws IOException {

	// This has to be set here, before the skip, since we can
	// hit EOS during the skip, in the case that our entire read
	// is smaller than the checksum chunk.
	boolean eosBefore = eos;

	//for the first read, skip the extra bytes at the front.
	if (lastChunkLen < 0 && startOffset > firstChunkOffset && len > 0) {
	// Skip these bytes. But don't call this.skip()!
	int toSkip = (int)(startOffset - firstChunkOffset);
	if ( skipBuf == null ) {
	skipBuf = new byte[bytesPerChecksum];
	}
	if ( super.read(skipBuf, 0, toSkip) != toSkip ) {
	// should never happen
	throw new IOException("Could not skip required number of bytes");
	}
	}

	int nRead = super.read(buf, off, len);

	// if eos was set in the previous read, send a status code to the DN
	if (eos && !eosBefore && nRead >= 0) {
	if (needChecksum()) {
	sendReadResult(dnSock, CHECKSUM_OK);
	} else {
	sendReadResult(dnSock, SUCCESS);
	}
	}
	return nRead;
	}

	@Override
	public synchronized long skip(long n) throws IOException {
	/* How can we make sure we don't throw a ChecksumException, at least
	* in majority of the cases?. This one throws. */
	if ( skipBuf == null ) {
	skipBuf = new byte[bytesPerChecksum];
	}

	long nSkipped = 0;
	while ( nSkipped < n ) {
	int toSkip = (int)Math.min(n-nSkipped, skipBuf.length);
	int ret = read(skipBuf, 0, toSkip);
	if ( ret <= 0 ) {
	return nSkipped;
	}
	nSkipped += ret;
	}
	return nSkipped;
	}

	@Override
	public int read() throws IOException {
	throw new IOException("read() is not expected to be invoked. " +
	"Use read(buf, off, len) instead.");
	}

	@Override
	public boolean seekToNewSource(long targetPos) throws IOException {
	/* Checksum errors are handled outside the BlockReader.
	* DFSInputStream does not always call 'seekToNewSource'. In the
	* case of pread(), it just tries a different replica without seeking.
	*/
	return false;
	}

	@Override
	public void seek(long pos) throws IOException {
	throw new IOException("Seek() is not supported in BlockInputChecker");
	}

	@Override
	protected long getChunkPosition(long pos) {
	throw new RuntimeException("getChunkPosition() is not supported, " +
	"since seek is not required");
	}

	/**
	* Makes sure that checksumBytes has enough capacity
	* and limit is set to the number of checksum bytes needed
	* to be read.
	*/
	private void adjustChecksumBytes(int dataLen) {
	int requiredSize =
	((dataLen + bytesPerChecksum - 1)/bytesPerChecksum)*checksumSize;
	if (checksumBytes == null \|\| requiredSize > checksumBytes.capacity()) {
	checksumBytes = ByteBuffer.wrap(new byte[requiredSize]);
	} else {
	checksumBytes.clear();
	}
	checksumBytes.limit(requiredSize);
	}

	@Override
	protected synchronized int readChunk(long pos, byte[] buf, int offset,
	int len, byte[] checksumBuf)
	throws IOException {
	// Read one chunk.
	if (eos) {
	// Already hit EOF
	return -1;
	}

	// Read one DATA_CHUNK.
	long chunkOffset = lastChunkOffset;
	if ( lastChunkLen > 0 ) {
	chunkOffset += lastChunkLen;
	}

	// pos is relative to the start of the first chunk of the read.
	// chunkOffset is relative to the start of the block.
	// This makes sure that the read passed from FSInputChecker is the
	// for the same chunk we expect to be reading from the DN.
	if ( (pos + firstChunkOffset) != chunkOffset ) {
	throw new IOException("Mismatch in pos : " + pos + " + " +
	firstChunkOffset + " != " + chunkOffset);
	}

	// Read next packet if the previous packet has been read completely.
	if (dataLeft <= 0) {
	//Read packet headers.
	PacketHeader header = new PacketHeader();
	header.readFields(in);

	if (LOG.isDebugEnabled()) {
	LOG.debug("DFSClient readChunk got header " + header);
	}

	// Sanity check the lengths
	if (!header.sanityCheck(lastSeqNo)) {
	throw new IOException("BlockReader: error in packet header " +
	header);
	}

	lastSeqNo = header.getSeqno();
	dataLeft = header.getDataLen();
	adjustChecksumBytes(header.getDataLen());
	if (header.getDataLen() > 0) {
	IOUtils.readFully(in, checksumBytes.array(), 0,
	checksumBytes.limit());
	}
	}

