Blog post for write_unprepared (facebook#5711)

Summary:
Introducing write_unprepared feature.
Pull Request resolved: facebook#5711

Differential Revision: D16838307

Pulled By: maysamyabandeh

fbshipit-source-id: d9a4daf63dd0f855bea49c14ce84e6299f1401c7

docs/_posts/2019-08-15-unordered-write.markdown

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+---
+title: Higher write throughput with `unordered_write` feature
+layout: post
+author: maysamyabandeh
+category: blog
+---
+
+Since RocksDB 6.3, The `unordered_write=`true option together with WritePrepared transactions offers 34-42% higher write throughput compared to vanilla RocksDB. If the application can handle more relaxed ordering guarantees, the gain in throughput would increase to 63-131%.
+
+### Background
+
+Currently RocksDB API delivers the following powerful guarantees:
+- Atomic reads: Either all of a write batch is visible to reads or none of it.
+- Read-your-own writes: When a write thread returns to the user, a subsequent read by the same thread will be able to see its own writes.
+- Immutable Snapshots: The reads visible to the snapshot are immutable in the sense that it will not be affected by any in-flight or future writes.
+
+### `unordered_write`
+
+The `unordered_write` feature, when turned on, relaxes the default guarantees of RocksDB. While it still gives read-your-own-write property, neither atomic reads nor the immutable snapshot properties are provided any longer. However, RocksDB users could still get read-your-own-write and immutable snapshots when using this feature in conjunction with TransactionDB configured with WritePrepared transactions and `two_write_queues`. You can read [here](https://github.com/facebook/rocksdb/wiki/unordered_write) to learn about the design of `unordered_write` and [here](https://github.com/facebook/rocksdb/wiki/WritePrepared-Transactions) to learn more about WritePrepared transactions.
+
+### How to use it?
+
+To get the same guarantees as vanilla RocksdB:
+
+    DBOptions db_options;
+    db_options.unordered_write = true;
+    db_options.two_write_queues = true;
+    DB* db;
+    {
+      TransactionDBOptions txn_db_options;
+      txn_db_options.write_policy = TxnDBWritePolicy::WRITE_PREPARED;
+      txn_db_options.skip_concurrency_control = true;
+      TransactionDB* txn_db;
+      TransactionDB::Open(options, txn_db_options, kDBPath, &txn_db);
+      db = txn_db;
+    }
+    db->Write(...);
+
+To get relaxed guarantees:
+
+    DBOptions db_options;
+    db_options.unordered_write = true;
+    DB* db;
+    DB::Open(db_options, kDBPath, &db);
+    db->Write(...);
+
+# Benchmarks
+
+    TEST_TMPDIR=/dev/shm/ ~/db_bench --benchmarks=fillrandom --threads=32 --num=10000000 -max_write_buffer_number=16 --max_background_jobs=64 --batch_size=8 --writes=3000000 -level0_file_num_compaction_trigger=99999 --level0_slowdown_writes_trigger=99999 --level0_stop_writes_trigger=99999 -enable_pipelined_write=false -disable_auto_compactions --transaction_db=true --unordered_write=1 --disable_wal=0
+
+Throughput with `unordered_write`=true and using WritePrepared transaction:
+- WAL: +42%
+- No-WAL: +34%
+Throughput with `unordered_write`=true
+- WAL: +63%
+- NoWAL: +131%