Add Bulkoading IngestExternalFile blog post

Summary:
new blog post for bulkoading
Closes https://github.com/facebook/rocksdb/pull/1883

Differential Revision: D4671984

Pulled By: IslamAbdelRahman

fbshipit-source-id: 3450860

docs/_posts/2017-02-17-bulkoad-ingest-sst-file.markdown

@@ -0,0 +1,50 @@
+---
+title: Bulkoading by ingesting external SST files
+layout: post
+author: IslamAbdelRahman
+category: blog
+---
+
+## Introduction
+
+One of the basic operations of RocksDB is writing to RocksDB, Writes happen when user call (DB::Put, DB::Write, DB::Delete ... ), but what happens when you write to RocksDB ? .. this is a brief description of what happens.
+- User insert a new key/value by calling DB::Put() (or DB::Write())
+- We create a new entry for the new key/value in our in-memory structure (memtable / SkipList by default) and we assign it a new sequence number.
+- When the memtable exceeds a specific size (64 MB for example), we convert this memtable to a SST file, and put this file in level 0 of our LSM-Tree
+- Later, compaction will kick in and move data from level 0 to level 1, and then from level 1 to level 2 .. and so on 
+
+But what if we can skip these steps and add data to the lowest possible level directly ? This is what bulk-loading does
+
+## Bulkloading
+
+- Write all of our keys and values into SST file outside of the DB
+- Add the SST file into the LSM directly
+
+This is bulk-loading, and in specific use-cases it allow users to achieve faster data loading and better write-amplification.
+
+and doing it is as simple as 
+```cpp
+Options options;
+SstFileWriter sst_file_writer(EnvOptions(), options, options.comparator);
+Status s = sst_file_writer.Open(file_path);
+assert(s.ok());
+
+// Insert rows into the SST file, note that inserted keys must be 
+// strictly increasing (based on options.comparator)
+for (...) {
+  s = sst_file_writer.Add(key, value);
+  assert(s.ok());
+}
+
+// Ingest the external SST file into the DB
+s = db_->IngestExternalFile({"/home/usr/file1.sst"}, IngestExternalFileOptions());
+assert(s.ok());
+```
+
+You can find more details about how to generate SST files and ingesting them into RocksDB in this [wiki page](https://github.com/facebook/rocksdb/wiki/Creating-and-Ingesting-SST-files)
+
+## Use cases
+There are multiple use cases where bulkloading could be useful, for example
+- Generating SST files in offline jobs in Hadoop, then downloading and ingesting the SST files into RocksDB
+- Migrating shards between machines by dumping key-range in SST File and loading the file in a different machine
+- Migrating from a different storage (InnoDB to RocksDB migration in MyRocks)