	// Sanity checks
	assert len >= bytesPerChecksum;
	assert checksum != null;
	assert checksumSize == 0 \|\| (checksumBuf.length % checksumSize == 0);


	int checksumsToRead, bytesToRead;

	if (checksumSize > 0) {

	// How many chunks left in our packet - this is a ceiling
	// since we may have a partial chunk at the end of the file
	int chunksLeft = (dataLeft - 1) / bytesPerChecksum + 1;

	// How many chunks we can fit in databuffer
	// - note this is a floor since we always read full chunks
	int chunksCanFit = Math.min(len / bytesPerChecksum,
	checksumBuf.length / checksumSize);

	// How many chunks should we read
	checksumsToRead = Math.min(chunksLeft, chunksCanFit);
	// How many bytes should we actually read
	bytesToRead = Math.min(
	checksumsToRead * bytesPerChecksum, // full chunks
	dataLeft); // in case we have a partial
	} else {
	// no checksum
	bytesToRead = Math.min(dataLeft, len);
	checksumsToRead = 0;
	}

	if ( bytesToRead > 0 ) {
	// Assert we have enough space
	assert bytesToRead <= len;
	assert checksumBytes.remaining() >= checksumSize * checksumsToRead;
	assert checksumBuf.length >= checksumSize * checksumsToRead;
	IOUtils.readFully(in, buf, offset, bytesToRead);
	checksumBytes.get(checksumBuf, 0, checksumSize * checksumsToRead);
	}

	dataLeft -= bytesToRead;
	assert dataLeft >= 0;

	lastChunkOffset = chunkOffset;
	lastChunkLen = bytesToRead;

	// If there's no data left in the current packet after satisfying
	// this read, and we have satisfied the client read, we expect
	// an empty packet header from the DN to signify this.
	// Note that pos + bytesToRead may in fact be greater since the
	// DN finishes off the entire last chunk.
	if (dataLeft == 0 &&
	pos + bytesToRead >= bytesNeededToFinish) {

	// Read header
	int packetLen = in.readInt();
	long offsetInBlock = in.readLong();
	long seqno = in.readLong();
	boolean lastPacketInBlock = in.readBoolean();
	int dataLen = in.readInt();

	if (!lastPacketInBlock \|\|
	dataLen != 0) {
	throw new IOException("Expected empty end-of-read packet! Header: " +
	"(packetLen : " + packetLen +
	", offsetInBlock : " + offsetInBlock +
	", seqno : " + seqno +
	", lastInBlock : " + lastPacketInBlock +
	", dataLen : " + dataLen);
	}

	eos = true;
	}

	if ( bytesToRead == 0 ) {
	return -1;
	}

	return bytesToRead;
	}

	private BlockReader(String file, String bpid, long blockId,
	DataInputStream in, DataChecksum checksum, boolean verifyChecksum,
	long startOffset, long firstChunkOffset, long bytesToRead, Socket dnSock) {
	// Path is used only for printing block and file information in debug
	super(new Path("/blk_" + blockId + ":" + bpid + ":of:"+ file)/too non path-like?/,
	1, verifyChecksum,
	checksum.getChecksumSize() > 0? checksum : null,
	checksum.getBytesPerChecksum(),
	checksum.getChecksumSize());

	this.dnSock = dnSock;
	this.in = in;
	this.checksum = checksum;
	this.startOffset = Math.max( startOffset, 0 );

	// The total number of bytes that we need to transfer from the DN is
	// the amount that the user wants (bytesToRead), plus the padding at
	// the beginning in order to chunk-align. Note that the DN may elect
	// to send more than this amount if the read starts/ends mid-chunk.
	this.bytesNeededToFinish = bytesToRead + (startOffset - firstChunkOffset);

	this.firstChunkOffset = firstChunkOffset;
	lastChunkOffset = firstChunkOffset;
	lastChunkLen = -1;

	bytesPerChecksum = this.checksum.getBytesPerChecksum();
	checksumSize = this.checksum.getChecksumSize();
	}

	public static BlockReader newBlockReader(Socket sock, String file,
	ExtendedBlock block, Token<BlockTokenIdentifier> blockToken,
	long startOffset, long len, int bufferSize) throws IOException {
	return newBlockReader(sock, file, block, blockToken, startOffset, len, bufferSize,
	true);
	}

	/** Java Doc required */
	public static BlockReader newBlockReader( Socket sock, String file,
	ExtendedBlock block,
	Token<BlockTokenIdentifier> blockToken,
	long startOffset, long len,
	int bufferSize, boolean verifyChecksum)
	throws IOException {
	return newBlockReader(sock, file, block, blockToken, startOffset,
	len, bufferSize, verifyChecksum, "");
	}

