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  1. 18 2月, 2021 1 次提交
  3. 05 12月, 2020 1 次提交
    • L
      Add blob support to DBIter (#7731) · 61932cdf
      由 Levi Tamasi 提交于 
      Summary:
The patch adds iterator support to the integrated BlobDB implementation.
Whenever a blob reference is encountered during iteration, the corresponding
blob is retrieved by calling `Version::GetBlob`, assuming the `expose_blob_index`
(formerly `allow_blob`) flag is *not* set. (Note: the flag is set by the old stacked
BlobDB implementation, which has its own blob file handling/blob retrieval logic.)

In addition, `DBIter` now uniformly returns `Status::NotSupported` with the error
message `"BlobDB does not support merge operator."` when encountering a
blob reference while performing a merge (instead of potentially returning a
message that implies the database should be opened using the stacked BlobDB's
`Open`.)

TODO: We can implement support for lazily retrieving the blob value (or in other
words, bypassing the retrieval of blob values based on key) by extending the `Iterator`
API with a new `PrepareValue` method (similarly to `InternalIterator`, which already
supports lazy values).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7731

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D25256293

Pulled By: ltamasi

fbshipit-source-id: c39cd782011495a526cdff99c16f5fca400c4811
      61932cdf
  5. 18 9月, 2020 1 次提交
  7. 07 8月, 2020 1 次提交
  9. 04 8月, 2020 1 次提交
    • A
      dedup ReadOptions in iterator hierarchy (#7210) · a4a4a2da
      由 Andrew Kryczka 提交于 
      Summary:
Previously, a `ReadOptions` object was stored in every `BlockBasedTableIterator`
and every `LevelIterator`. This redundancy consumes extra memory,
resulting in the `Arena` making more allocations, and iteration
observing worse cache performance.

This PR migrates callers of `NewInternalIterator()` and
`MakeInputIterator()` to provide a `ReadOptions` object guaranteed to
outlive the returned iterator. When the iterator's lifetime will be managed by the
user, this lifetime guarantee is achieved by storing the `ReadOptions`
value in `ArenaWrappedDBIter`. Then, sub-iterators of `NewInternalIterator()` and
`MakeInputIterator()` can hold a reference-to-const `ReadOptions`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7210

Test Plan:
- `make check` under ASAN and valgrind
- benchmark: on a DB with 2 L0 files and 3 L1+ levels, this PR reduced `Arena` allocation 4792 -> 4160 bytes.

Reviewed By: anand1976

Differential Revision: D22861323

Pulled By: ajkr

fbshipit-source-id: 54aebb3e89c872eeab0f5793b4b6e42878d093ce
      a4a4a2da
  11. 03 7月, 2020 1 次提交
  13. 04 6月, 2020 1 次提交
    • Z
      API change: DB::OpenForReadOnly will not write to the file system unless... · 02df00d9
      由 Zitan Chen 提交于 
      API change: DB::OpenForReadOnly will not write to the file system unless create_if_missing is true (#6900)

Summary:
DB::OpenForReadOnly will not write anything to the file system (i.e., create directories or files for the DB) unless create_if_missing is true.

This change also fixes some subcommands of ldb, which write to the file system even if the purpose is for readonly.

Two tests for this updated behavior of DB::OpenForReadOnly are also added.

Other minor changes:
1. Updated HISTORY.md to include this API change of DB::OpenForReadOnly;
2. Updated the help information for the put and batchput subcommands of ldb with the option [--create_if_missing];
3. Updated the comment of Env::DeleteDir to emphasize that it returns OK only if the directory to be deleted is empty.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6900

