Summary:
Our internal CI test caught RocksDB Lite build failures. The failures are due to a new test introduced in #4665 using `SSTFileWriter` and `IngestExternalFile`, but these is not exposed under lite mode. Fixed by #ifdef'ing out the test.
```
db/db_test2.cc: In member function ‘virtual void rocksdb::DBTest2_TestCompactFiles_Test::TestBody()’:
db/db_test2.cc:2907:3: error: ‘SstFileWriter’ is not a member of ‘rocksdb’
rocksdb::SstFileWriter sst_file_writer{rocksdb::EnvOptions(), options};
^
In file included from ./util/testharness.h:15:0,
from ./table/mock_table.h:23,
from ./db/db_test_util.h:44,
from db/db_test2.cc:13:
db/db_test2.cc:2912:13: error: ‘sst_file_writer’ was not declared in this scope
ASSERT_OK(sst_file_writer.Open(external_file1));
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4675
Differential Revision: D13035984
Pulled By: sagar0
fbshipit-source-id: c1ceac550dfac1a85eeea436693dc7dd467519a6
Summary:
In #4652 we are setting -Os for lite builds only when LITE=1 is specified. But currently almost all the users invoke lite build via OPT="-DROCKSDB_LITE=1". So this diff tries to set LITE=1 when users already pass in -DROCKSDB_LITE=1 via the command line.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4671
Differential Revision: D13033801
Pulled By: sagar0
fbshipit-source-id: e7b506cee574f9e3f42221ee6647915011c78d78
Summary:
Part of the test required that a compaction start before a
manual flush, but this was not enforced by the test. In some cases,
particularly when writing to tmpfs, this could lead to the compaction
starting after the flush, which caused the base level to be higher than
it was expected to be. Add a sync point in the test to ensure that the
flush and compaction happen simultaneously.
The test also had some stale comments, so those have been removed or
modified, and the test has been simplified so that it no longer uses sleeps
and writes uncompressed SSTs.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4668
Differential Revision: D13032440
Pulled By: abhimadan
fbshipit-source-id: 3f23b583a096454dafb8d8ea75678605dec80209
Summary:
`CompactFiles` gets `SuperVersion` before `WaitForIngestFile`, while `IngestExternalFile` may add files that overlap with `input_file_names`
The timeline of execution flow is as follow:
Let's say that level N has two file [1,2] and [5,6]
```
timeline user_thread1 user_thread2
t0 | CompactFiles([1, 2], [5, 6]) begin
t1 | GetReferencedSuperVersion()
t2 | IngestExternalFile([3,4]) to level N begin
t3 | CompactFiles resume
V
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4665
Differential Revision: D13030674
Pulled By: ajkr
fbshipit-source-id: 8be19477fd6e505032267a979d32f3097cc3be51
Summary:
In the past, both `DBImpl::atomic_flush_` and
`DBImpl::immutable_db_options_.atomic_flush` exist. However, we fail to set
`immutable_db_options_.atomic_flush`, but use `DBImpl::atomic_flush_` which is
set correctly. This does not lead to incorrect behavior, but is a duplicate of
information.
Since `immutable_db_options_` is always there and has `atomic_flush`, we should
use it as source of truth and remove `DBImpl::atomic_flush_`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4631
Differential Revision: D12928371
Pulled By: riversand963
fbshipit-source-id: f85a811959d3828aad4a3a1b05f71facf19c636d
Summary:
Hi, yiwu-arbug, I found that `DBImpl::GetColumnFamilyHandleUnlocked` still have data race condition, because `column_family_memtables_` has a stateful cache `current_` and `column_family_memtables_::Seek` maybe call without the protection of `mutex_` by a write thread
check 859dbda6e3/db/write_batch.cc (L1188) and 859dbda6e3/db/write_batch.cc (L1756) and 859dbda6e3/db/db_impl_write.cc (L318)
So it's better to use `versions_->GetColumnFamilySet()->GetColumnFamily` instead.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4666
Differential Revision: D13027117
Pulled By: yiwu-arbug
fbshipit-source-id: 4e3778eaf8e7f7c8577bbd78129b6a5fd7ce79fb
Summary:
Changes:
1. in current version, key size distribution is printed out as the result. In this change, the result will be output to a file to make further analyze easier
2. To understand how the unique keys are accessed over time, the total unique key number of each CF of each query type in each second over time is output to a file. In this way, user could know when the unique keys are accessed frequently or accessed rarely.
