Update documentation

Summary:
Added more options for compaction settings + thread pools.

Please check if thread pool description is correct.

Test Plan: -

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14043

doc/index.html

@@ -387,7 +387,8 @@ of point reads of small values may wish to switch to a smaller block
 size if performance measurements indicate an improvement.  There isn't
 much benefit in using blocks smaller than one kilobyte, or larger than
 a few megabytes.  Also note that compression will be more effective
-with larger block sizes.
+with larger block sizes. To change block size parameter, use
+<code>Options::block_size</code>.
 <p>
 <h2>Write buffer</h2>
 <p>
@@ -434,7 +435,7 @@ filesystem and each file stores a sequence of compressed blocks.  If
 used uncompressed block contents. If <code>options.block_cache_compressed</code>
 is non-NULL, it is used to cache frequently used compressed blocks. Compressed
 cache is an alternative to OS cache, which also caches compressed blocks. If
-compressed cache is used, you should disable OS cache by setting
+compressed cache is used, the OS cache will be disabled automatically by setting
 <code>options.allow_os_buffer</code> to false.
 <p>
 <pre>
@@ -588,7 +589,7 @@ Here we give overview of the options that impact behavior of Compactions:
 <ul>
 <p>
 <li><code>Options::compaction_style</code> - RocksDB currently supports two
-compaction algorithms - Compaction style and Level style. This option switches
+compaction algorithms - Universal  style and Level style. This option switches
 between the two.  Can be kCompactionStyleUniversal or kCompactionStyleLevel.
 If this is kCompactionStyleUniversal, then you can configure universal style
 parameters with <code>Options::compaction_options_universal</code>.
@@ -608,16 +609,126 @@ key-value during background compaction.
 </ul>
 <p>
 Other options impacting performance of compactions and when they get triggered
-are: <code>access_hint_on_compaction_start</code>,
-<code>level0_file_num_compaction_trigger</code>,
-<code>max_mem_compaction_level</code>, <code>target_file_size_base</code>,
-<code>target_file_size_multiplier</code>,
-<code>expanded_compaction_factor</code>, <code>source_compaction_factor</code>,
-<code>max_grandparent_overlap_factor</code>,
-<code>disable_seek_compaction</code>, <code>max_background_compactions</code>.
+are: 
+<ul>
+<p>
+<li> <code>Options::access_hint_on_compaction_start</code> - Specify the file access 
+pattern once a compaction is started. It will be applied to all input files of a compaction. Default: NORMAL
+<p>
+<li> <code>Options::level0_file_num_compaction_trigger</code> -  Number of files to trigger level-0 compaction. 
+A negative value means that level-0 compaction will not be triggered by number of files at all.
+<p>
+<li> <code>Options::max_mem_compaction_level</code> -  Maximum level to which a new compacted memtable is pushed if it
+does not create overlap.  We try to push to level 2 to avoid the relatively expensive level 0=>1 compactions and to avoid some
+expensive manifest file operations.  We do not push all the way to the largest level since that can generate a lot of wasted disk
+space if the same key space is being repeatedly overwritten.
+<p>
+<li> <code>Options::target_file_size_base</code> and <code>Options::target_file_size_multiplier</code> - 
+Target file size for compaction.  target_file_size_base is per-file size for level-1.
+Target file size for level L can be calculated by target_file_size_base * (target_file_size_multiplier ^ (L-1))
+For example, if target_file_size_base is 2MB and target_file_size_multiplier is 10, then each file on level-1 will
+be 2MB, and each file on level 2 will be 20MB, and each file on level-3 will be 200MB. Default target_file_size_base is 2MB
+and default target_file_size_multiplier is 1.
+<p>
+<li> <code>Options::expanded_compaction_factor</code> -  Maximum number of bytes in all compacted files.  We avoid expanding
+the lower level file set of a compaction if it would make the total compaction cover more than
+(expanded_compaction_factor * targetFileSizeLevel()) many bytes.
+<p>
+<li> <code>Options::source_compaction_factor</code> -    Maximum number of bytes in all source files to be compacted in a
+single compaction run. We avoid picking too many files in the source level so that we do not exceed the total source bytes
+for compaction to exceed (source_compaction_factor * targetFileSizeLevel()) many bytes.
+Default:1, i.e. pick maxfilesize amount of data as the source of a compaction.
+<p>
+<li> <code>Options::max_grandparent_overlap_factor</code> -   Control maximum bytes of overlaps in grandparent (i.e., level+2) before we
+stop building a single file in a level->level+1 compaction.
+<p>
+<li> <code>Options::disable_seek_compaction</code> -  Disable compaction triggered by seek.
+With bloomfilter and fast storage, a miss on one level is very cheap if the file handle is cached in table cache
+(which is true if max_open_files is large).
+<p>
+<li> <code>Options::max_background_compactions</code> - Maximum number of concurrent background jobs, submitted to
+the default LOW priority thread pool
+</ul>
+
 <p>
 You can learn more about all of those options in <code>rocksdb/options.h</code>
 