Test Plan: passed make check; also manually tested a few ldb subcommands

Reviewed By: pdillinger

Differential Revision: D21822188

Pulled By: gg814

fbshipit-source-id: 604cc0f0d0326a937ee25a32cdc2b512f9a3be6e
      02df00d9
  15. 16 4月, 2020 1 次提交
    • M
      Properly report IO errors when IndexType::kBinarySearchWithFirstKey is used (#6621) · e45673de
      由 Mike Kolupaev 提交于 
      Summary:
Context: Index type `kBinarySearchWithFirstKey` added the ability for sst file iterator to sometimes report a key from index without reading the corresponding data block. This is useful when sst blocks are cut at some meaningful boundaries (e.g. one block per key prefix), and many seeks land between blocks (e.g. for each prefix, the ranges of keys in different sst files are nearly disjoint, so a typical seek needs to read a data block from only one file even if all files have the prefix). But this added a new error condition, which rocksdb code was really not equipped to deal with: `InternalIterator::value()` may fail with an IO error or Status::Incomplete, but it's just a method returning a Slice, with no way to report error instead. Before this PR, this type of error wasn't handled at all (an empty slice was returned), and kBinarySearchWithFirstKey implementation was considered a prototype.

Now that we (LogDevice) have experimented with kBinarySearchWithFirstKey for a while and confirmed that it's really useful, this PR is adding the missing error handling.

It's a pretty inconvenient situation implementation-wise. The error needs to be reported from InternalIterator when trying to access value. But there are ~700 call sites of `InternalIterator::value()`, most of which either can't hit the error condition (because the iterator is reading from memtable or from index or something) or wouldn't benefit from the deferred loading of the value (e.g. compaction iterator that reads all values anyway). Adding error handling to all these call sites would needlessly bloat the code. So instead I made the deferred value loading optional: only the call sites that may use deferred loading have to call the new method `PrepareValue()` before calling `value()`. The feature is enabled with a new bool argument `allow_unprepared_value` to a bunch of methods that create iterators (it wouldn't make sense to put it in ReadOptions because it's completely internal to iterators, with virtually no user-visible effect). Lmk if you have better ideas.

Note that the deferred value loading only happens for *internal* iterators. The user-visible iterator (DBIter) always prepares the value before returning from Seek/Next/etc. We could go further and add an API to defer that value loading too, but that's most likely not useful for LogDevice, so it doesn't seem worth the complexity for now.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6621

Test Plan: make -j5 check . Will also deploy to some logdevice test clusters and look at stats.

Reviewed By: siying

Differential Revision: D20786930

Pulled By: al13n321

fbshipit-source-id: 6da77d918bad3780522e918f17f4d5513d3e99ee
      e45673de
  17. 03 3月, 2020 1 次提交
    • H
      return timestamp from get (#6409) · 904a60ff
      由 Huisheng Liu 提交于 
      Summary:
Added new Get() methods that return timestamp. Dummy implementation is given so that classes derived from DB don't need to be touched to provide their implementation. MultiGet is not included.

ReadRandom perf test (10 minutes) on the same development machine ram drive with the same DB data shows no regression (within marge of error). The test is adapted from https://github.com/facebook/rocksdb/wiki/RocksDB-In-Memory-Workload-Performance-Benchmarks.
    base line (commit 72ee067b):
        101.712 micros/op 314602 ops/sec;   36.0 MB/s (5658999 of 5658999 found)
    This PR:
        100.288 micros/op 319071 ops/sec;   36.5 MB/s (5674999 of 5674999 found)

./db_bench --db=r:\rocksdb.github --num_levels=6 --key_size=20 --prefix_size=20 --keys_per_prefix=0 --value_size=100 --cache_size=2147483648 --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=134217728 --max_bytes_for_level_base=1073741824 --disable_wal=0 --wal_dir=r:\rocksdb.github\WAL_LOG --sync=0 --verify_checksum=1 --delete_obsolete_files_period_micros=314572800 --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 --duration=600 --benchmarks=readrandom --use_existing_db=1 --num=25000000 --threads=32
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6409

Differential Revision: D20200086

Pulled By: riversand963

fbshipit-source-id: 490edd74d924f62bd8ae9c29c2a6bbbb8410ca50
      904a60ff
  19. 21 2月, 2020 1 次提交
    • S
      Replace namespace name "rocksdb" with ROCKSDB_NAMESPACE (#6433) · fdf882de
      由 sdong 提交于 
      Summary:
When dynamically linking two binaries together, different builds of RocksDB from two sources might cause errors. To provide a tool for user to solve the problem, the RocksDB namespace is changed to a flag which can be overridden in build time.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6433

Test Plan: Build release, all and jtest. Try to build with ROCKSDB_NAMESPACE with another flag.