3. output the total QPS of each CF to a file
4. Add the print result of total queries of each CF of each query type.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4646
Differential Revision: D12968156
Pulled By: zhichao-cao
fbshipit-source-id: 6c411c7ec47c7843a70929136efd71a150db0e4c
Summary:
The flakyness can be reproduced with the following patch:
```
--- a/db/db_impl_compaction_flush.cc
+++ b/db/db_impl_compaction_flush.cc
@@ -2013,6 +2013,9 @@ void DBImpl::BackgroundCallFlush() {
if (job_context.HaveSomethingToDelete()) {
PurgeObsoleteFiles(job_context);
}
+ static int f_count = 0;
+ printf("clean flush job context %d\n", ++f_count);
+ env_->SleepForMicroseconds(1000000);
job_context.Clean();
mutex_.Lock();
}
```
The issue is that FlushMemtable with opt.wait=true does not wait for `OnStallConditionsChanged` being called. The event listener is triggered on `JobContext::Clean`, which happens after flush result is installed. At the time we check for stall condition after flushing memtable, the job context cleanup may not be finished.
To fix the flaykyness, we use sync point to create a custom WaitForFlush that waits for context cleanup.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4658
Differential Revision: D13007301
Pulled By: yiwu-arbug
fbshipit-source-id: d98395ee7b0ad4c62e83e8d0e9b6028058c61712
Summary:
When running `make shared_lib` under fbcode, there's liblua link error: https://gist.github.com/yiwu-arbug/b796bff6b3d46d90c1ed878d983de50d
This is because we link liblua.a when building shared lib. If we want to link with liblua, we need to link with liblua_pic.a instead. Fixing by simply not link with lua.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4651
Differential Revision: D12964798
Pulled By: yiwu-arbug
fbshipit-source-id: 18d6cee94afe20748068822b76e29ef255cdb04d
Summary:
Updated the comments around all options which are currently dynamic.
Without explicitly specifying in the comments around the options, its hard for RocksDB users to know if an option is dynamically changeable or not unless they dig into the implementation details.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4653
Differential Revision: D12966735
Pulled By: sagar0
fbshipit-source-id: 1a01acbf6fe506b989e71629ce223f9803ebae27
Summary:
Ran the following commands to recursively change all the files under RocksDB:
```
find . -type f -name "*.cc" -exec sed -i 's/ unique_ptr/ std::unique_ptr/g' {} +
find . -type f -name "*.cc" -exec sed -i 's/<unique_ptr/<std::unique_ptr/g' {} +
find . -type f -name "*.cc" -exec sed -i 's/ shared_ptr/ std::shared_ptr/g' {} +
find . -type f -name "*.cc" -exec sed -i 's/<shared_ptr/<std::shared_ptr/g' {} +
```
Running `make format` updated some formatting on the files touched.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4638
Differential Revision: D12934992
Pulled By: sagar0
fbshipit-source-id: 45a15d23c230cdd64c08f9c0243e5183934338a8
Summary:
We already exercised backup functionality in `db_stress` according to the `-backup_one_in` flag. This PR verifies the backup can be restored/opened and sanity checks a few keys. Changes in this PR:
- Extracted existing backup-related logic to a helper function, `TestBackupRestore`
- Added restore logic, which targets a hidden directory named "./.restore\<thread number\>", similar to how backups target hidden directories named "./.backup\<thread number\>".
- After restore, check the existence/non-existence of a few keys.
- With this PR, backup is no longer compatible with clearing column families.
- Also included unrelated fixes to set `ReadOptions::total_order_seek=true` when using `-compare_full_db_state_snapshot`
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4655
Differential Revision: D12972496
Pulled By: ajkr
fbshipit-source-id: 481a40052d9a38d1bd5c5159aa4d7c5a4b546b80
Summary:
Ensure delete[] and not delete is called on buffer_, as it is reset with new char[buffer_size_].
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4647
Differential Revision: D12961327
Pulled By: ajkr
fbshipit-source-id: c1af373b98359edfdc291caebe4e0acdfb8afdd8
Summary:
valgrind tests with 1 thread run too long. To make it shorter, black list some long tests. These are already blacklisted in parallel valgrind tests, but they are not in non-parallel mode
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4642
Differential Revision: D12945237
Pulled By: siying
fbshipit-source-id: 04cf977d435996480fe87aa09f14b17975b74f7d
Summary:
Move the line `Add xxhash64 checksum support` to `Unreleased` section because it has not been released to 5.17.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4627
Differential Revision: D12944123
Pulled By: riversand963
fbshipit-source-id: 2762857065b9e741c64ff8b6116ca62fe31891d8
Summary:
When evicting an entry form the commit_cache, it is verified against the list of old snapshots to see if it overlaps with any. The list of old snapshots is split into two lists: an efficient concurrent cache and an slow vector protected by a lock. The patch fixes a bug that would stop the search in the cache if it finds any and yet would not include the larger snapshots in the slower list.
