Blog post for releasing 5.2.1

Summary: Closes https://github.com/facebook/rocksdb/pull/2025

Differential Revision: D4759957

Pulled By: siying

fbshipit-source-id: 5b5b625

docs/_posts/2017-03-02-rocksdb-5-2-1-released.markdown

@@ -0,0 +1,22 @@
+---
+title: RocksDB 5.2.1 Released!
+layout: post
+author: sdong
+category: blog
+---
+
+### Public API Change
+* NewLRUCache() will determine number of shard bits automatically based on capacity, if the user doesn't pass one. This also impacts the default block cache when the user doesn't explict provide one.
+* Change the default of delayed slowdown value to 16MB/s and further increase the L0 stop condition to 36 files.
+
+### New Features
+* Added new overloaded function GetApproximateSizes that allows to specify if memtable stats should be computed only without computing SST files' stats approximations.
+* Added new function GetApproximateMemTableStats that approximates both number of records and size of memtables.
+* (Experimental) Two-level indexing that partition the index and creates a 2nd level index on the partitions. The feature can be enabled by setting kTwoLevelIndexSearch as IndexType and configuring index_per_partition.
+
+### Bug Fixes
+* RangeSync() should work if ROCKSDB_FALLOCATE_PRESENT is not set
+* Fix wrong results in a data race case in Get()
+* Some fixes related to 2PC.
+* Fix several bugs in Direct I/O supports.
+* Fix a regression bug which can cause Seek() to miss some keys if the return key has been updated many times after the snapshot which is used by the iterator.