Summary:
Two more bug fixes in db_stress:
1. this is to complete the fix of the regression bug causing overflowing when supporting FLAGS_prefix_size = -1.
2. Fix regression bug in compare iterator itself:
(1) when creating control iterator, which used the same read option as the normal iterator by mistake; (2) the logic of comparing has some problems. Fix them.
(3) disable validation for lower bound now, which generated some wildly different results. Disabling it to make normal tests pass while investigating it.
3. Cleaning up snapshots in verification failure cases. Memory is leaked otherwise.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5867
Test Plan: Run "make crash_test" for a while and see at least 1 is fixed.
Differential Revision: D17671712
fbshipit-source-id: 011f98ea1a72aef23e19ff28656830c78699b402
Summary:
When prefix_size = -1, stress test crashes with run time error because of overflow. Fix it by not using -1 but 7 in prefix scan mode.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5862
Test Plan:
Run
python -u tools/db_crashtest.py --simple whitebox --random_kill_odd \
888887 --compression_type=zstd
and see it doesn't crash.
Differential Revision: D17642313
fbshipit-source-id: f029e7651498c905af1b1bee6d310ae50cdcda41
Summary:
For now, crash_test is not able to report any failure for the logic related to iterator upper, lower bounds or iterators, or reseek. These are features prone to errors. Improve db_stress in several ways:
(1) For each iterator run, reseek up to 3 times.
(2) For every iterator, create control iterator with upper or lower bound, with total order seek. Compare the results with the iterator.
(3) Make simple crash test to avoid prefix size to have more coverage.
(4) make prefix_size = 0 a valid size and -1 to indicate disabling prefix extractor.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5846
Test Plan: Manually hack the code to create wrong results and see they are caught by the tool.
Differential Revision: D17631760
fbshipit-source-id: acd460a177bd2124a5ffd7fff490702dba63030b
Summary:
Further apply formatter to more recent commits.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5830
Test Plan: Run all existing tests.
Differential Revision: D17488031
fbshipit-source-id: 137458fd94d56dd271b8b40c522b03036943a2ab
Summary:
Some recent commits might not have passed through the formatter. I formatted recent 45 commits. The script hangs for more commits so I stopped there.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5827
Test Plan: Run all existing tests.
Differential Revision: D17483727
fbshipit-source-id: af23113ee63015d8a43d89a3bc2c1056189afe8f
Summary:
PR https://github.com/facebook/rocksdb/issues/4020 enabled partitioned indexes/filters in stress tests; however,
this causes assertion failures in BatchedOpsStressTest. This patch
disables them until we can root cause the failures.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5811
Test Plan: Ran the script and made sure it only uses the binary search index.
Differential Revision: D17399366
Pulled By: ltamasi
fbshipit-source-id: adb116e6297f9c6ccd7ac15b6a16c9aa91f21ac5
Summary:
file_reader_writer.h and .cc contain several files and helper function, and it's hard to navigate. Separate it to multiple files and put them under file/
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5803
Test Plan: Build whole project using make and cmake.
Differential Revision: D17374550
fbshipit-source-id: 10efca907721e7a78ed25bbf74dc5410dea05987
Summary:
PR https://github.com/facebook/rocksdb/issues/4020 implicitly enabled the hash index as well in stress/crash
tests, resulting in assertion failures in Block. This patch disables
the hash index until we can pinpoint the root cause of these issues.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5792
Test Plan:
Ran tools/db_crashtest.py and made sure it only uses index types 0 and 2
(binary search and partitioned index).
Differential Revision: D17346777
Pulled By: ltamasi
fbshipit-source-id: b4318f37f1fda3ee1bbff4ef2c2f556ca9e6b551
Summary:
This is required to compile on Windows with Visual Studio 2015.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5786
Differential Revision: D17335994
fbshipit-source-id: 8f9568310bc6f697e312b5e24ad465e9084f0011
Summary:
- In `db_stress`, support choosing index type and whether to enable filter partitioning, and randomly set those options in crash test
- When partitioned filter is enabled by crash test, force partitioned index to also be enabled since it's a prerequisite
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4020
Test Plan:
currently this is blocked on fixing the bug that crash test caught:
```
$ TEST_TMPDIR=/data/compaction_bench python ./tools/db_crashtest.py blackbox --simple --interval=10 --max_key=10000000
...
