branch-0.23.1/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockSender.java

/**
 * 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.server.datanode;

import java.io.BufferedInputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.SocketException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.util.Arrays;

import org.apache.commons.logging.Log;
import org.apache.hadoop.fs.ChecksumException;
import org.apache.hadoop.hdfs.protocol.ExtendedBlock;
import org.apache.hadoop.hdfs.protocol.HdfsConstants;
import org.apache.hadoop.hdfs.protocol.datatransfer.PacketHeader;
import org.apache.hadoop.hdfs.util.DataTransferThrottler;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.io.ReadaheadPool;
import org.apache.hadoop.io.ReadaheadPool.ReadaheadRequest;
import org.apache.hadoop.io.nativeio.NativeIO;
import org.apache.hadoop.net.SocketOutputStream;
import org.apache.hadoop.util.DataChecksum;

/**
 * Reads a block from the disk and sends it to a recipient.
 * 
 * Data sent from the BlockeSender in the following format:
 * <br><b>Data format:</b> <pre>
 *    +--------------------------------------------------+
 *    | ChecksumHeader | Sequence of data PACKETS...     |
 *    +--------------------------------------------------+ 
 * </pre>   
 * <b>ChecksumHeader format:</b> <pre>
 *    +--------------------------------------------------+
 *    | 1 byte CHECKSUM_TYPE | 4 byte BYTES_PER_CHECKSUM |
 *    +--------------------------------------------------+ 
 * </pre>   
 * An empty packet is sent to mark the end of block and read completion.
 * 
 *  PACKET Contains a packet header, checksum and data. Amount of data
 *  carried is set by BUFFER_SIZE.
 *  <pre>
 *    +-----------------------------------------------------+
 *    | 4 byte packet length (excluding packet header)      |
 *    +-----------------------------------------------------+
 *    | 8 byte offset in the block | 8 byte sequence number |
 *    +-----------------------------------------------------+
 *    | 1 byte isLastPacketInBlock                          |
 *    +-----------------------------------------------------+
 *    | 4 byte Length of actual data                        |
 *    +-----------------------------------------------------+
 *    | x byte checksum data. x is defined below            |
 *    +-----------------------------------------------------+
 *    | actual data ......                                  |
 *    +-----------------------------------------------------+
 *    
 *    Data is made of Chunks. Each chunk is of length <= BYTES_PER_CHECKSUM.
 *    A checksum is calculated for each chunk.
 *    
 *    x = (length of data + BYTE_PER_CHECKSUM - 1)/BYTES_PER_CHECKSUM *
 *        CHECKSUM_SIZE
 *        
 *    CHECKSUM_SIZE depends on CHECKSUM_TYPE (usually, 4 for CRC32) 
 *    </pre>
 *  
 *  The client reads data until it receives a packet with 
 *  "LastPacketInBlock" set to true or with a zero length. If there is 
 *  no checksum error, it replies to DataNode with OP_STATUS_CHECKSUM_OK: 
 *  <pre>
 *    +------------------------------+
 *    | 2 byte OP_STATUS_CHECKSUM_OK |
 *    +------------------------------+
 *  </pre>
 */
class BlockSender implements java.io.Closeable {
  static final Log LOG = DataNode.LOG;
  static final Log ClientTraceLog = DataNode.ClientTraceLog;
  private static final boolean is32Bit = 
      System.getProperty("sun.arch.data.model").equals("32");
  /**
   * Minimum buffer used while sending data to clients. Used only if
   * transferTo() is enabled. 64KB is not that large. It could be larger, but
   * not sure if there will be much more improvement.
   */
  private static final int MIN_BUFFER_WITH_TRANSFERTO = 64*1024;
  private static final int TRANSFERTO_BUFFER_SIZE = Math.max(
      HdfsConstants.IO_FILE_BUFFER_SIZE, MIN_BUFFER_WITH_TRANSFERTO);
  
