apache/hadoop · error · IOException

BlockReader: error in first chunk offset ({}) startOffset is

Error message

BlockReader: error in first chunk offset ({}) startOffset is {} for file {}

What it means

After a successful READ_BLOCK response the datanode reports firstChunkOffset — the file offset where the chunk containing startOffset begins. The client validates 0 <= firstChunkOffset <= startOffset and firstChunkOffset > startOffset - bytesPerChecksum; a violation means the datanode's chunk offset is inconsistent with the requested start offset (bad offset arithmetic on the DN, a truncated replica, or a startOffset at the very end of the block).

Source

Thrown at hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/client/impl/BlockReaderRemote.java:432

    // Get bytes in block
    //
    DataInputStream in = new DataInputStream(peer.getInputStream());

    BlockOpResponseProto status = BlockOpResponseProto.parseFrom(
        PBHelperClient.vintPrefixed(in));
    checkSuccess(status, peer, block, file);
    ReadOpChecksumInfoProto checksumInfo =
        status.getReadOpChecksumInfo();
    DataChecksum checksum = DataTransferProtoUtil.fromProto(
        checksumInfo.getChecksum());
    //Warning when we get CHECKSUM_NULL?

    // Read the first chunk offset.
    long firstChunkOffset = checksumInfo.getChunkOffset();

    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 BlockReaderRemote(file, block.getBlockId(), checksum,
        verifyChecksum, startOffset, firstChunkOffset, len, peer, datanodeID,
        peerCache, networkDistance);
  }

  static void checkSuccess(
      BlockOpResponseProto status, Peer peer,
      ExtendedBlock block, String file)
      throws IOException {
    String logInfo = "for OP_READ_BLOCK"
        + ", self=" + peer.getLocalAddressString()
        + ", remote=" + peer.getRemoteAddressString()
        + ", for file " + file
        + ", for pool " + block.getBlockPoolId()

View on GitHub (pinned to 2add963021)

Solutions

1. Retry with refreshed block locations (DFSInputStream retries internally; add job-level retry for persistent failures).
2. Verify input-split offset arithmetic at file/block boundaries — offsets must be strictly inside the block length.
3. hdfs fsck the file; let re-replication heal a bad replica.
4. If one datanode is consistently involved, check its version and logs.

try {
  return openAtOffset(dfs, path, offset);
} catch (IOException e) {
  if (e.getMessage() != null && e.getMessage().contains("first chunk offset")) {
    // bad offset math or bad replica: revalidate offset, then retry once
    long safe = Math.min(offset, dfs.getFileStatus(path).getLen() - 1);
    return openAtOffset(dfs, path, safe);
  }
  throw e;
}

When it happens

Trigger: Opening a block at an offset near the block end, or reading a truncated/damaged replica, where the datanode computes a first chunk offset outside the allowed window relative to startOffset.

Common situations: Input-split offset math that computes offsets at or beyond the block's committed length (off-by-one at file/block boundaries); replicas damaged on disk; datanode version bugs in offset handling.

Related errors

• BlockReader: error in packet header {}
• Expected empty end-of-read packet! Header: {}
• Premature EOF from inputStream
• Invalid payload length ${payloadLen}
• Invalid header length ${headerLen}

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/42a6d0e30e6c4bf9. Report an issue: GitHub.