An extra info log entry is also removed. The condition to trigger that although very rare is still feasible and should not spam the LOG when that happens.
Fixes#4621
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4639
Differential Revision: D12934989
Pulled By: maysamyabandeh
fbshipit-source-id: 4e0fe8147ba292b554ae78e94c21c2ef31e03e2d
Summary:
This property can help debug why SST files aren't being deleted. Previously we only had the property "rocksdb.is-file-deletions-enabled". However, even when that returned true, obsolete SSTs may still not be deleted due to the coarse-grained mechanism we use to prevent newly created SSTs from being accidentally deleted. That coarse-grained mechanism uses a lower bound file number for SSTs that should not be deleted, and this property exposes that lower bound.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4618
Differential Revision: D12898179
Pulled By: ajkr
fbshipit-source-id: fe68acc041ddbcc9276bbd48976524d95aafc776
Summary:
ExternalSSTFileTest.IngestNonExistingFile occasionally fail for number of SST files after manual compaction doesn't go down as expected. Although I don't find a reason how this can happen, adding an extra waiting to make sure obsolete file purging has finished before we check the files doesn't hurt.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4625
Differential Revision: D12910586
Pulled By: siying
fbshipit-source-id: 2a5ddec6908c99cf3bcc78431c6f93151c2cab59
Summary:
fix current failing lite test:
> In file included from ./util/testharness.h:15:0,
from ./table/mock_table.h:23,
from ./db/db_test_util.h:44,
from db/db_flush_test.cc:10:
db/db_flush_test.cc: In member function ‘virtual void rocksdb::DBFlushTest_ManualFlushFailsInReadOnlyMode_Test::TestBody()’:
db/db_flush_test.cc:250:35: error: ‘Properties’ is not a member of ‘rocksdb::DB’
ASSERT_TRUE(db_->GetIntProperty(DB::Properties::kBackgroundErrors,
^
make: *** [db/db_flush_test.o] Error 1
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4619
Differential Revision: D12898319
Pulled By: miasantreble
fbshipit-source-id: 72de603b1f2e972fc8caa88611798c4e98e348c6
Summary:
In 'StatisticsCollector', the call of Thread.sleep() might be better outside the loop?
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4588
Differential Revision: D12903406
Pulled By: sagar0
fbshipit-source-id: 1647ed779e9972bc2cea03f4c38e37ab3ad7c361
Summary:
The new case is directIO = true, write_global_seqno = false in which we no longer write global_seqno to the external SST file.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4614
Differential Revision: D12885001
Pulled By: riversand963
fbshipit-source-id: 7541bdc608b3a0c93d3c3c435da1b162b36673d4
Summary:
Long absolute file names in log make it hard to read the LOG files.
So we shorter them to relative to the root of RocksDB project path.
In most cases, they will only have one level directory and one file name.
There was [a talk](#4316) about making "util/logging.h" a public header file.
But we concern the conflicts that might be introduced in for macros
named `STRINGIFY`, `TOSTRING`, and `PREPEND_FILE_LINE`.
So I prepend a prefix `ROCKS_LOG_` to them.
I also remove the line that includes "port.h" which seems unneccessary here.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4616
Differential Revision: D12892857
Pulled By: siying
fbshipit-source-id: af79aaf82153b8fd66b5966aced39a51fbca9c6c
Summary:
The logic to wait for stall conditions to clear before beginning a manual flush didn't take into account whether the DB was in read-only mode. In read-only mode the stall conditions would never clear since no background work is happening, so the wait would be never-ending. It's probably better to return an error to the user.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4615
Differential Revision: D12888008
Pulled By: ajkr
fbshipit-source-id: 1c474b42a7ac38d9fd0d0e2340ff1d53e684d83c
Summary:
A background compaction with pre-picked files (i.e., either a manual compaction or a bottom-pri compaction) fails when the DB is in read-only mode. In the failure handling, we forgot to unregister the compaction and the files it covered. Then subsequent manual compactions could conflict with this zombie compaction (possibly Halloween related) and wait forever for it to finish.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4611
Differential Revision: D12871217
Pulled By: ajkr
fbshipit-source-id: 9d24e921d5bbd2ee8c2c9536a30abfa42a220c6e
Summary:
Originally, the manual flush calls in db_stress flushes only a single column
family, which is not sufficient when atomic flush is enabled.
