Summary:
to workaround issue of http://tracker.ceph.com/issues/21422 .
and in tcmalloc aligned_alloc and posix_memalign() are basically the
same thing. the same applies to GNU glibc.
fixes#3175
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Closes https://github.com/facebook/rocksdb/pull/3862
Differential Revision: D8147930
Pulled By: yiwu-arbug
fbshipit-source-id: 355afe93c4dd0a96a0d711ef190e8b86fbe8d11d
Summary:
Update LRUCacheShard constructor so that adding new params to it don't need to add extra SetXXX() methods.
Closes https://github.com/facebook/rocksdb/pull/3896
Differential Revision: D8128618
Pulled By: yiwu-arbug
fbshipit-source-id: 6afa715de1493a50de413678761a765e3af9b83b
Summary:
Previously we were using -1 as the default for every library, which was legacy from our zlib options. That worked for a while, but after zstd introduced a146ee04ae, it started giving poor compression ratios by default in zstd.
This PR adds a constant to RocksDB public API, `CompressionOptions::kDefaultCompressionLevel`, which will get translated to the default value specific to the compression library being used in "util/compression.h". The constant uses a number that appears to be larger than any library's maximum compression level.
Closes https://github.com/facebook/rocksdb/pull/3895
Differential Revision: D8125780
Pulled By: ajkr
fbshipit-source-id: 2db157a89118cd4f94577c2f4a0a5ff31c8391c6
Summary:
Explicitly specify the underlying type of enums help developers understand the physical storage.
Closes https://github.com/facebook/rocksdb/pull/3892
Differential Revision: D8107027
Pulled By: riversand963
fbshipit-source-id: a00efecbba46df4a3c8eed0994a2d4972ad1a1d3
Summary:
Currently when ldb built from master tries to open a DB from version 2.2, there will be a segfault because table_properties didn't exist back then.
Closes https://github.com/facebook/rocksdb/pull/3890
Differential Revision: D8100914
Pulled By: miasantreble
fbshipit-source-id: b255e8aedc54695432be2e704839c857dabdd65a
Summary:
Fix Issue #3771 : Check for nullptr in Slice constructor
Slice ctor checks for nullptr and creates empty string if the string does not exist
Closes https://github.com/facebook/rocksdb/pull/3887
Differential Revision: D8098852
Pulled By: ajkr
fbshipit-source-id: 04471077defa9776ce7b8c389a61312ce31002fb
Summary:
DBTest.GroupCommitTest would often fail when run under valgrind because its sleeps were insufficient to guarantee a group commit had multiple entries. Instead we can use sync point to force a leader to wait until a non-leader thread has enqueued its work, thus guaranteeing a leader can do group commit work for multiple threads.
Closes https://github.com/facebook/rocksdb/pull/3883
Differential Revision: D8079429
Pulled By: ajkr
fbshipit-source-id: 61dc50fad29d2c85547842f681288de60fa29049
Summary:
I repro'd some of the "unexpected value" failures showing up in our CI lately and they always happened on keys that have a mix of single deletes and merge operands. The `SingleDelete()` API comment mentions it's incompatible with `Merge()`, so this PR prevents `db_stress` from mixing them.
Closes https://github.com/facebook/rocksdb/pull/3878
Differential Revision: D8097346
Pulled By: ajkr
fbshipit-source-id: 357a48c6a31156f4f8db3ce565638ad924c437a1
Summary:
By using WritableFileWriter rather than WritableFile directly, we can buffer multiple Append() calls to one write() file system call, which will be expensive to underlying Env without its own write buffering.
Closes https://github.com/facebook/rocksdb/pull/3882
Differential Revision: D8080673
Pulled By: siying
fbshipit-source-id: e0db900cb3c178166aa738f3985db65e3ae2cf1b
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
Summary:
Change `keys_` from `set<string>` to `vector<set<string>>` so that each column
family's keys are stored in one set.
ajkr When you have a chance, can you PTAL? Thanks!
Closes https://github.com/facebook/rocksdb/pull/3871
Differential Revision: D8056447
Pulled By: riversand963
fbshipit-source-id: 650d0f9cad02b1bc005fc329ad76edbf053e6386
Summary:
Previously it only printed percentiles, even though our histogram keeps track of count and sum (and more). There have been many times we want to know more than the percentiles. For example, we currently want sum of "rocksdb.compression.times.nanos" and sum of "rocksdb.decompression.times.nanos", which would allow us to know the relative cost of compression vs decompression.