Verification failed for column family 0 key 937501: Value not found: NotFound:
Crash-recovery verification failed :(
```
Differential Revision: D8508683
Pulled By: maysamyabandeh
fbshipit-source-id: 0337e5d0558bcef26b1f3699f47265a2c1e99629
Summary:
https://github.com/facebook/rocksdb/pull/5741 added compaction TTL to crash test, but it causes assertion fails for FIFO compaction. Disable this combination for now while we debug the assertion failure.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5749
Test Plan: Run crash test and observe that when compaction_style=2, compaction_ttl is always 0.
Differential Revision: D17078292
fbshipit-source-id: 446821a3b9739956094d5e4f9be1251a15b57f5d
Summary:
Covering periodic compaction and compaction TTL can help us expose potential issues. Add it there.
Randomly select value for these two options.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5741
Test Plan: Run crash_test and see the perameters generated.
Differential Revision: D17059515
fbshipit-source-id: 8213974846a0b6a22fc13be705825c9054d1d097
Summary:
MyRocks currently sets `max_write_buffer_number_to_maintain` in order to maintain enough history for transaction conflict checking. The effectiveness of this approach depends on the size of memtables. When memtables are small, it may not keep enough history; when memtables are large, this may consume too much memory.
We are proposing a new way to configure memtable list history: by limiting the memory usage of immutable memtables. The new option is `max_write_buffer_size_to_maintain` and it will take precedence over the old `max_write_buffer_number_to_maintain` if they are both set to non-zero values. The new option accounts for the total memory usage of flushed immutable memtables and mutable memtable. When the total usage exceeds the limit, RocksDB may start dropping immutable memtables (which is also called trimming history), starting from the oldest one.
The semantics of the old option actually works both as an upper bound and lower bound. History trimming will start if number of immutable memtables exceeds the limit, but it will never go below (limit-1) due to history trimming.
In order the mimic the behavior with the new option, history trimming will stop if dropping the next immutable memtable causes the total memory usage go below the size limit. For example, assuming the size limit is set to 64MB, and there are 3 immutable memtables with sizes of 20, 30, 30. Although the total memory usage is 80MB > 64MB, dropping the oldest memtable will reduce the memory usage to 60MB < 64MB, so in this case no memtable will be dropped.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5022
Differential Revision: D14394062
Pulled By: miasantreble
fbshipit-source-id: 60457a509c6af89d0993f988c9b5c2aa9e45f5c5
Summary:
AtomicFlushStressTest is a powerful test, but right now we only run it for atomic_flush=true + disable_wal=true. We further extend it to the case where atomic_flush=false + disable_wal = false. All the workload generation and validation can stay the same.
Atomic flush crash test is also changed to switch between the two test scenarios. It makes the name "atomic flush crash test" out of sync from what it really does. We leave it as it is to avoid troubles with continous test set-up.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5729
Test Plan: Run "CRASH_TEST_KILL_ODD=188 TEST_TMPDIR=/dev/shm/ USE_CLANG=1 make whitebox_crash_test_with_atomic_flush", observe the settings used and see it passed.
Differential Revision: D16969791
fbshipit-source-id: 56e37487000ae631e31b0100acd7bdc441c04163
Summary:
Atomic white box test's kill odd is the same as normal test. However, in the scenario that only WritableFileWriter::Append() is blacklisted, WritableFileWriter::Flush() dominates the killing odds. Normally, most of WritableFileWriter::Flush() are called in WAL writes, where every write triggers a WAL flush. In atomic test, WAL is disabled, so the kill happens less frequently than we antipated. In some rare cases, the kill didn't end up with happening (for reasons I still don't fully understand) and cause the stress test timeout.