  /** the block to read from */
  private final ExtendedBlock block;
  /** the replica to read from */
  private final Replica replica;
  /** The visible length of a replica. */
  private final long replicaVisibleLength;
  /** Stream to read block data from */
  private InputStream blockIn;
  /** updated while using transferTo() */
  private long blockInPosition = -1;
  /** Stream to read checksum */
  private DataInputStream checksumIn;
  /** Checksum utility */
  private final DataChecksum checksum;
  /** Initial position to read */
  private long initialOffset;
  /** Current position of read */
  private long offset;
  /** Position of last byte to read from block file */
  private final long endOffset;
  /** Number of bytes in chunk used for computing checksum */
  private final int chunkSize;
  /** Number bytes of checksum computed for a chunk */
  private final int checksumSize;
  /** If true, failure to read checksum is ignored */
  private final boolean corruptChecksumOk;
  /** Sequence number of packet being sent */
  private long seqno;
  /** Set to true if transferTo is allowed for sending data to the client */
  private final boolean transferToAllowed;
  /** Set to true once entire requested byte range has been sent to the client */
  private boolean sentEntireByteRange;
  /** When true, verify checksum while reading from checksum file */
  private final boolean verifyChecksum;
  /** Format used to print client trace log messages */
  private final String clientTraceFmt;
  private volatile ChunkChecksum lastChunkChecksum = null;
  
  /** The file descriptor of the block being sent */
  private FileDescriptor blockInFd;

  // Cache-management related fields
  private final long readaheadLength;
  private boolean shouldDropCacheBehindRead;
  private ReadaheadRequest curReadahead;
  private long lastCacheDropOffset;
  private static final long CACHE_DROP_INTERVAL_BYTES = 1024 * 1024; // 1MB
  /**
   * Minimum length of read below which management of the OS
   * buffer cache is disabled.
   */
  private static final long LONG_READ_THRESHOLD_BYTES = 256 * 1024;
  
  private static ReadaheadPool readaheadPool =
    ReadaheadPool.getInstance();

  /**
   * Constructor
   * 
   * @param block Block that is being read
   * @param startOffset starting offset to read from
   * @param length length of data to read
   * @param corruptChecksumOk
   * @param verifyChecksum verify checksum while reading the data
   * @param datanode datanode from which the block is being read
   * @param clientTraceFmt format string used to print client trace logs
   * @throws IOException
   */
  BlockSender(ExtendedBlock block, long startOffset, long length,
              boolean corruptChecksumOk, boolean verifyChecksum,
              DataNode datanode, String clientTraceFmt)
      throws IOException {
    try {
      this.block = block;
      this.corruptChecksumOk = corruptChecksumOk;
      this.verifyChecksum = verifyChecksum;
      this.clientTraceFmt = clientTraceFmt;
      this.readaheadLength = datanode.getDnConf().readaheadLength;
      this.shouldDropCacheBehindRead = datanode.getDnConf().dropCacheBehindReads;
      
      synchronized(datanode.data) { 
        this.replica = getReplica(block, datanode);
        this.replicaVisibleLength = replica.getVisibleLength();
      }
      // if there is a write in progress
      ChunkChecksum chunkChecksum = null;
      if (replica instanceof ReplicaBeingWritten) {
        long minEndOffset = startOffset + length;
        waitForMinLength((ReplicaBeingWritten)replica, minEndOffset);
        ReplicaInPipeline rip = (ReplicaInPipeline) replica;
        chunkChecksum = rip.getLastChecksumAndDataLen();
      }

      if (replica.getGenerationStamp() < block.getGenerationStamp()) {
        throw new IOException("Replica gen stamp < block genstamp, block="
            + block + ", replica=" + replica);
      }
      if (replicaVisibleLength < 0) {
        throw new IOException("Replica is not readable, block="
            + block + ", replica=" + replica);
      }
      if (DataNode.LOG.isDebugEnabled()) {
        DataNode.LOG.debug("block=" + block + ", replica=" + replica);
      }

      // transferToFully() fails on 32 bit platforms for block sizes >= 2GB,
      // use normal transfer in those cases
      this.transferToAllowed = datanode.getDnConf().transferToAllowed &&
        (!is32Bit || length <= Integer.MAX_VALUE);

      DataChecksum csum;
      if (!corruptChecksumOk || datanode.data.metaFileExists(block)) {
        checksumIn = new DataInputStream(new BufferedInputStream(datanode.data
            .getMetaDataInputStream(block), HdfsConstants.IO_FILE_BUFFER_SIZE));

