Summary:
With ColumnFamilyData::pending_flush_, we have the following code snippet in DBImpl::ScheedulePendingFlush
```
if (!cfd->pending_flush() && cfd->imm()->IsFlushPending()) {
...
}
```
`Pending` is ambiguous, and I feel `queued_for_flush` is a better name,
especially for the sake of readability.
Closes https://github.com/facebook/rocksdb/pull/3777
Differential Revision: D7783066
Pulled By: riversand963
fbshipit-source-id: f1bd8c8bfe5eafd2c94da0d8566c9b2b6bb57229
Summary:
This change adds a virtual `Truncate` method to `Env`, which truncates
the named file to the specified size. At the moment, this is only
supported for `MockEnv`, but other `Env's` could be extended to override
the method too. This is the same approach that methods like `LinkFile` and
`AreSameFile` have taken.
This is useful for any user of the in-memory `Env`. The implementation's
header is not exported, so before this change, it was impossible to
access it's already existing `Truncate` method.
Closes https://github.com/facebook/rocksdb/pull/3779
Differential Revision: D7785789
Pulled By: ajkr
fbshipit-source-id: 3bcdaeea7b7180529f7d9b496dc67b791a00bbf0
Summary:
- When options file is provided to db_stress, take supported options from the file instead of from flags
- Call `BuildOptionsTable` after `Open` so it can use `options_` once it has been populated either from flags or from file
- Allow options filename to be passed via `db_crashtest.py`
Closes https://github.com/facebook/rocksdb/pull/3768
Differential Revision: D7755331
Pulled By: ajkr
fbshipit-source-id: 5205cc5deb0d74d677b9832174153812bab9a60a
Summary:
7a6353bd1c prevents empty filter blocks from being written for SST files containing range deletions only. However the assertion this PR removes is still a problem as we could be reading from a DB generated by a RocksDB build without the 7a6353bd1c patch. So remove the assertion. We already don't do this check when `cache_index_and_filter_blocks=false`, so it should be safe.
Closes https://github.com/facebook/rocksdb/pull/3773
Differential Revision: D7769964
Pulled By: ajkr
fbshipit-source-id: 7285762446f2cd2ccf16efd7a988a106fbb0d8d3
Summary:
sync parent directory after deleting a file in delete scheduler. Otherwise, trim speed may not be as smooth as what we want.
Closes https://github.com/facebook/rocksdb/pull/3767
Differential Revision: D7760136
Pulled By: siying
fbshipit-source-id: ec131d53b61953f09c60d67e901e5eeb2716b05f
Summary:
bc0da4b512 optimized bloom filters by skipping duplicate entires when the whole key and prefixes are both added to the bloom. It however used empty string as the initial value of the last entry added to the bloom. This is incorrect since empty key/prefix are valid entires by themselves. This patch fixes that.
Closes https://github.com/facebook/rocksdb/pull/3776
Differential Revision: D7778803
Pulled By: maysamyabandeh
fbshipit-source-id: d5a065daebee17f9403cac51e9d5626aac87bfbc
Summary:
WritePrepared rollback implementation is not ready to be invoked in the middle of workload. This is due the lack of synchronization to obtain the cf handle from db. Temporarily disabling this until the problem with rollback is fixed.
Closes https://github.com/facebook/rocksdb/pull/3772
Differential Revision: D7769041
Pulled By: maysamyabandeh
fbshipit-source-id: 0e3b0ce679bc2afba82e653a40afa3f045722754
Summary:
Currently, the `rocksdb_options_set_ratelimiter` in `c.cc` will change the input to nil, which make it is
not possible to use the shared rate limiter create by `rocksdb_ratelimiter_create` in different rocksdb option.
In this pr, I changed it to shared ptr.
Closes https://github.com/facebook/rocksdb/pull/3758
Differential Revision: D7749740
Pulled By: ajkr
fbshipit-source-id: c6121f8ca75402afdb4b295ce63c2338d253a1b5
Summary:
In include/rocksdb/db.h, enum EntryType is redeclared even though
original declaration in types.h in included.