This PR adds count and sum to the string printed by `StatisticsImpl::ToString`. This is a bit risky as there are definitely parsers assuming the old format. I will mention it in HISTORY.md and hope for the best...
Closes https://github.com/facebook/rocksdb/pull/3863
Differential Revision: D8038831
Pulled By: ajkr
fbshipit-source-id: 0465b72e4b0cbf18ef965f4efe402601d16d5b5c
Summary:
`RangeDelAggregator` holds the pointers returned by `BlockIter::key()` and `BlockIter::value()` so requires the data to which they point is pinned. `BlockIter::key()` points into block memory and is guaranteed to be pinned if and only if prefix encoding is disabled (or, equivalently, restart interval is set to one). I think `BlockIter::value()` is always pinned. Added an assert for these and removed the wrong TODO about increasing restart interval, which would enable key prefix encoding and break the assertion.
Closes https://github.com/facebook/rocksdb/pull/3875
Differential Revision: D8063667
Pulled By: ajkr
fbshipit-source-id: 60b5ebcc0cdd610dd6aad9e74a23378793672c41
Summary:
We only generate the header dependency (".cc.d") files for files mentioned in "src.mk". When we don't generate them, changes to header dependencies do not cause `make` to recompile the dependent ".o". Then it takes a while for developers (or maybe just me) to realize `make clean` is necessary.
Closes https://github.com/facebook/rocksdb/pull/3876
Differential Revision: D8065389
Pulled By: ajkr
fbshipit-source-id: 0f62eee7bcab15b0215791564e6ab3775d46996b
Summary:
Right now, every Block::NewIterator() reads num_restarts_ from the block, which is already read in Block::Block(). This sometimes cause a CPU cache miss. Although fetching this cacheline can usually benefit follow-up block restart offset reading, as they are close to each other, it's almost free to get ride of this read by storing it in the Block class.
Closes https://github.com/facebook/rocksdb/pull/3869
Differential Revision: D8052493
Pulled By: siying
fbshipit-source-id: 9c72360f0c2d7329f3c198ce4eaedd2bc14b87c1
Summary:
In the past, the default value of max_manifest_file_size is uint64_t::MAX,
allowing a long running RocksDB process to grow its MANIFEST file to take up
the entire disk, as reported in [issue 3851](https://github.com/facebook/rocksdb/issues/3851). It is reasonable and common to provide a default non-max value for this option. Therefore, I set the value to 1GB.
siying miasantreble Please let me know whether this looks good to you. Thanks!
Closes https://github.com/facebook/rocksdb/pull/3867
Differential Revision: D8051524
Pulled By: riversand963
fbshipit-source-id: 50251f0804b1fa933a19a30d19d261ea8b9d2b72
Summary:
Right now ReverseBytewiseComparator::FindShortestSeparator() doesn't really shorten key, and ReverseBytewiseComparator::FindShortestSuccessor() seems to return wrong results. The code is confusing too as it uses BytewiseComparatorImpl::FindShortestSeparator() but the function actually won't do anything if the the first key is larger than the second.
Implement ReverseBytewiseComparator::FindShortestSeparator() and override ReverseBytewiseComparator::FindShortestSuccessor() to be empty.
Closes https://github.com/facebook/rocksdb/pull/3836
Differential Revision: D7959762
Pulled By: siying
fbshipit-source-id: 93acb621c16ce6f23e087ae4e19f7d84d1254683
Summary:
this will fix the failing clang_check test
Closes https://github.com/facebook/rocksdb/pull/3868
Differential Revision: D8050880
Pulled By: miasantreble
fbshipit-source-id: 749932e2e4025f835c961c068d601e522a126da6
Summary:
Before this PR, Iterator/InternalIterator may simultaneously have non-ok status() and Valid() = true. That state means that the last operation failed, but the iterator is nevertheless positioned on some unspecified record. Likely intended uses of that are:
* If some sst files are corrupted, a normal iterator can be used to read the data from files that are not corrupted.
* When using read_tier = kBlockCacheTier, read the data that's in block cache, skipping over the data that is not.
