richardstartin commented on a change in pull request #7930:
URL: https://github.com/apache/pinot/pull/7930#discussion_r772663250



##########
File path: 
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/io/writer/impl/BaseChunkSVForwardIndexWriter.java
##########
@@ -74,15 +71,13 @@ protected BaseChunkSVForwardIndexWriter(File file, 
ChunkCompressionType compress
       int numDocsPerChunk, int chunkSize, int sizeOfEntry, int version)
       throws IOException {
     Preconditions.checkArgument(version == DEFAULT_VERSION || version == 
CURRENT_VERSION);
-    _file = file;
-    _headerEntryChunkOffsetSize = getHeaderEntryChunkOffsetSize(version);
-    _dataOffset = headerSize(totalDocs, numDocsPerChunk, 
_headerEntryChunkOffsetSize);
     _chunkSize = chunkSize;
     _chunkCompressor = ChunkCompressorFactory.getCompressor(compressionType);
+    _headerEntryChunkOffsetSize = getHeaderEntryChunkOffsetSize(version);
+    _dataOffset = writeHeader(compressionType, totalDocs, numDocsPerChunk, 
sizeOfEntry, version);
     _chunkBuffer = ByteBuffer.allocateDirect(chunkSize);
-    _dataChannel = new RandomAccessFile(file, "rw").getChannel();
-    _header = _dataChannel.map(FileChannel.MapMode.READ_WRITE, 0, _dataOffset);
-    writeHeader(compressionType, totalDocs, numDocsPerChunk, sizeOfEntry, 
version);
+    _compressedBuffer = ByteBuffer.allocateDirect(chunkSize * 2);

Review comment:
       For posterity, here are links to the documentation of the functions we 
call in to via JNI:
   
   `LZ4_compressBound` is described 
[here](https://fossies.org/linux/lz4/doc/lz4_manual.html)
   `Snappy::MaxCompressedLength ` is described 
[here](https://github.com/google/snappy/blob/master/snappy.h)
   `ZSTD_compressBound` is described 
[here](https://zstd.docsforge.com/dev/api/ZSTD_compressBound/)




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