	/**
	* Create a new BlockReader specifically to satisfy a read.
	* This method also sends the OP_READ_BLOCK request.
	*
	* @param sock An established Socket to the DN. The BlockReader will not close it normally
	* @param file File location
	* @param block The block object
	* @param blockToken The block token for security
	* @param startOffset The read offset, relative to block head
	* @param len The number of bytes to read
	* @param bufferSize The IO buffer size (not the client buffer size)
	* @param verifyChecksum Whether to verify checksum
	* @param clientName Client name
	* @return New BlockReader instance, or null on error.
	*/
	public static BlockReader newBlockReader( Socket sock, String file,
	ExtendedBlock block,
	Token<BlockTokenIdentifier> blockToken,
	long startOffset, long len,
	int bufferSize, boolean verifyChecksum,
	String clientName)
	throws IOException {
	// in and out will be closed when sock is closed (by the caller)
	DataTransferProtocol.Sender.opReadBlock(
	new DataOutputStream(new BufferedOutputStream(
	NetUtils.getOutputStream(sock,HdfsConstants.WRITE_TIMEOUT))),
	block, startOffset, len, clientName, blockToken);

	//
	// Get bytes in block, set streams
	//

	DataInputStream in = new DataInputStream(
	new BufferedInputStream(NetUtils.getInputStream(sock),
	bufferSize));

	DataTransferProtocol.Status status = DataTransferProtocol.Status.read(in);
	if (status != SUCCESS) {
	if (status == ERROR_ACCESS_TOKEN) {
	throw new InvalidBlockTokenException(
	"Got access token error for OP_READ_BLOCK, self="
	+ sock.getLocalSocketAddress() + ", remote="
	+ sock.getRemoteSocketAddress() + ", for file " + file
	+ ", for pool " + block.getBlockPoolId() + " block "
	+ block.getBlockId() + "_" + block.getGenerationStamp());
	} else {
	throw new IOException("Got error for OP_READ_BLOCK, self="
	+ sock.getLocalSocketAddress() + ", remote="
	+ sock.getRemoteSocketAddress() + ", for file " + file
	+ ", for pool " + block.getBlockPoolId() + " block "
	+ block.getBlockId() + "_" + block.getGenerationStamp());
	}
	}
	DataChecksum checksum = DataChecksum.newDataChecksum( in );
	//Warning when we get CHECKSUM_NULL?

	// Read the first chunk offset.
	long firstChunkOffset = in.readLong();

	if ( firstChunkOffset < 0 \|\| firstChunkOffset > startOffset \|\|
	firstChunkOffset >= (startOffset + checksum.getBytesPerChecksum())) {
	throw new IOException("BlockReader: error in first chunk offset (" +
	firstChunkOffset + ") startOffset is " +
	startOffset + " for file " + file);
	}

	return new BlockReader(file, block.getBlockPoolId(), block.getBlockId(),
	in, checksum, verifyChecksum, startOffset, firstChunkOffset, len, sock);
	}

	@Override
	public synchronized void close() throws IOException {
	startOffset = -1;
	checksum = null;
	if (dnSock != null) {
	dnSock.close();
	}

	// in will be closed when its Socket is closed.
	}

	/** kind of like readFully(). Only reads as much as possible.
	* And allows use of protected readFully().
	*/
	public int readAll(byte[] buf, int offset, int len) throws IOException {
	return readFully(this, buf, offset, len);
	}

	/**
	* Take the socket used to talk to the DN.
	*/
	public Socket takeSocket() {
	assert hasSentStatusCode() :
	"BlockReader shouldn't give back sockets mid-read";
	Socket res = dnSock;
	dnSock = null;
	return res;
	}

	/**
	* Whether the BlockReader has reached the end of its input stream
	* and successfully sent a status code back to the datanode.
	*/
	public boolean hasSentStatusCode() {
	return sentStatusCode;
	}

	/**
	* When the reader reaches end of the read, it sends a status response
	* (e.g. CHECKSUM_OK) to the DN. Failure to do so could lead to the DN
	* closing our connection (which we will re-open), but won't affect
	* data correctness.
	*/
	void sendReadResult(Socket sock, DataTransferProtocol.Status statusCode) {
	assert !sentStatusCode : "already sent status code to " + sock;
	try {
	OutputStream out = NetUtils.getOutputStream(sock, HdfsConstants.WRITE_TIMEOUT);
	statusCode.writeOutputStream(out);
	out.flush();
	sentStatusCode = true;
	} catch (IOException e) {
	// It's ok not to be able to send this. But something is probably wrong.
	LOG.info("Could not send read status (" + statusCode + ") to datanode " +
	sock.getInetAddress() + ": " + e.getMessage());
	}
	}

	/**
	* File name to print when accessing a block directly (from servlets)
	* @param s Address of the block location
	* @param poolId Block pool ID of the block
	* @param blockId Block ID of the block
	* @return string that has a file name for debug purposes
	*/
	public static String getFileName(final InetSocketAddress s,
	final String poolId, final long blockId) {
	return s.toString() + ":" + poolId + ":" + blockId;
	}
	}