However, this behavior wasn't documented well (and until recently the wiki on github had misleading incorrect information). In the code there's a lot of confusion about the relationship between status() and Valid(), and about whether Seek()/SeekToLast()/etc reset the status or not. There were a number of bugs caused by this confusion, both inside rocksdb and in the code that uses rocksdb (including ours).
This PR changes the convention to:
* If status() is not ok, Valid() always returns false.
* Any seek operation resets status. (Before the PR, it depended on iterator type and on particular error.)
This does sacrifice the two use cases listed above, but siying said it's ok.
Overview of the changes:
* A commit that adds missing status checks in MergingIterator. This fixes a bug that actually affects us, and we need it fixed. `DBIteratorTest.NonBlockingIterationBugRepro` explains the scenario.
* Changes to lots of iterator types to make all of them conform to the new convention. Some bug fixes along the way. By far the biggest changes are in DBIter, which is a big messy piece of code; I tried to make it less big and messy but mostly failed.
* A stress-test for DBIter, to gain some confidence that I didn't break it. It does a few million random operations on the iterator, while occasionally modifying the underlying data (like ForwardIterator does) and occasionally returning non-ok status from internal iterator.
To find the iterator types that needed changes I searched for "public .*Iterator" in the code. Here's an overview of all 27 iterator types:
Iterators that didn't need changes:
* status() is always ok(), or Valid() is always false: MemTableIterator, ModelIter, TestIterator, KVIter (2 classes with this name anonymous namespaces), LoggingForwardVectorIterator, VectorIterator, MockTableIterator, EmptyIterator, EmptyInternalIterator.
* Thin wrappers that always pass through Valid() and status(): ArenaWrappedDBIter, TtlIterator, InternalIteratorFromIterator.
Iterators with changes (see inline comments for details):
* DBIter - an overhaul:
- It used to silently skip corrupted keys (`FindParseableKey()`), which seems dangerous. This PR makes it just stop immediately after encountering a corrupted key, just like it would for other kinds of corruption. Let me know if there was actually some deeper meaning in this behavior and I should put it back.
- It had a few code paths silently discarding subiterator's status. The stress test caught a few.
- The backwards iteration code path was expecting the internal iterator's set of keys to be immutable. It's probably always true in practice at the moment, since ForwardIterator doesn't support backwards iteration, but this PR fixes it anyway. See added DBIteratorTest.ReverseToForwardBug for an example.
- Some parts of backwards iteration code path even did things like `assert(iter_->Valid())` after a seek, which is never a safe assumption.
- It used to not reset status on seek for some types of errors.
- Some simplifications and better comments.
- Some things got more complicated from the added error handling. I'm open to ideas for how to make it nicer.
* MergingIterator - check status after every operation on every subiterator, and in some places assert that valid subiterators have ok status.
* ForwardIterator - changed to the new convention, also slightly simplified.
* ForwardLevelIterator - fixed some bugs and simplified.
* LevelIterator - simplified.
* TwoLevelIterator - changed to the new convention. Also fixed a bug that would make SeekForPrev() sometimes silently ignore errors from first_level_iter_.
* BlockBasedTableIterator - minor changes.
* BlockIter - replaced `SetStatus()` with `Invalidate()` to make sure non-ok BlockIter is always invalid.
* PlainTableIterator - some seeks used to not reset status.
* CuckooTableIterator - tiny code cleanup.
* ManagedIterator - fixed some bugs.
* BaseDeltaIterator - changed to the new convention and fixed a bug.
* BlobDBIterator - seeks used to not reset status.
* KeyConvertingIterator - some small change.
Closes https://github.com/facebook/rocksdb/pull/3810
Differential Revision: D7888019
Pulled By: al13n321
fbshipit-source-id: 4aaf6d3421c545d16722a815b2fa2e7912bc851d
Summary:
log_ contract specifies that it should not be modified unless both mutex_ and log_write_mutex_ are held. log_.erase however does that with only holding mutex_. This causes a race condition with two_write_queues since logs_.back is read with holding only log_write_mutex_ (which is correct according to logs_ contract) but logs_.erase is called concurrently. This is probably the cause of logs_.back returning nullptr in https://github.com/facebook/rocksdb/issues/3852 although I could not reproduce it.
