richardstartin commented on a change in pull request #7930:
URL: https://github.com/apache/pinot/pull/7930#discussion_r771977380



##########
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:
       I suggest to use 
`ByteBuffer.allocateDirect(_chunkCompressor.maxCompressedSize(chunkSize))` 
instead because most compressors can produce a much tighter bound than 2x. E.g. 
LZ4 (default) produces 8240 for 8192 and 1052704 for 1MB.




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