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file/file_prefetch_buffer.cc

@@ -141,8 +141,7 @@ Status FilePrefetchBuffer::Prefetch(const IOOptions& opts,
   TEST_SYNC_POINT("FilePrefetchBuffer::Prefetch:Start");
 
   if (offset + n <= buf->offset_ + buf->buffer_.CurrentSize()) {
-    // All requested bytes are already in the curr_ buffer. So no need to Read
-    // again.
+    // All requested bytes are already in the buffer. So no need to Read again.
     return Status::OK();
   }
 
@@ -413,7 +412,7 @@ Status FilePrefetchBuffer::HandleOverlappingData(
   // call ReadAsync on freed buffer.
   if (!buf->async_read_in_progress_ && DoesBufferContainData(buf) &&
       IsOffsetInBuffer(buf, offset) &&
-      (/*Data extends over curr_ buffer and second buffer either has data or in
+      (/*Data extends over two buffers and second buffer either has data or in
          process of population=*/
        (offset + length > next_buf->offset_) &&
        (next_buf->async_read_in_progress_ ||
@@ -463,26 +462,20 @@ Status FilePrefetchBuffer::HandleOverlappingData(
 }
 
 // If async_io is enabled in case of sequential reads, PrefetchAsyncInternal is
-// called. When buffers are switched, we clear the curr_ buffer as we assume the
+// called. When data is outdated, we clear the first buffer and free it as the
 // data has been consumed because of sequential reads.
-// Data in buffers will always be sequential with curr_ following second and
-// not vice versa.
 //
 // Scenarios for prefetching asynchronously:
-// Case1: If both buffers are empty, prefetch n + readahead_size_/2 bytes
-//        synchronously in curr_ and prefetch readahead_size_/2 async in second
-//        buffer.
-// Case2: If second buffer has partial or full data, make it current and
-//        prefetch readahead_size_/2 async in second buffer. In case of
-//        partial data, prefetch remaining bytes from size n synchronously to
-//        fulfill the requested bytes request.
-// Case3: If curr_ has partial data, prefetch remaining bytes from size n
-//        synchronously in curr_ to fulfill the requested bytes request and
-//        prefetch readahead_size_/2 bytes async in second buffer.
-// Case4: (Special case) If data is in both buffers, copy requested data from
-//        curr_, send async request on curr_, wait for poll to fill second
-//        buffer (if any), and copy remaining data from second buffer to third
-//        buffer.
+// Case1: If all buffers are in free_bufs_, prefetch n + readahead_size_/2 bytes
+//        synchronously in first buffer and prefetch readahead_size_/2 async in
+//        remaining buffers (num_buffers_ -1 ).
+// Case2: If first buffer has partial data, prefetch readahead_size_/2 async in
+//        remaining buffers. In case of partial data, prefetch remaining bytes
+//        from size n synchronously to fulfill the requested bytes request.
+// Case5: (Special case) If data is overlapping in two buffers, copy requested
+//        data from first, free that buffer to send for async request, wait for
+//        poll to fill next buffer (if any), and copy remaining data from that
+//        buffer to overlap buffer.
 Status FilePrefetchBuffer::PrefetchAsyncInternal(const IOOptions& opts,
                                                  RandomAccessFileReader* reader,
                                                  uint64_t offset, size_t length,