        // read and handle the common header here. For now just a version
        BlockMetadataHeader header = BlockMetadataHeader.readHeader(checksumIn);
        short version = header.getVersion();
        if (version != BlockMetadataHeader.VERSION) {
          LOG.warn("Wrong version (" + version + ") for metadata file for "
              + block + " ignoring ...");
        }
        csum = header.getChecksum();
      } else {
        LOG.warn("Could not find metadata file for " + block);
        // This only decides the buffer size. Use BUFFER_SIZE?
        csum = DataChecksum.newDataChecksum(DataChecksum.CHECKSUM_NULL,
            16 * 1024);
      }

      /*
       * If chunkSize is very large, then the metadata file is mostly
       * corrupted. For now just truncate bytesPerchecksum to blockLength.
       */       
      int size = csum.getBytesPerChecksum();
      if (size > 10*1024*1024 && size > replicaVisibleLength) {
        csum = DataChecksum.newDataChecksum(csum.getChecksumType(),
            Math.max((int)replicaVisibleLength, 10*1024*1024));
        size = csum.getBytesPerChecksum();        
      }
      chunkSize = size;
      checksum = csum;
      checksumSize = checksum.getChecksumSize();
      length = length < 0 ? replicaVisibleLength : length;

      // end is either last byte on disk or the length for which we have a 
      // checksum
      long end = chunkChecksum != null ? chunkChecksum.getDataLength()
          : replica.getBytesOnDisk();
      if (startOffset < 0 || startOffset > end
          || (length + startOffset) > end) {
        String msg = " Offset " + startOffset + " and length " + length
        + " don't match block " + block + " ( blockLen " + end + " )";
        LOG.warn(datanode.getDNRegistrationForBP(block.getBlockPoolId()) +
            ":sendBlock() : " + msg);
        throw new IOException(msg);
      }
      
      // Ensure read offset is position at the beginning of chunk
      offset = startOffset - (startOffset % chunkSize);
      if (length >= 0) {
        // Ensure endOffset points to end of chunk.
        long tmpLen = startOffset + length;
        if (tmpLen % chunkSize != 0) {
          tmpLen += (chunkSize - tmpLen % chunkSize);
        }
        if (tmpLen < end) {
          // will use on-disk checksum here since the end is a stable chunk
          end = tmpLen;
        } else if (chunkChecksum != null) {
          // last chunk is changing. flag that we need to use in-memory checksum 
          this.lastChunkChecksum = chunkChecksum;
        }
      }
      endOffset = end;

      // seek to the right offsets
      if (offset > 0) {
        long checksumSkip = (offset / chunkSize) * checksumSize;
        // note blockInStream is seeked when created below
        if (checksumSkip > 0) {
          // Should we use seek() for checksum file as well?
          IOUtils.skipFully(checksumIn, checksumSkip);
        }
      }
      seqno = 0;

      if (DataNode.LOG.isDebugEnabled()) {
        DataNode.LOG.debug("replica=" + replica);
      }
      blockIn = datanode.data.getBlockInputStream(block, offset); // seek to offset
      if (blockIn instanceof FileInputStream) {
        blockInFd = ((FileInputStream)blockIn).getFD();
      } else {
        blockInFd = null;
      }
    } catch (IOException ioe) {
      IOUtils.closeStream(this);
      IOUtils.closeStream(blockIn);
      throw ioe;
    }
  }

  /**
   * close opened files.
   */
  public void close() throws IOException {
    if (blockInFd != null && shouldDropCacheBehindRead && isLongRead()) {
      // drop the last few MB of the file from cache
      try {
        NativeIO.posixFadviseIfPossible(
            blockInFd, lastCacheDropOffset, offset - lastCacheDropOffset,
            NativeIO.POSIX_FADV_DONTNEED);
      } catch (Exception e) {
        LOG.warn("Unable to drop cache on file close", e);
      }
    }
    if (curReadahead != null) {
      curReadahead.cancel();
    }
    