Differential Revision: D19977691

fbshipit-source-id: aa7f2d0972e1c31d75339ac48478f34f6cfcfb3e
      fdf882de
  21. 20 9月, 2019 1 次提交
  23. 14 9月, 2019 1 次提交
  25. 01 6月, 2019 2 次提交
  27. 01 3月, 2019 1 次提交
    • S
      Add two more StatsLevel (#5027) · 5e298f86
      由 Siying Dong 提交于 
      Summary:
Statistics cost too much CPU for some use cases. Add two stats levels
so that people can choose to skip two types of expensive stats, timers and
histograms.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5027

Differential Revision: D14252765

Pulled By: siying

fbshipit-source-id: 75ecec9eaa44c06118229df4f80c366115346592
      5e298f86
  29. 18 12月, 2018 1 次提交
  31. 29 11月, 2018 1 次提交
    • A
      Clean up FragmentedRangeTombstoneList (#4692) · 8fe1e06c
      由 Abhishek Madan 提交于 
      Summary:
Removed `one_time_use` flag, which removed the need for some
tests, and changed all `NewRangeTombstoneIterator` methods to return
`FragmentedRangeTombstoneIterators`.

These changes also led to removing `RangeDelAggregatorV2::AddUnfragmentedTombstones`
and one of the `MemTableListVersion::AddRangeTombstoneIterators` methods.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4692

Differential Revision: D13106570

Pulled By: abhimadan

fbshipit-source-id: cbab5432d7fc2d9cdfd8d9d40361a1bffaa8f845
      8fe1e06c
  33. 25 10月, 2018 1 次提交
    • A
      Use only "local" range tombstones during Get (#4449) · 8c78348c
      由 Abhishek Madan 提交于 
      Summary:
Previously, range tombstones were accumulated from every level, which
was necessary if a range tombstone in a higher level covered a key in a lower
level. However, RangeDelAggregator::AddTombstones's complexity is based on
the number of tombstones that are currently stored in it, which is wasteful in
the Get case, where we only need to know the highest sequence number of range
tombstones that cover the key from higher levels, and compute the highest covering
sequence number at the current level. This change introduces this optimization, and
removes the use of RangeDelAggregator from the Get path.

In the benchmark results, the following command was used to initialize the database:
```
./db_bench -db=/dev/shm/5k-rts -use_existing_db=false -benchmarks=filluniquerandom -write_buffer_size=1048576 -compression_type=lz4 -target_file_size_base=1048576 -max_bytes_for_level_base=4194304 -value_size=112 -key_size=16 -block_size=4096 -level_compaction_dynamic_level_bytes=true -num=5000000 -max_background_jobs=12 -benchmark_write_rate_limit=20971520 -range_tombstone_width=100 -writes_per_range_tombstone=100 -max_num_range_tombstones=50000 -bloom_bits=8
```

...and the following command was used to measure read throughput:
```
./db_bench -db=/dev/shm/5k-rts/ -use_existing_db=true -benchmarks=readrandom -disable_auto_compactions=true -num=5000000 -reads=100000 -threads=32
```

The filluniquerandom command was only run once, and the resulting database was used
to measure read performance before and after the PR. Both binaries were compiled with
`DEBUG_LEVEL=0`.

Readrandom results before PR:
```
readrandom   :       4.544 micros/op 220090 ops/sec;   16.9 MB/s (63103 of 100000 found)
```

Readrandom results after PR:
```
readrandom   :      11.147 micros/op 89707 ops/sec;    6.9 MB/s (63103 of 100000 found)
```

So it's actually slower right now, but this PR paves the way for future optimizations (see #4493).