Fixes https://github.com/facebook/rocksdb/issues/3852
Closes https://github.com/facebook/rocksdb/pull/3859
Differential Revision: D8026103
Pulled By: maysamyabandeh
fbshipit-source-id: ee394e00fe4aa520d884c5ef87981e9d6b5ccb28
Summary:
TSAN reports a false alarm for lock-order-inversion in DBWriteTest.IOErrorOnWALWritePropagateToWriteThreadFollower but Open and FlushWAL are not run concurrently. Suppressing the error by skipping FlushWAL in the test until TSAN is fixed.
The alternative would be to use
```
TSAN_OPTIONS="suppressions=tsan-suppressions.txt" ./db_write_test
```
but it does not seem straightforward to integrate it to our test infra.
Closes https://github.com/facebook/rocksdb/pull/3854
Differential Revision: D8000202
Pulled By: maysamyabandeh
fbshipit-source-id: fde33483d963a7ad84d3145123821f64960a4802
Summary:
This feature was introduced for universal compaction in cc01985d. At that point we thought it'd be used only to prevent long-running universal full compactions from blocking short-lived upper-level compactions. Now we have a level compaction user who could benefit from it since they use more expensive compression algorithm in the bottom level. So enable it for level.
Closes https://github.com/facebook/rocksdb/pull/3835
Differential Revision: D7957179
Pulled By: ajkr
fbshipit-source-id: 177285d2cef3b650b6a4d81dc5db84bc441c9fe4
Summary:
I noticed, while debugging an unrelated issue, that db_stress is failing to build on mac, leading to a failed `make all`.
```
$ make db_stress -j4
...
tools/db_stress.cc:862:69: error: cannot initialize a parameter of type 'uint64_t *' (aka 'unsigned long long *') with an rvalue of type 'size_t *' (aka 'unsigned long *')
status = FLAGS_env->GetFileSize(FLAGS_expected_values_path, &size);
^~~~~
./include/rocksdb/env.h:277:66: note: passing argument to parameter 'file_size' here
virtual Status GetFileSize(const std::string& fname, uint64_t* file_size) = 0;
^
1 error generated.
make: *** [tools/db_stress.o] Error 1
make: *** Waiting for unfinished jobs....
```
Closes https://github.com/facebook/rocksdb/pull/3839
Differential Revision: D7979236
Pulled By: sagar0
fbshipit-source-id: 0615e7bb5405bade71e4203803bf723720422d62
Summary:
Currently manual_wal_flush if set in the options will be used only for the wal files created during wal switch. The configuration thus does not affect the first wal file. The patch fixes that and also update the related unit tests.
This PR is built on top of https://github.com/facebook/rocksdb/pull/3756
Closes https://github.com/facebook/rocksdb/pull/3824
Differential Revision: D7909153
Pulled By: maysamyabandeh
fbshipit-source-id: 024ed99d2555db06bf096c902b998e432bb7b9ce
Summary:
The patch clarifies the ownership of the root db after TransactionDB::Open. If it is a success the ownership if with the TransactionDB, and the root db will be deleted when the destructor of the base class, StackableDB, is called. If it is failure, the temporarily created root db will also be deleted properly.
The patch also includes lots of useful formatting changes.
Closes https://github.com/facebook/rocksdb/pull/3714 upon which this patch is built.
Closes https://github.com/facebook/rocksdb/pull/3806
Differential Revision: D7878010
Pulled By: maysamyabandeh
fbshipit-source-id: f54f3942e29434143ae5a2423ceec9c7072cd4c2
Summary:
Previously `DBOptions::use_direct_io_for_flush_and_compaction=true` combined with `DBOptions::use_direct_reads=false` could cause RocksDB to simultaneously read from two file descriptors for the same file, where background reads used direct I/O and foreground reads used buffered I/O. Our measurements found this mixed-mode I/O negatively impacted foreground read perf, compared to when only buffered I/O was used.
This PR makes the mixed-mode I/O situation impossible by repurposing `DBOptions::use_direct_io_for_flush_and_compaction` to only apply to background writes, and `DBOptions::use_direct_reads` to apply to all reads. There is no risk of direct background direct writes happening simultaneously with buffered reads since we never read from and write to the same file simultaneously.