If WAL is disabled, make the odds 5x likely to trigger.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5717
Test Plan: Run whitebox_crash_test_with_atomic_flush and whitebox_crash_test and observe the kill odds printed out.
Differential Revision: D16897237
fbshipit-source-id: cbf5d96f6fc0e980523d0f1f94bf4e72cdb82d1c
Summary:
Right now VerifyChecksum() doesn't do read-ahead. In some use cases, users won't be able to achieve good performance. With this change, by default, RocksDB will do a default readahead, and users will be able to overwrite the readahead size by passing in a ReadOptions.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5713
Test Plan: Add a new unit test.
Differential Revision: D16860874
fbshipit-source-id: 0cff0fe79ac855d3d068e6ccd770770854a68413
Summary:
Add a command in ldb so that users can print out tombstones in SST files.
In order to test the code, change the interface of LDBCommandRunner::RunCommand() so that it doesn't return from the program, but return the status code.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5615
Test Plan: Add a new unit test
Differential Revision: D16550326
fbshipit-source-id: 88ddfe6984bdcbb3a528abdd115089df09eba52e
Summary:
This PR adds support in block cache trace analyzer to read from human readable trace file. This is needed when a user does not have access to the binary trace file.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5679
Test Plan: USE_CLANG=1 make check -j32
Differential Revision: D16697239
Pulled By: HaoyuHuang
fbshipit-source-id: f2e29d7995816c389b41458f234ec8e184a924db
Summary:
This PR adds four more eviction policies.
- OPT [1]
- Hyperbolic caching [2]
- ARC [3]
- GreedyDualSize [4]
[1] L. A. Belady. 1966. A Study of Replacement Algorithms for a Virtual-storage Computer. IBM Syst. J. 5, 2 (June 1966), 78-101. DOI=http://dx.doi.org/10.1147/sj.52.0078
[2] Aaron Blankstein, Siddhartha Sen, and Michael J. Freedman. 2017. Hyperbolic caching: flexible caching for web applications. In Proceedings of the 2017 USENIX Conference on Usenix Annual Technical Conference (USENIX ATC '17). USENIX Association, Berkeley, CA, USA, 499-511.
[3] Nimrod Megiddo and Dharmendra S. Modha. 2003. ARC: A Self-Tuning, Low Overhead Replacement Cache. In Proceedings of the 2nd USENIX Conference on File and Storage Technologies (FAST '03). USENIX Association, Berkeley, CA, USA, 115-130.
[4] N. Young. The k-server dual and loose competitiveness for paging. Algorithmica, June 1994, vol. 11,(no.6):525-41. Rewritten version of ''On-line caching as cache size varies'', in The 2nd Annual ACM-SIAM Symposium on Discrete Algorithms, 241-250, 1991.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5644
Differential Revision: D16548817
Pulled By: HaoyuHuang
fbshipit-source-id: 838f76db9179f07911abaab46c97e1c929cfcd63
Summary:
This is a new API added to db.h to allow for fetching all merge operands associated with a Key. The main motivation for this API is to support use cases where doing a full online merge is not necessary as it is performance sensitive. Example use-cases:
1. Update subset of columns and read subset of columns -
Imagine a SQL Table, a row is encoded as a K/V pair (as it is done in MyRocks). If there are many columns and users only updated one of them, we can use merge operator to reduce write amplification. While users only read one or two columns in the read query, this feature can avoid a full merging of the whole row, and save some CPU.
2. Updating very few attributes in a value which is a JSON-like document -
Updating one attribute can be done efficiently using merge operator, while reading back one attribute can be done more efficiently if we don't need to do a full merge.
