Add comment explaining why RandomAccessFileReader* is not passed in at construction time

file/file_prefetch_buffer.h

@@ -300,6 +300,14 @@ class FilePrefetchBuffer {
   // offset                : the file offset to start reading from.
   // n                     : the number of bytes to read.
   //
+  // Note: Why do we pass in the RandomAccessFileReader* for every single call
+  // to Prefetch/PrefetchAsync/TryReadFromCache? Why can't we just pass it in at
+  // construction time?
+  // Although the RandomAccessFileReader* is often available when creating
+  // the FilePrefetchBuffer, this is not true for BlobDB (see
+  // BlobSource::GetBlob). The file reader gets retrieved or created inside
+  // BlobFileCache::GetBlobFileReader, after we have already allocated a new
+  // FilePrefetchBuffer.
   Status Prefetch(const IOOptions& opts, RandomAccessFileReader* reader,
                   uint64_t offset, size_t n);
 
@@ -488,6 +496,10 @@ class FilePrefetchBuffer {
   // Whether we reuse the file system provided buffer
   // Until we also handle the async read case, only enable this optimization
   // for the synchronous case when num_buffers_ = 1.
+  // Note: Although it would be more convenient if we could determine
+  // whether we want to reuse the file system buffer at construction time,
+  // this would not work in all cases, because not all clients (BlobDB in
+  // particular) have a RandomAccessFileReader* available at construction time.
   bool UseFSBuffer(RandomAccessFileReader* reader) {
     return reader->file() != nullptr && !reader->use_direct_io() &&
            fs_ != nullptr &&