Closes https://github.com/facebook/rocksdb/pull/3829
Differential Revision: D7915443
Pulled By: ajkr
fbshipit-source-id: 78bcbf276449b7e7766ab6b0db246f789fb1b279
Summary:
- Any options unknown to `db_crashtest.py` are now passed directly to `db_stress`. This way, we won't need to update `db_crashtest.py` every time `db_stress` gets a new option.
- Remove `db_crashtest.py` redundant arguments where the value is the same as `db_stress`'s default
- Remove `db_crashtest.py` redundant arguments where the value is the same in a previously applied options map. For example, default_params are always applied before whitebox_default_params, so if they require the same value for an argument, that value only needs to be provided in default_params.
- Made the simple option maps applied in addition to the regular option maps. Previously they were exclusive which led to lots of duplication
Closes https://github.com/facebook/rocksdb/pull/3809
Differential Revision: D7885779
Pulled By: ajkr
fbshipit-source-id: 3a3243b55724d6d5bff36e939b582b9b62c538a8
Summary:
The only use of RandomRW is to change seqno when bulkloading, and in this use case, the file should exist. We should fail the file opening in this case.
Closes https://github.com/facebook/rocksdb/pull/3827
Differential Revision: D7913719
Pulled By: siying
fbshipit-source-id: 62cf6734f1a6acb9e14f715b927da388131c3492
Summary:
Now BlockBasedTableIterator directly uses BlockIter. By making BlockIter final, we can prevent unintended virtual function overriding.
Closes https://github.com/facebook/rocksdb/pull/3828
Differential Revision: D7933816
Pulled By: siying
fbshipit-source-id: 026a08cb5c5b6d3d6f44743152b4251da4756f2c
Summary:
`ReadaheadRandomAccessFile` had an unwritten assumption, which was that its wrapped file's `Read()` function always copies into the provided scratch buffer. Actually this was not true when the wrapped file was `PosixMmapReadableFile`, whose `Read()` implementation does no copying and instead returns a `Slice` pointing directly into the `mmap`'d memory region. This PR:
- prevents `ReadaheadRandomAccessFile` from ever wrapping mmap readable files
- adds an assert for the assumption `ReadaheadRandomAccessFile` makes about the wrapped file's use of scratch buffer
Closes https://github.com/facebook/rocksdb/pull/3813
Differential Revision: D7891513
Pulled By: ajkr
fbshipit-source-id: dc64a55222d6af280c39a1852ee39e9e9d7cde7d
Summary:
People also use ON/OFF, TRUE/FALSE and other switch options that is allowed by cmake.
Closes https://github.com/facebook/rocksdb/pull/3814
Differential Revision: D7899032
Pulled By: ajkr
fbshipit-source-id: b71511af59e0a78eedafb639b5002c47050bf3c2
Summary:
TBBROOT and LIBRARY_PATH are set in env by the script.
With TBB 2018 the library path is $TBBROOT/lib/intel64/gcc4.7 for anything above gcc 4.7, which is both compiler and architecture related. We cannot simply do ${TBB_ROOT_DIR}/lib.
Closes https://github.com/facebook/rocksdb/pull/3815
Differential Revision: D7899006
Pulled By: ajkr
fbshipit-source-id: 159ab1f6a5c40452ed6aa8d79300206953d916c2
Summary:
tsan flavor of this test occasionally times out in our test infra. The patch split the test to two, each working on half of the option range.
Before:
[ OK ] FaultTest/FaultInjectionTest.FaultTest/0 (5918 ms)
[ OK ] FaultTest/FaultInjectionTest.FaultTest/1 (5336 ms)
After:
[ OK ] FaultTest/FaultInjectionTestSplitted.FaultTest/0 (2930 ms)
[ OK ] FaultTest/FaultInjectionTestSplitted.FaultTest/1 (2676 ms)
[ OK ] FaultTest/FaultInjectionTestSplitted.FaultTest/2 (2759 ms)
[ OK ] FaultTest/FaultInjectionTestSplitted.FaultTest/3 (2546 ms)
Closes https://github.com/facebook/rocksdb/pull/3819
Differential Revision: D7894975
Pulled By: maysamyabandeh
fbshipit-source-id: 809f1411cbcc27f8aa71a6b29a16b039f51b67c9
Summary:
The origin commit #3635 will hurt performance for users who aren't using range deletions， because unneeded std::set operations, so it was reverted by commit 44653c7b7a. (see #3672)
To fix this, move the set to and add a check in , i.e., file will be added only if is non-nullptr.