----
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4449

Differential Revision: D10370575

Pulled By: abhimadan

fbshipit-source-id: 9a2e152be1ef36969055c0e9eb4beb0d96c11f4d
      8c78348c
  35. 23 8月, 2018 1 次提交
    • Z
      add missing counters in readonly mode (#4260) · f1f5ba08
      由 Zhongyi Xie 提交于 
      Summary:
User reported (https://github.com/facebook/rocksdb/issues/4168) that when opening RocksDB in read-only mode, some statistics are not correctly reported. After some investigation, we believe the following counters are indeed not reported during Get() call in a read-only DB:
rocksdb.memtable.hit
rocksdb.memtable.miss
rocksdb.number.keys.read
rocksdb.bytes.read
As well as histogram rocksdb.bytes.per.read
and perf context get_read_bytes
This PR will add the necessary counter reporting logic in the Get() call path
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4260

Differential Revision: D9476431

Pulled By: miasantreble

fbshipit-source-id: 7ab409d4e59df05d09ae8b69fe75554e5aa240d6
      f1f5ba08
  37. 01 8月, 2018 1 次提交
    • S
      Trace and Replay for RocksDB (#3837) · 12b6cdee
      由 Sagar Vemuri 提交于 
      Summary:
A framework for tracing and replaying RocksDB operations.

A binary trace file is created by capturing the DB operations, and it can be replayed back at the same rate using db_bench.

- Column-families are supported
- Multi-threaded tracing is supported.
- TraceReader and TraceWriter are exposed to the user, so that tracing to various destinations can be enabled (say, to other messaging/logging services). By default, a FileTraceReader and FileTraceWriter are implemented to capture to a file and replay from it.
- This is not yet ideal to be enabled in production due to large performance overhead, but it can be safely tried out in a shadow setup, say, for analyzing RocksDB operations.

Currently supported DB operations:
- Writes:
-- Put
-- Merge
-- Delete
-- SingleDelete
-- DeleteRange
-- Write
- Reads:
-- Get (point lookups)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/3837

Differential Revision: D7974837

Pulled By: sagar0

fbshipit-source-id: 8ec65aaf336504bc1f6ed0feae67f6ed5ef97a72
      12b6cdee
  39. 22 5月, 2018 1 次提交
    • Z
      Move prefix_extractor to MutableCFOptions · c3ebc758
      由 Zhongyi Xie 提交于 
      Summary:
Currently it is not possible to change bloom filter config without restart the db, which is causing a lot of operational complexity for users.
This PR aims to make it possible to dynamically change bloom filter config.
Closes https://github.com/facebook/rocksdb/pull/3601

Differential Revision: D7253114

Pulled By: miasantreble

fbshipit-source-id: f22595437d3e0b86c95918c484502de2ceca120c
      c3ebc758
  41. 13 4月, 2018 1 次提交
  43. 06 3月, 2018 1 次提交
  45. 23 2月, 2018 2 次提交
  47. 10 10月, 2017 1 次提交
    • Y
      WritePrepared Txn: Iterator · 8c392a31
      由 Yi Wu 提交于 
      Summary:
On iterator create, take a snapshot, create a ReadCallback and pass the ReadCallback to the underlying DBIter to check if key is committed.
Closes https://github.com/facebook/rocksdb/pull/2981

Differential Revision: D6001471

Pulled By: yiwu-arbug

fbshipit-source-id: 3565c4cdaf25370ba47008b0e0cb65b31dfe79fe
      8c392a31
  49. 06 10月, 2017 1 次提交
  51. 25 7月, 2017 1 次提交
    • S
      Add Iterator::Refresh() · e67b35c0
      由 Siying Dong 提交于 
      Summary:
Add and implement Iterator::Refresh(). When this function is called, if the super version doesn't change, update the sequence number of the iterator to the latest one and invalidate the iterator. If the super version changed, recreated the whole iterator. This can help users reuse the iterator more easily.
Closes https://github.com/facebook/rocksdb/pull/2621

Differential Revision: D5464500

Pulled By: siying

fbshipit-source-id: f548bd35e85c1efca2ea69273802f6704eba6ba9
      e67b35c0
  53. 22 7月, 2017 2 次提交
  55. 16 7月, 2017 1 次提交
  57. 25 6月, 2017 1 次提交
    • M
      Optimize for serial commits in 2PC · 499ebb3a
      由 Maysam Yabandeh 提交于 
      Summary:
Throughput: 46k tps in our sysbench settings (filling the details later)

The idea is to have the simplest change that gives us a reasonable boost
in 2PC throughput.