    IOException ioe = null;
    if(checksumIn!=null) {
      try {
        checksumIn.close(); // close checksum file
      } catch (IOException e) {
        ioe = e;
      }
      checksumIn = null;
    }   
    if(blockIn!=null) {
      try {
        blockIn.close(); // close data file
      } catch (IOException e) {
        ioe = e;
      }
      blockIn = null;
      blockInFd = null;
    }
    // throw IOException if there is any
    if(ioe!= null) {
      throw ioe;
    }
  }
  
  private static Replica getReplica(ExtendedBlock block, DataNode datanode)
      throws ReplicaNotFoundException {
    Replica replica = datanode.data.getReplica(block.getBlockPoolId(),
        block.getBlockId());
    if (replica == null) {
      throw new ReplicaNotFoundException(block);
    }
    return replica;
  }
  
  /**
   * Wait for rbw replica to reach the length
   * @param rbw replica that is being written to
   * @param len minimum length to reach
   * @throws IOException on failing to reach the len in given wait time
   */
  private static void waitForMinLength(ReplicaBeingWritten rbw, long len)
      throws IOException {
    // Wait for 3 seconds for rbw replica to reach the minimum length
    for (int i = 0; i < 30 && rbw.getBytesOnDisk() < len; i++) {
      try {
        Thread.sleep(100);
      } catch (InterruptedException ie) {
        throw new IOException(ie);
      }
    }
    long bytesOnDisk = rbw.getBytesOnDisk();
    if (bytesOnDisk < len) {
      throw new IOException(
          String.format("Need %d bytes, but only %d bytes available", len,
              bytesOnDisk));
    }
  }
  
  /**
   * Converts an IOExcpetion (not subclasses) to SocketException.
   * This is typically done to indicate to upper layers that the error 
   * was a socket error rather than often more serious exceptions like 
   * disk errors.
   */
  private static IOException ioeToSocketException(IOException ioe) {
    if (ioe.getClass().equals(IOException.class)) {
      // "se" could be a new class in stead of SocketException.
      IOException se = new SocketException("Original Exception : " + ioe);
      se.initCause(ioe);
      /* Change the stacktrace so that original trace is not truncated
       * when printed.*/ 
      se.setStackTrace(ioe.getStackTrace());
      return se;
    }
    // otherwise just return the same exception.
    return ioe;
  }

  /**
   * @param datalen Length of data 
   * @return number of chunks for data of given size
   */
  private int numberOfChunks(long datalen) {
    return (int) ((datalen + chunkSize - 1)/chunkSize);
  }
  
  /**
   * Sends a packet with up to maxChunks chunks of data.
   * 
   * @param pkt buffer used for writing packet data
   * @param maxChunks maximum number of chunks to send
   * @param out stream to send data to
   * @param transferTo use transferTo to send data
   * @param throttler used for throttling data transfer bandwidth
   */
  private int sendPacket(ByteBuffer pkt, int maxChunks, OutputStream out,
      boolean transferTo, DataTransferThrottler throttler) throws IOException {
    int dataLen = (int) Math.min(endOffset - offset,
                             (chunkSize * (long) maxChunks));
    
    int numChunks = numberOfChunks(dataLen); // Number of chunks be sent in the packet
    int checksumDataLen = numChunks * checksumSize;
    int packetLen = dataLen + checksumDataLen + 4;
    boolean lastDataPacket = offset + dataLen == endOffset && dataLen > 0;

    writePacketHeader(pkt, dataLen, packetLen);

    int checksumOff = pkt.position();
    byte[] buf = pkt.array();
    
    if (checksumSize > 0 && checksumIn != null) {
      readChecksum(buf, checksumOff, checksumDataLen);

      // write in progress that we need to use to get last checksum
      if (lastDataPacket && lastChunkChecksum != null) {
        int start = checksumOff + checksumDataLen - checksumSize;
        byte[] updatedChecksum = lastChunkChecksum.getChecksum();
        
        if (updatedChecksum != null) {
          System.arraycopy(updatedChecksum, 0, buf, start, checksumSize);
        }
      }
    }
    
    int dataOff = checksumOff + checksumDataLen;
    if (!transferTo) { // normal transfer
      IOUtils.readFully(blockIn, buf, dataOff, dataLen);

      if (verifyChecksum) {
        verifyChecksum(buf, dataOff, dataLen, numChunks, checksumOff);
      }
    }
    
    try {
      if (transferTo) {
        SocketOutputStream sockOut = (SocketOutputStream)out;
        sockOut.write(buf, 0, dataOff); // First write checksum
        