The db_bench command which find the performance regression:
> ./db_bench --benchmarks=fillrandom,seekrandomwhilewriting --threads=1 --num=1000000 --reads=150000 --key_size=66 > --value_size=1262 --statistics=0 --compression_ratio=0.5 --histogram=1 --seek_nexts=1 --stats_per_interval=1 > --stats_interval_seconds=600 --max_background_flushes=4 --num_multi_db=1 --max_background_compactions=16 --seed=1522388277 > -write_buffer_size=1048576 --level0_file_num_compaction_trigger=10000 --compression_type=none
Before and after the modification, I re-run this command on the machine, the results of are as follows：
**fillrandom**
Table | P50 | P75 | P99 | P99.9 | P99.99 |
---- | --- | --- | --- | ----- | ------ |
before commit | 5.92 | 8.57 | 19.63 | 980.97 | 12196.00 |
after commit | 5.91 | 8.55 | 19.34 | 965.56 | 13513.56 |
**seekrandomwhilewriting**
Table | P50 | P75 | P99 | P99.9 | P99.99 |
---- | --- | --- | --- | ----- | ------ |
before commit | 1418.62 | 1867.01 | 3823.28 | 4980.99 | 9240.00 |
after commit | 1450.54 | 1880.61 | 3962.87 | 5429.60 | 7542.86 |
Closes https://github.com/facebook/rocksdb/pull/3800
Differential Revision: D7874245
Pulled By: ajkr
fbshipit-source-id: 2e8bec781b3f7399246babd66395c88619534a17
Summary:
In case `--expected_values_path` is unset, we allocate a buffer internally to hold the expected DB state. This PR makes sure it is freed.
Closes https://github.com/facebook/rocksdb/pull/3804
Differential Revision: D7874694
Pulled By: ajkr
fbshipit-source-id: a8f7655e009507c4e639ceebfc3525d69c856e3b
Summary:
Currently HarnessTest.Randomized is already split but some of the splits are faster than the others. The reason is that each split takes a continuous range of the generated args and the test with later args takes longer to finish. The patch evenly split the args among splits in a round robin fashion.
Before:
```
[ OK ] HarnessTest.Randomized1n2 (2278 ms)
[ OK ] HarnessTest.Randomized3n4 (1095 ms)
[ OK ] HarnessTest.Randomized5 (658 ms)
[ OK ] HarnessTest.Randomized6 (1258 ms)
[ OK ] HarnessTest.Randomized7 (6476 ms)
[ OK ] HarnessTest.Randomized8 (8182 ms)
```
After
```
[ OK ] HarnessTest.Randomized1 (2649 ms)
[ OK ] HarnessTest.Randomized2 (2645 ms)
[ OK ] HarnessTest.Randomized3 (2577 ms)
[ OK ] HarnessTest.Randomized4 (2490 ms)
[ OK ] HarnessTest.Randomized5 (2553 ms)
[ OK ] HarnessTest.Randomized6 (2560 ms)
[ OK ] HarnessTest.Randomized7 (2501 ms)
[ OK ] HarnessTest.Randomized8 (2574 ms)
```
Closes https://github.com/facebook/rocksdb/pull/3808
Differential Revision: D7882663
Pulled By: maysamyabandeh
fbshipit-source-id: 09b749a9684b6d7d65466aa4b00c5334a49e833e
Summary:
We were seeing the following error: "ThreadSanitizer: DenseSlabAllocator overflow. Dying."
It is fixable by mmap'ing a smaller region for keys' expected values, which this PR achieves by reducing the number of keys.
Closes https://github.com/facebook/rocksdb/pull/3803
Differential Revision: D7874478
Pulled By: ajkr
fbshipit-source-id: 433939f5cb92410ab4777d540cb0cc2ee0fe6c2e
Summary:
Delete archive directory before WAL folder
since archive may be contained as a subfolder.
Also improve loop readability.
Closes https://github.com/facebook/rocksdb/pull/3797
Differential Revision: D7866378
Pulled By: riversand963
fbshipit-source-id: 0c45d97677ce6fbefa3f8d602ef5e2a2a925e6f5
Kefu Chai