----------------------------------------------------------------------------------------------------
API :
Status GetMergeOperands(
const ReadOptions& options, ColumnFamilyHandle* column_family,
const Slice& key, PinnableSlice* merge_operands,
GetMergeOperandsOptions* get_merge_operands_options,
int* number_of_operands)
Example usage :
int size = 100;
int number_of_operands = 0;
std::vector<PinnableSlice> values(size);
GetMergeOperandsOptions merge_operands_info;
db_->GetMergeOperands(ReadOptions(), db_->DefaultColumnFamily(), "k1", values.data(), merge_operands_info, &number_of_operands);
Description :
Returns all the merge operands corresponding to the key. If the number of merge operands in DB is greater than merge_operands_options.expected_max_number_of_operands no merge operands are returned and status is Incomplete. Merge operands returned are in the order of insertion.
merge_operands-> Points to an array of at-least merge_operands_options.expected_max_number_of_operands and the caller is responsible for allocating it. If the status returned is Incomplete then number_of_operands will contain the total number of merge operands found in DB for key.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5604
Test Plan:
Added unit test and perf test in db_bench that can be run using the command:
./db_bench -benchmarks=getmergeoperands --merge_operator=sortlist
Differential Revision: D16657366
Pulled By: vjnadimpalli
fbshipit-source-id: 0faadd752351745224ee12d4ae9ef3cb529951bf
Summary:
This PR updated the python script to plot graphs for stats output from block cache analyzer.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5673
Test Plan: Manually run the script to generate graphs.
Differential Revision: D16657145
Pulled By: HaoyuHuang
fbshipit-source-id: fd510b5fd4307835f9a986fac545734dbe003d28
Summary:
This PR implements cache eviction using reinforcement learning. It includes two implementations:
1. An implementation of Thompson Sampling for the Bernoulli Bandit [1].
2. An implementation of LinUCB with disjoint linear models [2].
The idea is that a cache uses multiple eviction policies, e.g., MRU, LRU, and LFU. The cache learns which eviction policy is the best and uses it upon a cache miss.
Thompson Sampling is contextless and does not include any features.
LinUCB includes features such as level, block type, caller, column family id to decide which eviction policy to use.
[1] Daniel J. Russo, Benjamin Van Roy, Abbas Kazerouni, Ian Osband, and Zheng Wen. 2018. A Tutorial on Thompson Sampling. Found. Trends Mach. Learn. 11, 1 (July 2018), 1-96. DOI: https://doi.org/10.1561/2200000070
[2] Lihong Li, Wei Chu, John Langford, and Robert E. Schapire. 2010. A contextual-bandit approach to personalized news article recommendation. In Proceedings of the 19th international conference on World wide web (WWW '10). ACM, New York, NY, USA, 661-670. DOI=http://dx.doi.org/10.1145/1772690.1772758
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5610
Differential Revision: D16435067
Pulled By: HaoyuHuang
fbshipit-source-id: 6549239ae14115c01cb1e70548af9e46d8dc21bb
Summary:
The ObjectRegistry class replaces the Registrar and NewCustomObjects. Objects are registered with the registry by Type (the class must implement the static const char *Type() method).
This change is necessary for a few reasons:
- By having a class (rather than static template instances), the class can be passed between compilation units, meaning that objects could be registered and shared from a dynamic library with an executable.
- By having a class with instances, different units could have different objects registered. This could be useful if, for example, one Option allowed for a dynamic library and one did not.
When combined with some other PRs (being able to load shared libraries, a Configurable interface to configure objects to/from string), this code will allow objects in external shared libraries to be added to a RocksDB image at run-time, rather than requiring every new extension to be built into the main library and called explicitly by every program.
Test plan (on riversand963's devserver)
```
$COMPILE_WITH_ASAN=1 make -j32 all && sleep 1 && make check
```
All tests pass.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5293
Differential Revision: D16363396
Pulled By: riversand963
fbshipit-source-id: fbe4acb615bfc11103eef40a0b288845791c0180
Summary:
Right now, ldb cannot scan a DB with merge operands with default ldb. There is no hard to give a general merge operator so that it can at least print out something
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5607
Test Plan: Run ldb against a DB with merge operands and see the outputs.