        // no need to flush. since we know out is not a buffered stream. 
        sockOut.transferToFully(((FileInputStream)blockIn).getChannel(), 
                                blockInPosition, dataLen);
        blockInPosition += dataLen;
      } else { 
        // normal transfer
        out.write(buf, 0, dataOff + dataLen);
      }
    } catch (IOException e) {
      /* Exception while writing to the client. Connection closure from
       * the other end is mostly the case and we do not care much about
       * it. But other things can go wrong, especially in transferTo(),
       * which we do not want to ignore.
       *
       * The message parsing below should not be considered as a good
       * coding example. NEVER do it to drive a program logic. NEVER.
       * It was done here because the NIO throws an IOException for EPIPE.
       */
      String ioem = e.getMessage();
      if (!ioem.startsWith("Broken pipe") && !ioem.startsWith("Connection reset")) {
        LOG.error("BlockSender.sendChunks() exception: ", e);
      }
      throw ioeToSocketException(e);
    }

    if (throttler != null) { // rebalancing so throttle
      throttler.throttle(packetLen);
    }

    return dataLen;
  }
  
  /**
   * Read checksum into given buffer
   * @param buf buffer to read the checksum into
   * @param checksumOffset offset at which to write the checksum into buf
   * @param checksumLen length of checksum to write
   * @throws IOException on error
   */
  private void readChecksum(byte[] buf, final int checksumOffset,
      final int checksumLen) throws IOException {
    if (checksumSize <= 0 && checksumIn == null) {
      return;
    }
    try {
      checksumIn.readFully(buf, checksumOffset, checksumLen);
    } catch (IOException e) {
      LOG.warn(" Could not read or failed to veirfy checksum for data"
          + " at offset " + offset + " for block " + block, e);
      IOUtils.closeStream(checksumIn);
      checksumIn = null;
      if (corruptChecksumOk) {
        if (checksumOffset < checksumLen) {
          // Just fill the array with zeros.
          Arrays.fill(buf, checksumOffset, checksumLen, (byte) 0);
        }
      } else {
        throw e;
      }
    }
  }
  
  /**
   * Compute checksum for chunks and verify the checksum that is read from
   * the metadata file is correct.
   * 
   * @param buf buffer that has checksum and data
   * @param dataOffset position where data is written in the buf
   * @param datalen length of data
   * @param numChunks number of chunks corresponding to data
   * @param checksumOffset offset where checksum is written in the buf
   * @throws ChecksumException on failed checksum verification
   */
  public void verifyChecksum(final byte[] buf, final int dataOffset,
      final int datalen, final int numChunks, final int checksumOffset)
      throws ChecksumException {
    int dOff = dataOffset;
    int cOff = checksumOffset;
    int dLeft = datalen;

    for (int i = 0; i < numChunks; i++) {
      checksum.reset();
      int dLen = Math.min(dLeft, chunkSize);
      checksum.update(buf, dOff, dLen);
      if (!checksum.compare(buf, cOff)) {
        long failedPos = offset + datalen - dLeft;
        throw new ChecksumException("Checksum failed at " + failedPos,
            failedPos);
      }
      dLeft -= dLen;
      dOff += dLen;
      cOff += checksumSize;
    }
  }
  
  /**
   * sendBlock() is used to read block and its metadata and stream the data to
   * either a client or to another datanode. 
   * 
   * @param out  stream to which the block is written to
   * @param baseStream optional. if non-null, <code>out</code> is assumed to 
   *        be a wrapper over this stream. This enables optimizations for
   *        sending the data, e.g. 
   *        {@link SocketOutputStream#transferToFully(FileChannel, 
   *        long, int)}.
   * @param throttler for sending data.
   * @return total bytes read, including checksum data.
   */
  long sendBlock(DataOutputStream out, OutputStream baseStream, 
                 DataTransferThrottler throttler) throws IOException {
    if (out == null) {
      throw new IOException( "out stream is null" );
    }
    initialOffset = offset;
    long totalRead = 0;
    OutputStream streamForSendChunks = out;
    
    lastCacheDropOffset = initialOffset;

    if (isLongRead() && blockInFd != null) {
      // Advise that this file descriptor will be accessed sequentially.
      NativeIO.posixFadviseIfPossible(blockInFd, 0, 0, NativeIO.POSIX_FADV_SEQUENTIAL);
    }
    