Closes https://github.com/facebook/rocksdb/pull/3766
Differential Revision: D7765504
Pulled By: anand1976
fbshipit-source-id: 622a8ecb306993915be1b9dd5cdd79dbc6a4ea05
Summary:
It seems clear to me that the variable is initialized before line 492, but it wasn't clear to UBSAN. The failure was:
```
In file included from ./env/io_posix.h:14:0,
from env/env_posix.cc:44:
./include/rocksdb/env.h: In member function ‘virtual rocksdb::Status rocksdb::{anonymous}::PosixEnv::NewMemoryMappedFileBuffer(const string&, std::unique_ptr<rocksdb::MemoryMappedFileBuffer>*)’:
./include/rocksdb/env.h:822:36: error: ‘base’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
: base(_base), length(_length) {}
^
env/env_posix.cc:482:11: note: ‘base’ was declared here
void* base;
```
We can just initialize to nullptr to keep UBSAN happy.
Closes https://github.com/facebook/rocksdb/pull/3770
Differential Revision: D7756287
Pulled By: ajkr
fbshipit-source-id: 0f2efb9594e2d3a30706a4ca7e1d4a6328031bf2
Summary:
clang compilation is failing due to a4fb1f8c04. In that commit I added a call to `std::atomic::is_lock_free` which was evidently relying on a compiler builtin only present in gcc.
Drawbacks to this fix are:
- users may need to install libatomic
- there might be cases where clang is used even though USE_CLANG is unset (e.g., when clang is the only available compiler). I didn't figure out how to add -latomic in those cases...
An alternative fix mentioned in http://lists.llvm.org/pipermail/llvm-bugs/2017-August/057263.html is using -stdlib=libc++ with clang.
Closes https://github.com/facebook/rocksdb/pull/3769
Differential Revision: D7756261
Pulled By: ajkr
fbshipit-source-id: 26888300683fa9970ab5950239d1aa217e8efd49
Summary:
Previously, our `db_stress` tool held the expected state of the DB in-memory, so after crash-recovery, there was no way to verify data correctness. This PR adds an option, `--expected_values_file`, which specifies a file holding the expected values.
In black-box testing, the `db_stress` process can be killed arbitrarily, so updates to the `--expected_values_file` must be atomic. We achieve this by `mmap`ing the file and relying on `std::atomic<uint32_t>` for atomicity. Actually this doesn't provide a total guarantee on what we want as `std::atomic<uint32_t>` could, in theory, be translated into multiple stores surrounded by a mutex. We can verify our assumption by looking at `std::atomic::is_always_lock_free`.
For the `mmap`'d file, we didn't have an existing way to expose its contents as a raw memory buffer. This PR adds it in the `Env::NewMemoryMappedFileBuffer` function, and `MemoryMappedFileBuffer` class.
`db_crashtest.py` is updated to use an expected values file for black-box testing. On the first iteration (when the DB is created), an empty file is provided as `db_stress` will populate it when it runs. On subsequent iterations, that same filename is provided so `db_stress` can check the data is as expected on startup.
Closes https://github.com/facebook/rocksdb/pull/3629
Differential Revision: D7463144
Pulled By: ajkr
fbshipit-source-id: c8f3e82c93e045a90055e2468316be155633bd8b
Summary:
Currently we rely on FilterBitsBuilder to skip the duplicate keys. It does that by comparing that hash of the key to the hash of the last added entry. This logic breaks however when we have whole_key_filtering mixed with prefix blooms as their addition to FilterBitsBuilder will be interleaved. The patch fixes that by comparing the last whole key and last prefix with the whole key and prefix of the new key respectively and skip the call to FilterBitsBuilder if it is a duplicate.
Closes https://github.com/facebook/rocksdb/pull/3764
Differential Revision: D7744413
Pulled By: maysamyabandeh
fbshipit-source-id: 15df73bbbafdfd754d4e1f42ea07f47b03bc5eb8
Summary:
Background activities like compaction can negatively affect
latency of higher-priority tasks like request processing. To avoid this,
rocksdb already lowers the IO priority of background threads on Linux
systems. While this takes care of typical IO-bound systems, it does not
help much when CPU (temporarily) becomes the bottleneck. This is
especially likely when using more expensive compression settings.
This patch adds an API to allow for lowering the CPU priority of
background threads, modeled on the IO priority API. Benchmarks (see
below) show significant latency and throughput improvements when CPU
bound. As a result, workloads with some CPU usage bursts should benefit
from lower latencies at a given utilization, or should be able to push
utilization higher at a given request latency target.
A useful side effect is that compaction CPU usage is now easily visible
in common tools, allowing for an easier estimation of the contribution
of compaction vs. request processing threads.