+<h2> Universal style compaction specific settings</h2>
+<p>
+If you're using Universal style compaction, there is an object <code>CompactionOptionsUniversal</code>
+that hold all the different options for that compaction. The exact definition is in
+<code>rocksdb/universal_compaction.h</code> and you can set it in <code>Options::compaction_options_universal</code>.
+Here we give short overview of options in <code>CompactionOptionsUniversal</code>:
+<ul>
+<p>
+<li> <code>CompactionOptionsUniversal::size_ratio</code> - Percentage flexibilty while comparing file size. If the candidate file(s)
+   size is 1% smaller than the next file's size, then include next file into
+   this candidate set.  Default: 1
+<p>
+<li> <code>CompactionOptionsUniversal::min_merge_width</code> - The minimum number of files in a single compaction run. Default: 2
+<p>
+<li> <code>CompactionOptionsUniversal::max_merge_width</code> - The maximum number of files in a single compaction run. Default: UINT_MAX
+<p>
+<li> <code>CompactionOptionsUniversal::max_size_amplification_percent</code> - The size amplification is defined as the amount (in percentage) of
+additional storage needed to store a single byte of data in the database.  For example, a size amplification of 2% means that a database that
+contains 100 bytes of user-data may occupy upto 102 bytes of physical storage. By this definition, a fully compacted database has
+a size amplification of 0%. Rocksdb uses the following heuristic to calculate size amplification: it assumes that all files excluding
+the earliest file contribute to the size amplification.  Default: 200, which means that a 100 byte database could require upto
+300 bytes of storage.
+<p>
+<li> <code>CompactionOptionsUniversal::compression_size_percent</code> - If this option is set to be -1 (the default value), all the output files
+will follow compression type specified.  If this option is not negative, we will try to make sure compressed
+size is just above this value. In normal cases, at least this percentage
+of data will be compressed.
+When we are compacting to a new file, here is the criteria whether
+it needs to be compressed: assuming here are the list of files sorted
+by generation time: [ A1...An B1...Bm C1...Ct ],
+where A1 is the newest and Ct is the oldest, and we are going to compact
+B1...Bm, we calculate the total size of all the files as total_size, as
+well as  the total size of C1...Ct as total_C, the compaction output file
+will be compressed iff total_C / total_size < this percentage
+<p>
+<li> <code>CompactionOptionsUniversal::stop_style</code> - The algorithm used to stop picking files into a single compaction run.
+Can be kCompactionStopStyleSimilarSize (pick files of similar size) or kCompactionStopStyleTotalSize (total size of picked files > next file).
+Default: kCompactionStopStyleTotalSize
+</ul>
+
+<h1>Thread pools</h1>
+<p>
+A thread pool is associated with Env environment object. The client has to create a thread pool by setting the number of background
+threads using method <code>Env::SetBackgroundThreads()</code> defined in <code>rocksdb/env.h</code>.
+We use the thread pool for compactions and memtable flushes.
+Since memtable flushes are in critical code path (stalling memtable flush can stall writes, increasing p99), we suggest 
+having two thread pools - with priorities HIGH and LOW. Memtable flushes can be set up to be scheduled on HIGH thread pool.
+There are two options available for configuration of background compactions and flushes:
+<ul>
+<p>
+<li> <code>Options::max_background_compactions</code> - Maximum number of concurrent background jobs,
+submitted to the default LOW priority thread pool
+<p>
+<li> <code>Options::max_background_flushes</code> - Maximum number of concurrent background memtable flush jobs, submitted to
+the HIGH priority thread pool.  By default, all background jobs (major compaction and memtable flush) go
+to the LOW priority pool. If this option is set to a positive number, memtable flush jobs will be submitted to the HIGH priority pool.
+It is important when the same Env is shared by multiple db instances.  Without a separate pool, long running major compaction jobs could
+potentially block memtable flush jobs of other db instances, leading to unnecessary Put stalls.
+</ul>
+<p>
+<pre>
+  #include "rocksdb/env.h"
+  #include "rocksdb/db.h"
+
+  auto env = rocksdb::Env::Default();
+  env->SetBackgroundThreads(2, rocksdb::Env::LOW);
+  env->SetBackgroundThreads(1, rocksdb::Env::HIGH);
+  rocksdb::DB* db;
+  rocksdb::Options options;
+  options.env = env;
+  options.max_background_compactions = 2;
+  options.max_background_flushes = 1;
+  rocksdb::Status status = rocksdb::DB::Open(options, "/tmp/testdb", &amp;db);
+  assert(status.ok());
+  ...
+</pre>
 <h1>Approximate Sizes</h1>
 <p>
 The <code>GetApproximateSizes</code> method can used to get the approximate