Add class comment for BlockFetcher

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5337

Differential Revision: D15482289

Pulled By: ltamasi

fbshipit-source-id: 8639ca78c1b8dfcc337a742d4d81d5752f12545f

table/block_fetcher.h

@@ -13,14 +13,28 @@
 #include "util/memory_allocator.h"
 
 namespace rocksdb {
+
+// Retrieves a single block of a given file. Utilizes the prefetch buffer and/or
+// persistent cache provided (if any) to try to avoid reading from the file
+// directly. Note that both the prefetch buffer and the persistent cache are
+// optional; also, note that the persistent cache may be configured to store either
+// compressed or uncompressed blocks.
+//
+// If the retrieved block is compressed and the do_uncompress flag is set,
+// BlockFetcher uncompresses the block (using the uncompression dictionary,
+// if provided, to prime the compression algorithm), and returns the resulting
+// uncompressed block data. Otherwise, it returns the original block.
+//
+// Two read options affect the behavior of BlockFetcher: if verify_checksums is
+// true, the checksum of the (original) block is checked; if fill_cache is true,
+// the block is added to the persistent cache if needed.
+//
+// Memory for uncompressed and compressed blocks is allocated as needed
+// using memory_allocator and memory_allocator_compressed, respectively
+// (if provided; otherwise, the default allocator is used).
+
 class BlockFetcher {
  public:
-  // Read the block identified by "handle" from "file".
-  // The only relevant option is options.verify_checksums for now.
-  // On failure return non-OK.
-  // On success fill *result and return OK - caller owns *result
-  // @param uncompression_dict Data for presetting the compression library's
-  //    dictionary.
   BlockFetcher(RandomAccessFileReader* file,
                FilePrefetchBuffer* prefetch_buffer, const Footer& footer,
                const ReadOptions& read_options, const BlockHandle& handle,