Release note for #9546 (#9872)

Summary:
We don't really have a mechanism for internal-only release
notes, so adding this to the standard release notes. For picking into
7.2 release.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9872

Test Plan: release note only

Reviewed By: jay-zhuang

Differential Revision: D35761307

Pulled By: pdillinger

fbshipit-source-id: 5d1932767fff48456323df948604dbb956ac27b2

HISTORY.md

@@ -22,6 +22,7 @@
 * Added a dedicated integer DB property `rocksdb.live-blob-file-garbage-size` that exposes the total amount of garbage in the blob files in the current version.
 * RocksDB does internal auto prefetching if it notices sequential reads. It starts with readahead size `initial_auto_readahead_size` which now can be configured through BlockBasedTableOptions.
 * Add a merge operator that allows users to register specific aggregation function so that they can does aggregation using different aggregation types for different keys. See comments in include/rocksdb/utilities/agg_merge.h for actual usage. The feature is experimental and the format is subject to change and we won't provide a migration tool.
+* Meta-internal / Experimental: Improve CPU performance by replacing many uses of std::unordered_map with folly::F14FastMap when RocksDB is compiled together with Folly.
 
 ### Behavior changes
 * Disallow usage of commit-time-write-batch for write-prepared/write-unprepared transactions if TransactionOptions::use_only_the_last_commit_time_batch_for_recovery is false to prevent two (or more) uncommitted versions of the same key in the database. Otherwise, bottommost compaction may violate the internal key uniqueness invariant of SSTs if the sequence numbers of both internal keys are zeroed out (#9794).