As with IO priority, the implementation is limited to Linux, degrading
to a no-op on other systems.
Closes https://github.com/facebook/rocksdb/pull/3763
Differential Revision: D7740096
Pulled By: gwicke
fbshipit-source-id: e5d32373e8dc403a7b0c2227023f9ce4f22b413c
Summary:
There's a group of stats in PerfContext for profiling the write path. They break down the write time into WAL write, memtable insert, throttling, and everything else. We use these stats a lot for figuring out the cause of slow writes.
These stats got a bit out of date and are now categorizing some interesting things as "everything else", and also do some double counting. This PR fixes it and adds two new stats: time spent waiting for other threads of the batch group, and time spent waiting for scheduling flushes/compactions. Probably these will be enough to explain all the occasional abnormally slow (multiple seconds) writes that we're seeing.
Closes https://github.com/facebook/rocksdb/pull/3602
Differential Revision: D7251562
Pulled By: al13n321
fbshipit-source-id: 0a2d0f5a4fa5677455e1f566da931cb46efe2a0d
Summary:
This reverts commit 73f21a7b21.
It breaks compatibility. When created a DB using a build with this new change, opening the DB and reading the data will fail with this error:
"Corruption: Can't access /000000.sst: IO error: while stat a file for size: /tmp/xxxx/000000.sst: No such file or directory"
This is because the dummy AddFile4 entry generated by the new code will be treated as a real entry by an older build. The older build will think there is a real file with number 0, but there isn't such a file.
Closes https://github.com/facebook/rocksdb/pull/3762
Differential Revision: D7730035
Pulled By: siying
fbshipit-source-id: f2051859eff20ef1837575ecb1e1bb96b3751e77
Summary:
We used to name private directories like "1.tmp" while BackupEngine populated them, and then rename without the ".tmp" suffix (i.e., rename "1.tmp" to "1") after all files were copied. On glusterfs, directory renames like this require operations across many hosts, and partial failures have caused operational problems.
Fortunately we don't need to rename private directories. We already have a meta-file that uses the tempfile-rename pattern to commit a backup atomically after all its files have been successfully copied. So we can copy private files directly to their final location, so now there's no directory rename.
Closes https://github.com/facebook/rocksdb/pull/3749
Differential Revision: D7705610
Pulled By: ajkr
fbshipit-source-id: fd724a28dd2bf993ce323a5f2cb7e7d6980cc346
Summary:
The test is flaky in our CI but could not be reproduce manually on the same CI host. Disabling it.
Closes https://github.com/facebook/rocksdb/pull/3753
Differential Revision: D7716320
Pulled By: yiwu-arbug
fbshipit-source-id: 6bed3b05880c1d24e8dc86bc970e5181bc98fb45
Summary:
Currently WritePrepared rolls back a transaction with prepare sequence number prepare_seq by i) write a single rollback batch with rollback_seq, ii) add <rollback_seq, rollback_seq> to commit cache, iii) remove prepare_seq from PrepareHeap.
This is correct assuming that there is no snapshot taken when a transaction is rolled back. This is the case the way MySQL does rollback which is after recovery. Otherwise if max_evicted_seq advances the prepare_seq, the live snapshot might assume data as committed since it does not find them in CommitCache.
The change is to simply add <prepare_seq. rollback_seq> to commit cache before removing prepare_seq from PrepareHeap. In this way if max_evicted_seq advances prpeare_seq, the existing mechanism that we have to check evicted entries against live snapshots will make sure that the live snapshot will not see the data of rolled back transaction.
Closes https://github.com/facebook/rocksdb/pull/3745
Differential Revision: D7696193
Pulled By: maysamyabandeh
fbshipit-source-id: c9a2d46341ddc03554dded1303520a1cab74ef9c
Summary:
1. Add a new ticker stat rocksdb.number.multiget.keys.found to track the
number of keys successfully read
2. Update rocksdb.memtable.hit/miss in DBImpl::MultiGet(). It was being done in
DBImpl::GetImpl(), but not MultiGet
Closes https://github.com/facebook/rocksdb/pull/3730
Differential Revision: D7677364
Pulled By: anand1976
fbshipit-source-id: af22bd0ef8ddc5cf2b4244b0a024e539fe48bca5
Summary:
The WriteBatch::Iterate will try with a larger sequence number if the memtable reports a duplicate. This status is specified with TryAgain status. So far the assumption was that the last entry in the batch will never return TryAgain, which is correct when WAL is created via WritePrepared since it always appends a batch separator if a natural one does not exist. However when reading a WAL generated by WriteCommitted this batch separator might not exist. Although WritePrepared is not supposed to be able to read the WAL generated by WriteCommitted we should avoid confusing scenarios in which the behavior becomes unpredictable. The path fixes that by allowing TryAgain even for the last entry of the write batch.
