Improve Put()'s comment to indicate that the key is overwritten if existing

Summary: As title

Test Plan: Not needed for comment only.

Reviewers: yhchiang, ljin, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: xjin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19887
Remove stats logger

Summary: Browsing through the code, looks like StatsLogger is not used at all!

Test Plan: compiles

Reviewers: ljin, sdong, yhchiang, dhruba

Reviewed By: dhruba

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19827
Support multiple DB directories in universal compaction style

Summary:
This patch adds a target size parameter in options.db_paths and universal compaction will base it to determine which DB path to place a new file.
Level-style stays the same.

Test Plan: Add new unit tests

Reviewers: ljin, yhchiang

Reviewed By: yhchiang

Subscribers: MarkCallaghan, dhruba, igor, leveldb

Differential Revision: https://reviews.facebook.net/D19869
Add Prev() for merge operator

Summary: Implement Prev() with merge operator for DBIterator. Request from mongoDB. Task 4673663.

Test Plan: make all check

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19743
Fix compressed cache
store file_indexer info in sequential memory

Summary:
  use arena to allocate space for next_level_index_ and level_rb_
  Thus increasing data locality and make Version::Get faster.

Benchmark detail
Base version: commit d2a727c182338514af955bbcb1d92db4af83b41c

command used:
./db_bench --db=/mnt/db/rocksdb --num_levels=6 --key_size=20 --prefix_size=20 --keys_per_prefix=0 --value_size=100 --block_size=4096 --cache_size=17179869184 --cache_numshardbits=6 --compression_type=none --compression_ratio=1 --min_level_to_compress=-1 --disable_seek_compaction=1 --hard_rate_limit=2 --write_buffer_size=134217728 --max_write_buffer_number=2 --level0_file_num_compaction_trigger=8 --target_file_size_base=2097152 --max_bytes_for_level_base=1073741824 --disable_wal=0 --sync=0 --disable_data_sync=1 --verify_checksum=1 --delete_obsolete_files_period_micros=314572800 --max_grandparent_overlap_factor=10 --max_background_compactions=4 --max_background_flushes=0 --level0_slowdown_writes_trigger=16 --level0_stop_writes_trigger=24 --statistics=0 --stats_per_interval=0 --stats_interval=1048576 --histogram=0 --use_plain_table=1 --open_files=-1 --mmap_read=1 --mmap_write=0 --memtablerep=prefix_hash --bloom_bits=10 --bloom_locality=1 --perf_level=0 --benchmarks=fillseq, readrandom,readrandom,readrandom --use_existing_db=0 --num=52428800 --threads=1

Result:
cpu running percentage:
Version::Get, improved from 7.98% to 7.42%
FileIndexer::GetNextLevelIndex, improved from 1.18% to 0.68%.

Test Plan:
  make all check

Reviewers: ljin, haobo, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, igor

Differential Revision: https://reviews.facebook.net/D19845
Refactoring Version::Get()

Summary: Refactoring Version::Get() method to move file picker logic to a separate class.

Test Plan: make check all

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19713
Fix db_test

Summary: Added deletion of DBIterators in DBIterator's tests

Test Plan: make valgrind_check

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20043
fix bug in LOG for flush memtable

Summary:
  One line change to fix a bug in the LOG when flush memtable

Test Plan:
  NONE

Reviewers: sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D20049
enable kHashSearch for blocktable in db_bench

Summary:
  add a flag called use_hash_search in db_bench

Test Plan:
  make all check
  ./db_bench --use_hash_search=1

Reviewers: ljin, haobo, yhchiang, sdong

Reviewed By: sdong

Subscribers: igor, dhruba

Differential Revision: https://reviews.facebook.net/D20067
Add struct CompactionInputFiles to manage compaction input files.

Summary: Add struct CompactionInputFiles to manage compaction input files.

Test Plan:
export ROCKSDB_TESTS=Compact
make db_test
./db_test

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20061
Allow class Compaction to handle input files from multiple levels.

Summary:
Allow class Compaction to handle input files from multiple levels.
This diff is a subset of https://reviews.facebook.net/D19263 where
only db/compaction.cc and db/compaction.h are changed.

Test Plan:
make db_test
export ROCKSDB_TESTS=Compaction
./db_test

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19923
Guarding files_ attribute with #ifndef NDEBUG guard in FilePicker class.

Summary: Adding guards to files_ attribute of FilePicker class. This attribute is used only in DEBUG mode. This fixes build of static_lib in mac.

Test Plan:
make static_lib in mac
make check all in devserver

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D20163
Fixed a signed and unsigned comparison in Compaction

Summary:
Fixed a signed and unsigned comparison in Compaction

Test Plan:
make db_test
export ROCKSDB_TESTS=Compaction
./db_test
Add -Wsign-compare to WARNING_FLAGS in Makefile

Summary:
Add -Wsign-compare to WARNING_FLAGS in Makefile as not all g++ compiler
include -Wsign-compare in -Wall when compiling '.h' file.

Test Plan: make -j32

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D20169
Add MaxInputLevel() to CompactionPicker

Summary:
Having if-then branch for different compaction strategies is considered
hacky and make CompactionPicker less pluggable.  This diff removes two
of such if-then branches in version_set.cc by adding MaxInputLevel() to
CompactionPicker.

    // Given the current number of levels, returns the lowest allowed level
    // for compaction input.
    virtual int MaxInputLevel(int current_num_levels) const;

Test Plan:
make db_test
export ROCKSDB_TESTS=Compaction
./db_test

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19971
Add PlainTableOptions

Summary:
Since we have a lot of options for PlainTable, add a struct PlainTableOptions
to manage them

Test Plan: make all check

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20175
Adding option to save PlainTable index and bloom filter in SST file.

Summary:
Adding option to save PlainTable index and bloom filter in SST file.
If there is no bloom block and/or index block, PlainTableReader builds
new ones. Otherwise PlainTableReader just use these blocks.

Test Plan: make all check

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19527
Update HISTORY.md

Summary: Updating HISTORY.md to reflect changes in PlainTable index

Test Plan: no need

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20229
Fix clang compiler warnings