    // Trigger readahead of beginning of file if configured.
    manageOsCache();

    final long startTime = ClientTraceLog.isInfoEnabled() ? System.nanoTime() : 0;
    try {
      int maxChunksPerPacket;
      int pktSize = PacketHeader.PKT_HEADER_LEN;
      boolean transferTo = transferToAllowed && !verifyChecksum
          && baseStream instanceof SocketOutputStream
          && blockIn instanceof FileInputStream;
      if (transferTo) {
        FileChannel fileChannel = ((FileInputStream)blockIn).getChannel();
        blockInPosition = fileChannel.position();
        streamForSendChunks = baseStream;
        maxChunksPerPacket = numberOfChunks(TRANSFERTO_BUFFER_SIZE);
        
        // Smaller packet size to only hold checksum when doing transferTo
        pktSize += checksumSize * maxChunksPerPacket;
      } else {
        maxChunksPerPacket = Math.max(1,
            numberOfChunks(HdfsConstants.IO_FILE_BUFFER_SIZE));
        // Packet size includes both checksum and data
        pktSize += (chunkSize + checksumSize) * maxChunksPerPacket;
      }

      ByteBuffer pktBuf = ByteBuffer.allocate(pktSize);

      while (endOffset > offset) {
        manageOsCache();
        long len = sendPacket(pktBuf, maxChunksPerPacket, streamForSendChunks,
            transferTo, throttler);
        offset += len;
        totalRead += len + (numberOfChunks(len) * checksumSize);
        seqno++;
      }
      try {
        // send an empty packet to mark the end of the block
        sendPacket(pktBuf, maxChunksPerPacket, streamForSendChunks, transferTo,
            throttler);
        out.flush();
      } catch (IOException e) { //socket error
        throw ioeToSocketException(e);
      }

      sentEntireByteRange = true;
    } finally {
      if (clientTraceFmt != null) {
        final long endTime = System.nanoTime();
        ClientTraceLog.info(String.format(clientTraceFmt, totalRead,
            initialOffset, endTime - startTime));
      }
      close();
    }
    return totalRead;
  }

  /**
   * Manage the OS buffer cache by performing read-ahead
   * and drop-behind.
   */
  private void manageOsCache() throws IOException {
    if (!isLongRead() || blockInFd == null) {
      // don't manage cache manually for short-reads, like
      // HBase random read workloads.
      return;
    }

    // Perform readahead if necessary
    if (readaheadLength > 0 && readaheadPool != null) {
      curReadahead = readaheadPool.readaheadStream(
          clientTraceFmt, blockInFd,
          offset, readaheadLength, Long.MAX_VALUE,
          curReadahead);
    }

    // Drop what we've just read from cache, since we aren't
    // likely to need it again
    long nextCacheDropOffset = lastCacheDropOffset + CACHE_DROP_INTERVAL_BYTES;
    if (shouldDropCacheBehindRead &&
        offset >= nextCacheDropOffset) {
      long dropLength = offset - lastCacheDropOffset;
      if (dropLength >= 1024) {
        NativeIO.posixFadviseIfPossible(blockInFd,
            lastCacheDropOffset, dropLength,
            NativeIO.POSIX_FADV_DONTNEED);
      }
      lastCacheDropOffset += CACHE_DROP_INTERVAL_BYTES;
    }
  }

  private boolean isLongRead() {
    return (endOffset - offset) > LONG_READ_THRESHOLD_BYTES;
  }

  /**
   * Write packet header into {@code pkt}
   */
  private void writePacketHeader(ByteBuffer pkt, int dataLen, int packetLen) {
    pkt.clear();
    PacketHeader header = new PacketHeader(packetLen, offset, seqno,
        (dataLen == 0), dataLen);
    header.putInBuffer(pkt);
  }
  
  boolean didSendEntireByteRange() {
    return sentEntireByteRange;
  }

  /**
   * @return the checksum type that will be used with this block transfer.
   */
  DataChecksum getChecksum() {
    return checksum;
  }

  /**
   * @return the offset into the block file where the sender is currently
   * reading.
   */
  long getOffset() {
    return offset;
  }
}