Major design changes:
1. The WAL file internal buffer is not flushed after each write. Instead
it is flushed before critical operations (WAL copy via fs) or when
FlushWAL is called by MySQL. Flushing the WAL buffer is also protected
via mutex_.
2. Use two sequence numbers: last seq, and last seq for write. Last seq
is the last visible sequence number for reads. Last seq for write is the
next sequence number that should be used to write to WAL/memtable. This
allows to have a memtable write be in parallel to WAL writes.
3. BatchGroup is not used for writes. This means that we can have
parallel writers which changes a major assumption in the code base. To
accommodate for that i) allow only 1 WriteImpl that intends to write to
memtable via mem_mutex_--which is fine since in 2PC almost all of the memtable writes
come via group commit phase which is serial anyway, ii) make all the
parts in the code base that assumed to be the only writer (via
EnterUnbatched) to also acquire mem_mutex_, iii) stat updates are
protected via a stat_mutex_.

Note: the first commit has the approach figured out but is not clean.
Submitting the PR anyway to get the early feedback on the approach. If
we are ok with the approach I will go ahead with this updates:
0) Rebase with Yi's pipelining changes
1) Currently batching is disabled by default to make sure that it will be
consistent with all unit tests. Will make this optional via a config.
2) A couple of unit tests are disabled. They need to be updated with the
serial commit of 2PC taken into account.
3) Replacing BatchGroup with mem_mutex_ got a bit ugly as it requires
releasing mutex_ beforehand (the same way EnterUnbatched does). This
needs to be cleaned up.
Closes https://github.com/facebook/rocksdb/pull/2345

Differential Revision: D5210732

Pulled By: maysamyabandeh

fbshipit-source-id: 78653bd95a35cd1e831e555e0e57bdfd695355a4
      499ebb3a
  59. 28 4月, 2017 1 次提交
  61. 11 4月, 2017 1 次提交
    • S
      Reduce the number of params needed to construct DBIter · 7124268a
      由 Sagar Vemuri 提交于 
      Summary:
DBIter, and in-turn NewDBIterator and NewArenaWrappedDBIterator, take a  bunch of params. They can be reduced by passing in ReadOptions directly instead of passing in every new param separately. It also seems much cleaner as a bunch of the params towards the end seem to be optional.

(Recently I introduced max_skippable_internal_keys, which added one more to the already huge count).

Idea courtesy IslamAbdelRahman
Closes https://github.com/facebook/rocksdb/pull/2116

Differential Revision: D4857128

Pulled By: sagar0

fbshipit-source-id: 7d239df094b94bd9ea79d145cdf825478ac037a8
      7124268a
  63. 06 4月, 2017 1 次提交
  65. 31 3月, 2017 1 次提交
    • S
      Option to fail a request as incomplete when skipping too many internal keys · c6d04f2e
      由 Sagar Vemuri 提交于 
      Summary:
Operations like Seek/Next/Prev sometimes take too long to complete when there are many internal keys to be skipped. Adding an option, max_skippable_internal_keys -- which could be used to set a threshold for the maximum number of keys that can be skipped, will help to address these cases where it is much better to fail a request (as incomplete) than to wait for a considerable time for the request to complete.

This feature -- to fail an iterator seek request as incomplete, is disabled by default when max_skippable_internal_keys = 0. It is enabled only when max_skippable_internal_keys > 0.

This feature is based on the discussion mentioned in the PR https://github.com/facebook/rocksdb/pull/1084.
Closes https://github.com/facebook/rocksdb/pull/2000