Closes https://github.com/facebook/rocksdb/pull/3747
Differential Revision: D7708391
Pulled By: maysamyabandeh
fbshipit-source-id: bfaddaa9b14a4cdaff6977f6f63c789a6ab1ee0d
Summary:
Currently the partitioned index iterator creates a new ReadOptions which ignores the fill_cache config set to ReadOptions passed by the user. The patch propagates fill_cache from the user's ReadOptions to that of partition index iterator.
Also it clarifies the contract of fill_cache that i) it does not apply to filters, ii) it still charges block cache for the size of the data block, it still pin the block if it is already in the block cache.
Closes https://github.com/facebook/rocksdb/pull/3739
Differential Revision: D7678308
Pulled By: maysamyabandeh
fbshipit-source-id: 53ed96424ae922e499e2d4e3580ddc3f0db893da
Summary:
Fix the following gcc-8 warnings:
- conflicting C language linkage declaration [-Werror]
- writing to an object with no trivial copy-assignment [-Werror=class-memaccess]
- array subscript -1 is below array bounds [-Werror=array-bounds]
Solves https://github.com/facebook/rocksdb/issues/3716
Closes https://github.com/facebook/rocksdb/pull/3736
Differential Revision: D7684161
Pulled By: yiwu-arbug
fbshipit-source-id: 47c0423d26b74add251f1d3595211eee1e41e54a
Summary:
This replaces reading the rocksdb version by external shell script. This does not work reliably on Windows (I wander how it works on AppVeyor).
Closes https://github.com/facebook/rocksdb/pull/3737
Differential Revision: D7703106
Pulled By: ajkr
fbshipit-source-id: 4079c7c77431757e9ddc801363ed896b18fdbf23
Summary:
Right now in `SyncClosedLogs`, `CopyFile`, and `AddRecord`, where `Sync` and `Append` are invoked in a loop, the error status are not checked. This could lead to potential corruption as later calls will overwrite the error status.
Closes https://github.com/facebook/rocksdb/pull/3740
Differential Revision: D7678848
Pulled By: miasantreble
fbshipit-source-id: 4b0b412975989dfe80348f73217b9c4122a4bd77
Summary:
Previously threads were named "rocksdb:bg\<index in thread pool\>", so the first thread in all thread pools would be named "rocksdb:bg0". Users want to be able to distinguish threads used for flush (high-pri) vs regular compaction (low-pri) vs compaction to bottom-level (bottom-pri). So I changed the thread naming convention to include the thread-pool priority.
Closes https://github.com/facebook/rocksdb/pull/3702
Differential Revision: D7581415
Pulled By: ajkr
fbshipit-source-id: ce04482b6acd956a401ef22dc168b84f76f7d7c1
Summary:
db_stress was already capable running transactions by setting use_txn. Running it under stress showed a couple of problems fixed in this patch.
- The uncommitted transaction must be either rolled back or commit after recovery.
- Current implementation of WritePrepared transaction cannot handle cf drop before crash. Clarified that in the comments and added safety checks. When running with use_txn, clear_column_family_one_in must be set to 0.
Closes https://github.com/facebook/rocksdb/pull/3733
Differential Revision: D7654419
Pulled By: maysamyabandeh
fbshipit-source-id: a024bad80a9dc99677398c00d29ff17d4436b7f3
Summary:
The reason for this initialization is that LLVM UBSAN check will fail due to
uninitialized bool. [StackOverflow post](https://stackoverflow.com/questions/31420154/runtime-error-load-of-value-127-which-is-not-a-valid-value-for-type-bool).
UBSAN log:
> ===== Running external_sst_file_basic_test
[==========] Running 7 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 7 tests from ExternalSSTFileBasicTest
[ RUN ] ExternalSSTFileBasicTest.Basic
[ OK ] ExternalSSTFileBasicTest.Basic (6 ms)
[ RUN ] ExternalSSTFileBasicTest.NoCopy
db/external_sst_file_ingestion_job.h:23:8: runtime error: load of value 253, which is not a valid value for type 'bool'
miasantreble I've tested this locally using the following command.
