{"record":{"id":"42a6d0e30e6c4bf9","repo":"apache/hadoop","slug":"blockreader-error-in-first-chunk-offset-star","errorCode":null,"errorMessage":"BlockReader: error in first chunk offset ({}) startOffset is {} for file {}","messagePattern":"BlockReader: error in first chunk offset \\((.+?)\\) startOffset is (.+?) for file (.+?)","errorType":"exception","errorClass":"IOException","httpStatus":null,"severity":"error","filePath":"hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/client/impl/BlockReaderRemote.java","lineNumber":432,"sourceCode":"    // Get bytes in block\n    //\n    DataInputStream in = new DataInputStream(peer.getInputStream());\n\n    BlockOpResponseProto status = BlockOpResponseProto.parseFrom(\n        PBHelperClient.vintPrefixed(in));\n    checkSuccess(status, peer, block, file);\n    ReadOpChecksumInfoProto checksumInfo =\n        status.getReadOpChecksumInfo();\n    DataChecksum checksum = DataTransferProtoUtil.fromProto(\n        checksumInfo.getChecksum());\n    //Warning when we get CHECKSUM_NULL?\n\n    // Read the first chunk offset.\n    long firstChunkOffset = checksumInfo.getChunkOffset();\n\n    if ( firstChunkOffset < 0 || firstChunkOffset > startOffset ||\n        firstChunkOffset <= (startOffset - checksum.getBytesPerChecksum())) {\n      throw new IOException(\"BlockReader: error in first chunk offset (\" +\n          firstChunkOffset + \") startOffset is \" +\n          startOffset + \" for file \" + file);\n    }\n\n    return new BlockReaderRemote(file, block.getBlockId(), checksum,\n        verifyChecksum, startOffset, firstChunkOffset, len, peer, datanodeID,\n        peerCache, networkDistance);\n  }\n\n  static void checkSuccess(\n      BlockOpResponseProto status, Peer peer,\n      ExtendedBlock block, String file)\n      throws IOException {\n    String logInfo = \"for OP_READ_BLOCK\"\n        + \", self=\" + peer.getLocalAddressString()\n        + \", remote=\" + peer.getRemoteAddressString()\n        + \", for file \" + file\n        + \", for pool \" + block.getBlockPoolId()","sourceCodeStart":414,"sourceCodeEnd":450,"githubUrl":"https://github.com/apache/hadoop/blob/2add9630210752f88ceb1bb74eb65e37bf41da8e/hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/client/impl/BlockReaderRemote.java#L414-L450","documentation":"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).","triggerScenarios":"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.","commonSituations":"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.","solutions":["Retry with refreshed block locations (DFSInputStream retries internally; add job-level retry for persistent failures).","Verify input-split offset arithmetic at file/block boundaries — offsets must be strictly inside the block length.","hdfs fsck the file; let re-replication heal a bad replica.","If one datanode is consistently involved, check its version and logs."],"exampleFix":null,"handlingStrategy":"retry","validationCode":null,"typeGuard":null,"tryCatchPattern":"try {\n  return openAtOffset(dfs, path, offset);\n} catch (IOException e) {\n  if (e.getMessage() != null && e.getMessage().contains(\"first chunk offset\")) {\n    // bad offset math or bad replica: revalidate offset, then retry once\n    long safe = Math.min(offset, dfs.getFileStatus(path).getLen() - 1);\n    return openAtOffset(dfs, path, safe);\n  }\n  throw e;\n}","preventionTips":["Assert input-split offsets are strictly less than the block/file length before opening.","Reuse Hadoop's split computation (FileInputFormat) instead of hand-rolled offset math at block boundaries.","Recurring first-chunk-offset failures on one file: fsck it — one replica is likely damaged."],"tags":["hdfs","data-transfer","chunk-offset","block-read"],"backgroundTag":"data-transfer-protocol-error","analyzedSha":"2add9630210752f88ceb1bb74eb65e37bf41da8e","analyzedAt":"2026-08-22T19:55:07.957Z","schemaVersion":2},"datasetVersion":"2026-08-22T20:17:22.307Z"}