With atomic flush, we should call `Flush(flush_opts, cfhs)` to better test this
new feature. Specifically, we manuall flush all column families so that
database verification is easier.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4608
Differential Revision: D12849160
Pulled By: riversand963
fbshipit-source-id: ae1f0dd825247b42c0aba520a5c967335102c876
Summary:
Add unit tests to demonstrate that `VersionSet::Recover` is able to detect and handle cases in which the MANIFEST has valid atomic group, incomplete trailing atomic group, atomic group mixed with normal version edits and atomic group with incorrect size.
With this capability, RocksDB identifies non-valid groups of version edits and do not apply them, thus guaranteeing that the db is restored to a state consistent with the most recent successful atomic flush before applying WAL.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4433
Differential Revision: D10079202
Pulled By: riversand963
fbshipit-source-id: a0e0b8bf4da1cf68e044d397588c121b66c68876
Summary:
Since the number of range deletions are reported in
TableProperties, it is confusing to not report the number of merge
operands and point deletions as top-level properties; they are
accessible through the public API, but since they are not the "main"
properties, they do not appear in aggregated table properties, or the
string representation of table properties.
This change promotes those two property keys to
`rocksdb/table_properties.h`, adds corresponding uint64 members for
them, deprecates the old access methods `GetDeletedKeys()` and
`GetMergeOperands()` (though they are still usable for now), and removes
`InternalKeyPropertiesCollector`. The property key strings are the same
as before this change, so this should be able to read DBs written from older
versions (though I haven't tested this yet).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4594
Differential Revision: D12826893
Pulled By: abhimadan
fbshipit-source-id: 9e4e4fbdc5b0da161c89582566d184101ba8eb68
Summary:
EnableFileDeletions() does info logging inside db mutex. This is not recommended in the code base, since there could be I/O involved. Move this outside the DB mutex.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4604
Differential Revision: D12834432
Pulled By: siying
fbshipit-source-id: ffe5c2626fcfdb4c54a661a3c3b0bc95054816cf
Summary:
When there's a gap between files, we do not need to output tombstones starting at the next output file's begin key to the current output file.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4592
Differential Revision: D12808627
Pulled By: ajkr
fbshipit-source-id: 77c8b2e7523a95b1cd6611194144092c06acb505
Summary:
Since ErrorHandler::RecoverFromNoSpace is no-op in LITE mode, then we should
not have this test in LITE mode. If we do keep it, it will cause the test
thread to wait on bg_cv_ that will not be signalled.
How to reproduce
```
$make clean && git checkout a27fce408e
$OPT="-DROCKSDB_LITE -g" make -j20
$./db_io_failure_test --gtest_filter=DBIOFailureTest.NoSpaceCompactRange
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4596
Differential Revision: D12818516
Pulled By: riversand963
fbshipit-source-id: bc83524f40fff1e29506979017f7f4c2b70322f3
Summary:
For flush triggered by RocksDB due to memory usage approaching certain
threshold (WriteBufferManager or Memtable full), we should cut the memtable
only when the current active memtable is not empty, i.e. contains data. This is
what we do for non-atomic flush. If we always cut memtable even when the active
memtable is empty, we will generate extra, empty immutable memtable.
This is not ideal since it may cause write stall. It also causes some
DBAtomicFlushTest to fail because cfd->imm()->NumNotFlushed() is different from
expectation.
Test plan
```
$make clean && make J=1 -j32 all check
$make clean && OPT="-DROCKSDB_LITE -g" make J=1 -j32 all check
$make clean && TEST_TMPDIR=/dev/shm/rocksdb OPT=-g make J=1 -j32 valgrind_test
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4595
Differential Revision: D12818520
Pulled By: riversand963
fbshipit-source-id: d867bdbeacf4199fdd642debb085f94703c41a18
Summary:
Introduce `JemallocNodumpAllocator`, which allow exclusion of block cache usage from core dump. It utilize custom hook of jemalloc arena, and when jemalloc arena request memory from system, the allocator use the hook to set `MADV_DONTDUMP ` to the memory. The implementation is basically the same as `folly::JemallocNodumpAllocator`, except for some minor difference:
1. It only support jemalloc >= 5.0
2. When the allocator destruct, it explicitly destruct the corresponding arena via `arena.<i>.destroy` via `mallctl`.
Depending on #4502.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4534
Differential Revision: D10435474
Pulled By: yiwu-arbug
fbshipit-source-id: e80edea755d3853182485d2be710376384ce0bb4
Summary:
Rename the interface, as it is mean to be a generic interface for memory allocation.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4590
Differential Revision: D10866340
Pulled By: yiwu-arbug
fbshipit-source-id: 85cb753351a40cb856c046aeaa3f3b369eef3d16
Summary:
This allows tombstone fragmenting to only be performed when the table is opened, and cached for subsequent accesses.