Differential Revision: D16442634
fbshipit-source-id: c66c414ec07f219cfc6e6ec2cc14c783ee95df54
Summary:
- Compute correlation between a few features and predictions, e.g., number of accesses since the last access vs number of accesses till the next access on a block.
- Output human readable trace file so python can consume it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5596
Test Plan: make clean && USE_CLANG=1 make check -j32
Differential Revision: D16373200
Pulled By: HaoyuHuang
fbshipit-source-id: c848d26bc2e9210461f317d7dbee42d55be5a0cc
Summary:
Atomic flush test started to fail after https://github.com/facebook/rocksdb/issues/5099. Then https://github.com/facebook/rocksdb/issues/5278 provided a fix after
which the same error occurred much less frequently. However it still occur
occasionally. Not sure what the root cause is. This PR disables the feature of
snapshot list refresh, and we should keep an eye on the failure in the future.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5581
Differential Revision: D16295985
Pulled By: riversand963
fbshipit-source-id: c9e62e65133c52c21b07097de359632ca62571e4
Summary:
ldb idump now only works for default column family. Extend it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5594
Test Plan: Compile and run the tool against a multiple CF DB.
Differential Revision: D16380684
fbshipit-source-id: bfb8af36fdad1806837c90aaaab492d71528aceb
Summary:
This PR traces the referenced key for Get for all types of blocks. This is useful when evaluating hybrid row-block caches.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5548
Test Plan: make clean && USE_CLANG=1 make check -j32
Differential Revision: D16157979
Pulled By: HaoyuHuang
fbshipit-source-id: f6327411c9deb74e35e22a35f66cdbae09ab9d87
Summary:
Currently, when the block cache is used for the filter block, it is not
really the block itself that is stored in the cache but a FilterBlockReader
object. Since this object is not pure data (it has, for instance, pointers that
might dangle, including in one case a back pointer to the TableReader), it's not
really sharable. To avoid the issues around this, the current code erases the
cache entries when the TableReader is closed (which, BTW, is not sufficient
since a concurrent TableReader might have picked up the object in the meantime).
Instead of doing this, the patch moves the FilterBlockReader out of the cache
altogether, and decouples the filter reader object from the filter block.
In particular, instead of the TableReader owning, or caching/pinning the
FilterBlockReader (based on the customer's settings), with the change the
TableReader unconditionally owns the FilterBlockReader, which in turn
owns/caches/pins the filter block. This change also enables us to reuse the code
paths historically used for data blocks for filters as well.
Note:
Eviction statistics for filter blocks are temporarily broken. We plan to fix this in a
separate phase.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5504
Test Plan: make asan_check
Differential Revision: D16036974
Pulled By: ltamasi
fbshipit-source-id: 770f543c5fb4ed126fd1e04bfd3809cf4ff9c091
Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5563
Test Plan: Manually run the script on files generated by block_cache_trace_analyzer.
Differential Revision: D16214400
Pulled By: HaoyuHuang
fbshipit-source-id: 94485eed995e9b2b63e197c5dfeb80129fa7897f
Summary:
This PR adds a ghost cache for admission control. Specifically, it admits an entry on its second access.
It also adds a hybrid row-block cache that caches the referenced key-value pairs of a Get/MultiGet request instead of its blocks.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5534
Test Plan: make clean && COMPILE_WITH_ASAN=1 make check -j32
Differential Revision: D16101124
Pulled By: HaoyuHuang
fbshipit-source-id: b99edda6418a888e94eb40f71ece45d375e234b1
Summary:
When atomic flush stress test fails, we print internal keys within the range with mismatched key/values for all column families.
Test plan (on devserver)
Manually hack the code to randomly insert wrong data. Run the test.
