5.6.1 release blog post

Summary:
5.6.1 release blog post
Closes https://github.com/facebook/rocksdb/pull/2638

Differential Revision: D5491168

Pulled By: yiwu-arbug

fbshipit-source-id: 14e3a92a03684afa4bd19bfb3ffb053cc09f5d4a

docs/_posts/2017-07-25-rocksdb-5-6-1-released.markdown

@@ -0,0 +1,22 @@
+---
+title: RocksDB 5.6.1 Released!
+layout: post
+author: yiwu
+category: blog
+---
+
+### Public API Change
+* Scheduling flushes and compactions in the same thread pool is no longer supported by setting `max_background_flushes=0`. Instead, users can achieve this by configuring their high-pri thread pool to have zero threads. See https://github.com/facebook/rocksdb/wiki/Thread-Pool for more details.
+* Replace `Options::max_background_flushes`, `Options::max_background_compactions`, and `Options::base_background_compactions` all with `Options::max_background_jobs`, which automatically decides how many threads to allocate towards flush/compaction.
+* options.delayed_write_rate by default take the value of options.rate_limiter rate.
+* Replace global variable `IOStatsContext iostats_context` with `IOStatsContext* get_iostats_context()`; replace global variable `PerfContext perf_context` with `PerfContext* get_perf_context()`.
+
+### New Features
+* Change ticker/histogram statistics implementations to use core-local storage. This improves aggregation speed compared to our previous thread-local approach, particularly for applications with many threads. See http://rocksdb.org/blog/2017/05/14/core-local-stats.html for more details.
+* Users can pass a cache object to write buffer manager, so that they can cap memory usage for memtable and block cache using one single limit.
+* Flush will be triggered when 7/8 of the limit introduced by write_buffer_manager or db_write_buffer_size is triggered, so that the hard threshold is hard to hit. See https://github.com/facebook/rocksdb/wiki/Write-Buffer-Manager for more details.
+* Introduce WriteOptions.low_pri. If it is true, low priority writes will be throttled if the compaction is behind. See https://github.com/facebook/rocksdb/wiki/Low-Priority-Write for more details.
+* `DB::IngestExternalFile()` now supports ingesting files into a database containing range deletions.
+
+### Bug Fixes
+* Shouldn't ignore return value of fsync() in flush.