```
TEST_TMPDIR=/dev/shm/rocksdb COMPILE_WITH_UBSAN=1 OPT=-g make J=1 -j8 ubsan_check
```
ajkr This PR is related to your review comment in [PR](https://github.com/facebook/rocksdb/pull/3713/). It turns out that, with UBSAN enabled, we must provide a default value for boolean member variables.
Closes https://github.com/facebook/rocksdb/pull/3728
Differential Revision: D7642476
Pulled By: riversand963
fbshipit-source-id: 4c09a4b8d271151cb99ae7393db9e4ad9f29762e
Summary:
fix a bug in `db_stress` where an int array was incorrectly deallocated using delete instead of delete[]
Closes https://github.com/facebook/rocksdb/pull/3725
Differential Revision: D7634749
Pulled By: miasantreble
fbshipit-source-id: 489b776f5f4c03de1824edac5495787ec19cc910
Summary:
this PR fixes a few failed contbuild:
1. ASAN memory leak in Block::NewIterator (table/block.cc:429). the proper destruction of first_level_iter_ and second_level_iter_ of two_level_iterator.cc is missing from the code after the refactoring in https://github.com/facebook/rocksdb/pull/3406
2. various unused param errors introduced by https://github.com/facebook/rocksdb/pull/3662
3. updated comment for `ForceReleaseCachedEntry` to emphasize the use of `force_erase` flag.
Closes https://github.com/facebook/rocksdb/pull/3718
Reviewed By: maysamyabandeh
Differential Revision: D7621192
Pulled By: miasantreble
fbshipit-source-id: 476c94264083a0730ded957c29de7807e4f5b146
Summary:
otherwise the env_librados_test executable will fail to link against
librados.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Closes https://github.com/facebook/rocksdb/pull/3724
Differential Revision: D7631542
Pulled By: ajkr
fbshipit-source-id: 38afbf21f9aeb7dedfb840aba8b2f8b421f9edb0
Summary:
…verwrite_keys. Also changed each no_overwrite_key set to an unordered set, otherwise Knuth shuffle only gets you 2x time improvement, because insertion (and subsequent internal sorting) into an ordered set is the bottleneck.
With this change, each iteration of permutation construction and prefix selection takes around 40 secs, as opposed to 360 secs previously. However, this still means that with the default 10 CF per blackbox test case, the test is going to time out given the default interval of 200 secs.
Also, there is currently an assertion error affecting all blackbox tests in db_crashtest.py; this assertion error will be fixed in a future PR.
Closes https://github.com/facebook/rocksdb/pull/3699
Differential Revision: D7624616
Pulled By: amytai
fbshipit-source-id: ea64fbe83407ff96c1c0ecabbc6c830576939393
Summary:
When there are many range deletions in a range, we want to trigger manual compaction on this range to reclaim disk space as soon as possible and speed up read.
After this change, we can collect informations of range deletions and store them into user properties which can guide our manual compaction.
Closes https://github.com/facebook/rocksdb/pull/3695
Differential Revision: D7570322
Pulled By: ajkr
fbshipit-source-id: c358fa43b0aac6cc954d2eadc7d3bd8015373369
Summary:
Calling rocksdb::Log, rocksdb::Info, etc with a `shared_ptr<Logger>` should behave the same as calling those functions with a `Logger *`. This PR achieves it by making the `shared_ptr<Logger>` versions delegate to the `Logger *` versions.
Closes#3689
Closes https://github.com/facebook/rocksdb/pull/3710
Differential Revision: D7595557
Pulled By: ajkr
fbshipit-source-id: 64dd7f20fd42dc821bac7b8032705c35b483e00d
Summary:
RocksDB supports ingestion of external ssts. If ingestion_options.move_files is true, when performing ingestion, RocksDB first tries to link external ssts. If external SST file resides on a different FS, or the underlying FS does not support hard link, then RocksDB performs actual file copy. However, no matter which choice is made, current code increase bytes-written when updating compaction stats, which is inaccurate when RocksDB does NOT copy file.
Rename a sync point.