```
$make clean && COMPILE_WITH_TSAN=1 make -j32 db_stress
$./db_stress -test_atomic_flush=true -ops_per_thread=10000
```
Check that proper error messages are printed, as follows:
```
2019/07/08-17:40:14 Starting verification
Verification failed
Latest Sequence Number: 190903
[default] 000000000000050B => 56290000525350515E5F5C5D5A5B5859
[3] 0000000000000533 => EE100000EAEBE8E9E6E7E4E5E2E3E0E1FEFFFCFDFAFBF8F9
Internal keys in CF 'default', [000000000000050B, 0000000000000533] (max 8)
key 000000000000050B seq 139920 type 1
key 0000000000000533 seq 0 type 1
Internal keys in CF '3', [000000000000050B, 0000000000000533] (max 8)
key 0000000000000533 seq 0 type 1
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5549
Differential Revision: D16158709
Pulled By: riversand963
fbshipit-source-id: f07fa87763f87b3bd908da03c956709c6456bcab
Summary:
Right now ldb can open running DB through read-only DB. However, it might leave info logs files to the read-only DB directory. Add an option to open the DB as secondary to avoid it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5537
Test Plan:
Run
./ldb scan --max_keys=10 --db=/tmp/rocksdbtest-2491/dbbench --secondary_path=/tmp --no_value --hex
and
./ldb get 0x00000000000000103030303030303030 --hex --db=/tmp/rocksdbtest-2491/dbbench --secondary_path=/tmp
against a normal db_bench run and observe the output changes. Also observe that no new info logs files are created under /tmp/rocksdbtest-2491/dbbench.
Run without --secondary_path and observe that new info logs created under /tmp/rocksdbtest-2491/dbbench.
Differential Revision: D16113886
fbshipit-source-id: 4e09dec47c2528f6ca08a9e7a7894ba2d9daebbb
Summary:
Print out some more information when db_tress fails with verification failures to help debugging problems.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5543
Test Plan:
Manually ingest some failures and observe the outputs are like this:
Verification failed
[default] 0000000000199A5A => 7C3D000078797A7B74757677707172736C6D6E6F68696A6B
[6] 000000000019C8BD => 65380000616063626D6C6F6E69686B6A
internal keys in default CF [0000000000199A5A, 000000000019C8BD] (max 8)
key 0000000000199A5A seq 179246 type 1
key 000000000019C8BD seq 163970 type 1
Lastest Sequence Number: 292234
Differential Revision: D16153717
fbshipit-source-id: b33fa50a828c190cbf8249a37955432044f92daf
Summary:
Sometimes it is helpful to fetch the whole history of stats after benchmark runs. Add such an option
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5532
Test Plan: Run the benchmark manually and observe the output is as expected.
Differential Revision: D16097764
fbshipit-source-id: 10b5b735a22a18be198b8f348be11f11f8806904
Summary:
Formatting fixes in db_bench_tool that were accidentally omitted
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5525
Test Plan: Unit tests
Differential Revision: D16078516
Pulled By: elipoz
fbshipit-source-id: bf8df0e3f08092a91794ebf285396d9b8a335bb9
Summary:
This PR creates cache_simulator.h file. It contains a CacheSimulator that runs against a block cache trace record. We can add alternative cache simulators derived from CacheSimulator later. For example, this PR adds a PrioritizedCacheSimulator that inserts filter/index/uncompressed dictionary blocks with high priority.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5517
Test Plan: make clean && COMPILE_WITH_ASAN=1 make check -j32
Differential Revision: D16043689
Pulled By: HaoyuHuang
fbshipit-source-id: 65f28ed52b866ffb0e6eceffd7f9ca7c45bb680d
Summary:
`create_column_family` cmd already exists but was somehow missed in the help message.
also add `drop_column_family` cmd which can drop a cf without opening db.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5503
Test Plan: Updated existing ldb_test.py to test deleting a column family.
Differential Revision: D16018414
Pulled By: lightmark
fbshipit-source-id: 1fc33680b742104fea86b10efc8499f79e722301
sdong