On the same DB used in #4449, running `readrandom` results in the following:
```
readrandom : 0.983 micros/op 1017076 ops/sec; 78.3 MB/s (63103 of 100000 found)
```
Now that Get performance in the presence of range tombstones is reasonable, I also compared the performance between a DB with range tombstones, "expanded" range tombstones (several point tombstones that cover the same keys the equivalent range tombstone would cover, a common workaround for DeleteRange), and no range tombstones. The created DBs had 5 million keys each, and DeleteRange was called at regular intervals (depending on the total number of range tombstones being written) after 4.5 million Puts. The table below summarizes the results of a `readwhilewriting` benchmark (in order to provide somewhat more realistic results):
```
Tombstones? | avg micros/op | stddev micros/op | avg ops/s | stddev ops/s
----------------- | ------------- | ---------------- | ------------ | ------------
None | 0.6186 | 0.04637 | 1,625,252.90 | 124,679.41
500 Expanded | 0.6019 | 0.03628 | 1,666,670.40 | 101,142.65
500 Unexpanded | 0.6435 | 0.03994 | 1,559,979.40 | 104,090.52
1k Expanded | 0.6034 | 0.04349 | 1,665,128.10 | 125,144.57
1k Unexpanded | 0.6261 | 0.03093 | 1,600,457.50 | 79,024.94
5k Expanded | 0.6163 | 0.05926 | 1,636,668.80 | 154,888.85
5k Unexpanded | 0.6402 | 0.04002 | 1,567,804.70 | 100,965.55
10k Expanded | 0.6036 | 0.05105 | 1,667,237.70 | 142,830.36
10k Unexpanded | 0.6128 | 0.02598 | 1,634,633.40 | 72,161.82
25k Expanded | 0.6198 | 0.04542 | 1,620,980.50 | 116,662.93
25k Unexpanded | 0.5478 | 0.0362 | 1,833,059.10 | 121,233.81
50k Expanded | 0.5104 | 0.04347 | 1,973,107.90 | 184,073.49
50k Unexpanded | 0.4528 | 0.03387 | 2,219,034.50 | 170,984.32
```
After a large enough quantity of range tombstones are written, range tombstone Gets can become faster than reading from an equivalent DB with several point tombstones.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4493
Differential Revision: D10842844
Pulled By: abhimadan
fbshipit-source-id: a7d44534f8120e6aabb65779d26c6b9df954c509
Summary:
Currently there are two contrun test failures:
* rocksdb-contrun-lite:
> tools/db_bench_tool.cc: In function ‘int rocksdb::db_bench_tool(int, char**)’:
tools/db_bench_tool.cc:5814:5: error: ‘DumpMallocStats’ is not a member of ‘rocksdb’
rocksdb::DumpMallocStats(&stats_string);
^
make: *** [tools/db_bench_tool.o] Error 1
* rocksdb-contrun-unity:
> In file included from unity.cc:44:0:
db/range_tombstone_fragmenter.cc: In member function ‘void rocksdb::FragmentedRangeTombstoneIterator::FragmentTombstones(std::unique_ptr<rocksdb::InternalIteratorBase<rocksdb::Slice> >, rocksdb::SequenceNumber)’:
db/range_tombstone_fragmenter.cc:90:14: error: reference to ‘ParsedInternalKeyComparator’ is ambiguous
auto cmp = ParsedInternalKeyComparator(icmp_);
This PR will fix them
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4587
Differential Revision: D10846554
Pulled By: miasantreble
fbshipit-source-id: 8d3358879e105060197b1379c84aecf51b352b93
Summary:
1. `WriteBufferManager` should have a reference alive in Java side through `Options`/`DBOptions` otherwise, if it's GC'ed at java side, native side can seg fault.
2. native method `setWriteBufferManager()` in `DBOptions.java` doesn't have it's jni method invocation in rocksdbjni which is added in this PR
3. `DBOptionsTest.java` is referencing object of `Options`. Instead it should be testing against `DBOptions`. Seems like a copy paste error.
4. Add a getter for WriteBufferManager.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4579
Differential Revision: D10561150
Pulled By: sagar0
fbshipit-source-id: 139a15c7f051a9f77b4200215b88267b48fbc487
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