Closes https://github.com/facebook/rocksdb/pull/3713
Differential Revision: D7604151
Pulled By: riversand963
fbshipit-source-id: dd0c0d9b9a69c7d9ffceafc3d9c23371aa413586
Summary:
This PR comments out the rest of the unused arguments which allow us to turn on the -Wunused-parameter flag. This is the second part of a codemod relating to https://github.com/facebook/rocksdb/pull/3557.
Closes https://github.com/facebook/rocksdb/pull/3662
Differential Revision: D7426121
Pulled By: Dayvedde
fbshipit-source-id: 223994923b42bd4953eb016a0129e47560f7e352
Summary:
This is a hack as temporary fix of MyRocks with rollbacking the merge operands. The way MyRocks uses merge operands is without protection of locks, which violates the assumption behind the rollback algorithm. They are ok with not being rolled back as it would just create a gap in the autoincrement column. The hack add an option to disable the rollback of merge operands by default and only enables it to let the unit test pass.
Closes https://github.com/facebook/rocksdb/pull/3711
Differential Revision: D7597177
Pulled By: maysamyabandeh
fbshipit-source-id: 544be0f666c7e7abb7f651ec8b23124e05056728
Summary:
We introduced smallest_prep optimization in this commit b225de7e10, which enables storing the smallest uncommitted sequence number along with the snapshot. This enables the readers that read from the snapshot to skip further checks and safely assumed the data is committed if its sequence number is less than smallest uncommitted when the snapshot was taken. The problem was that smallest uncommitted and the snapshot must be taken atomically, and the lack of atomicity had led to readers using a smallest uncommitted after the snapshot was taken and hence mistakenly skipping some data.
This patch fixes the problem by i) separating the process of removing of prepare entries from the AddCommitted function, ii) removing the prepare entires AFTER the committed sequence number is published, iii) getting smallest uncommitted (from the prepare list) BEFORE taking a snapshot. This guarantees that the smallest uncommitted that is accompanied with a snapshot is less than or equal of such number if it was obtained atomically.
Tested by running MySQLStyleTransactionTest/MySQLStyleTransactionTest.TransactionStressTest that was failing sporadically.
Closes https://github.com/facebook/rocksdb/pull/3703
Differential Revision: D7581934
Pulled By: maysamyabandeh
fbshipit-source-id: dc9d6f4fb477eba75d4d5927326905b548a96a32
Summary:
Add `compaction_reason` as part of event log for event `compaction started`.
Add counters for each `CompactionReason`.
Closes https://github.com/facebook/rocksdb/pull/3679
Differential Revision: D7550348
Pulled By: riversand963
fbshipit-source-id: a19cff3a678c785aa5ef41aac78b9a5968fcc34d
Summary:
In `cf_options_type_info`, the deprecated options are all considered to have offset zero in the `MutableCFOptions` struct. Previously we weren't checking in `GetMutableOptionsFromStrings` whether the provided option was deprecated or not and simply writing the provided value to the offset specified by `cf_options_type_info`. That meant setting any deprecated option would overwrite the first element in the struct, which is `write_buffer_size`. `db_stress` hit this often since it calls `SetOptions` with `soft_rate_limit=0` and `hard_rate_limit=0`, which are both deprecated so cause `write_buffer_size` to be set to zero, which causes it to crash on the following assertion:
```
db_stress: db/memtable.cc:106: rocksdb::MemTable::MemTable(const rocksdb::InternalKeyComparator&, const rocksdb::ImmutableCFOptions&, const rocksdb::MutableCFOptions&, rocksdb::WriteBufferManager*, rocksdb::SequenceNumber, uint32_t): Assertion `!ShouldScheduleFlush()' failed.
```
We fix it by skipping deprecated options (and logging a warning) when users provide them to `SetOptions`. I didn't want to fail the call for compatibility reasons.
Closes https://github.com/facebook/rocksdb/pull/3700
Differential Revision: D7572596
Pulled By: ajkr
fbshipit-source-id: bd5d84e14c0c39f30c5d4c6df7c1503d2c28ecf1
Summary:
1. Remove redundant text.
2. Make terminology consistent across all comments and doc of RocksDB. Also do
our best to conform to conventions. Specifically, use 'callback' instead of
'call-back' [wikipedia](https://en.wikipedia.org/wiki/Callback_(computer_programming)).
Closes https://github.com/facebook/rocksdb/pull/3693
Differential Revision: D7560396
Pulled By: riversand963
fbshipit-source-id: ba8c251c487f4e7d1872a1a8dc680f9e35a6ffb8