Differential Revision: D4753223

Pulled By: sagar0

fbshipit-source-id: 1c973f7
      c6d04f2e
  67. 16 3月, 2017 1 次提交
    • I
      Add macros to include file name and line number during Logging · e1916368
      由 Islam AbdelRahman 提交于 
      Summary:
current logging
```
2017/03/14-14:20:30.393432 7fedde9f5700 (Original Log Time 2017/03/14-14:20:30.393414) [default] Level summary: base level 1 max bytes base 268435456 files[1 0 0 0 0 0 0] max score 0.25
2017/03/14-14:20:30.393438 7fedde9f5700 [JOB 2] Try to delete WAL files size 61417909, prev total WAL file size 73820858, number of live WAL files 2.
2017/03/14-14:20:30.393464 7fedde9f5700 [DEBUG] [JOB 2] Delete /dev/shm/old_logging//MANIFEST-000001 type=3 #1 -- OK
2017/03/14-14:20:30.393472 7fedde9f5700 [DEBUG] [JOB 2] Delete /dev/shm/old_logging//000003.log type=0 #3 -- OK
2017/03/14-14:20:31.427103 7fedd49f1700 [default] New memtable created with log file: #9. Immutable memtables: 0.
2017/03/14-14:20:31.427179 7fedde9f5700 [JOB 3] Syncing log #6
2017/03/14-14:20:31.427190 7fedde9f5700 (Original Log Time 2017/03/14-14:20:31.427170) Calling FlushMemTableToOutputFile with column family [default], flush slots available 1, compaction slots allowed 1, compaction slots scheduled 1
2017/03/14-14:20:31.
Closes https://github.com/facebook/rocksdb/pull/1990

Differential Revision: D4708695

Pulled By: IslamAbdelRahman

fbshipit-source-id: cb8968f
      e1916368
  69. 14 3月, 2017 1 次提交
    • M
      Pinnableslice (2nd attempt) · 11526252
      由 Maysam Yabandeh 提交于 
      Summary:
PinnableSlice

    Summary:
    Currently the point lookup values are copied to a string provided by the
    user. This incures an extra memcpy cost. This patch allows doing point lookup
    via a PinnableSlice which pins the source memory location (instead of
    copying their content) and releases them after the content is consumed
    by the user. The old API of Get(string) is translated to the new API
    underneath.

    Here is the summary for improvements:

    value 100 byte: 1.8% regular, 1.2% merge values
    value 1k byte: 11.5% regular, 7.5% merge values
    value 10k byte: 26% regular, 29.9% merge values
    The improvement for merge could be more if we extend this approach to
    pin the merge output and delay the full merge operation until the user
    actually needs it. We have put that for future work.

    PS:
    Sometimes we observe a small decrease in performance when switching from
    t5452014 to this patch but with the old Get(string) API. The d
Closes https://github.com/facebook/rocksdb/pull/1756

Differential Revision: D4391738

Pulled By: maysamyabandeh

fbshipit-source-id: 6f3edd3
      11526252
  71. 09 1月, 2017 2 次提交
    • M
      Revert "PinnableSlice" · d0ba8ec8
      由 Maysam Yabandeh 提交于 
      Summary:
This reverts commit 54d94e9c.

The pull request was landed by mistake.
Closes https://github.com/facebook/rocksdb/pull/1755

Differential Revision: D4391678

Pulled By: maysamyabandeh

fbshipit-source-id: 36d5149
      d0ba8ec8
    • M
      PinnableSlice · 54d94e9c
      由 Maysam Yabandeh 提交于 
      Summary:
Currently the point lookup values are copied to a string provided by the user.
This incures an extra memcpy cost. This patch allows doing point lookup
via a PinnableSlice which pins the source memory location (instead of
copying their content) and releases them after the content is consumed
by the user. The old API of Get(string) is translated to the new API
underneath.

 Here is the summary for improvements:
 1. value 100 byte: 1.8%  regular, 1.2% merge values
 2. value 1k   byte: 11.5% regular, 7.5% merge values
 3. value 10k byte: 26% regular,    29.9% merge values

 The improvement for merge could be more if we extend this approach to
 pin the merge output and delay the full merge operation until the user
 actually needs it. We have put that for future work.

PS:
Sometimes we observe a small decrease in performance when switching from
t5452014 to this patch but with the old Get(string) API. The difference
is a little and could be noise. More importantly it is safely
cancelled
Closes https://github.com/facebook/rocksdb/pull/1732

Differential Revision: D4374613

Pulled By: maysamyabandeh

fbshipit-source-id: a077f1a
      54d94e9c