Update HISTORY.md for block cache redundant adds (#6764)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6764

Reviewed By: ltamasi

Differential Revision: D21267108

Pulled By: pdillinger

fbshipit-source-id: a3dfe2dbe4e8f6309a53eb72903ef58d52308f97

HISTORY.md

@@ -19,6 +19,7 @@
 * Provide an allocator for memkind to be used with block cache. This is to work with memory technologies (Intel DCPMM is one such technology currently available) that require different libraries for allocation and management (such as PMDK and memkind). The high capacities available make it possible to provision large caches (up to several TBs in size) beyond what is achievable with DRAM.
 * Option `max_background_flushes` can be set dynamically using DB::SetDBOptions().
 * Added functionality in sst_dump tool to check the compressed file size for different compression levels and print the time spent on compressing files with each compression type. Added arguments `--compression_level_from` and `--compression_level_to` to report size of all compression levels and one compression_type must be specified with it so that it will report compressed sizes of one compression type with different levels.
+* Added statistics for redundant insertions into block cache: rocksdb.block.cache.*add.redundant. (There is currently no coordination to ensure that only one thread loads a table block when many threads are trying to access that same table block.)
 
 ### Bug Fixes
 * Fix a bug when making options.bottommost_compression, options.compression_opts and options.bottommost_compression_opts dynamically changeable: the modified values are not written to option files or returned back to users when being queried.