Summary:
Currently, the following interleaving of events can lead to SuperVersion containing both immutable memtables as well as the resulting L0. This can cause Get to return incorrect result if there are merge operands. This may also affect other operations such as single deletes.
```
time main_thr bg_flush_thr bg_compact_thr compact_thr set_opts_thr
0 | WriteManifest:0
1 | issue compact
2 | wait
3 | Merge(counter)
4 | issue flush
5 | wait
6 | WriteManifest:1
7 | wake up
8 | write manifest
9 | wake up
10 | Get(counter)
11 | remove imm
V
```
The reason behind is that: one bg flush thread's installing new `Version` can be batched and performed by another thread that is the "leader" MANIFEST writer. This bg thread removes the memtables from current super version only after `LogAndApply` returns. After the leader MANIFEST writer signals (releasing mutex) this bg flush thread, it is possible that another thread sees this cf with both memtables (whose data have been flushed to the newest L0) and the L0 before this bg flush thread removes the memtables.
To address this issue, each bg flush thread can pass a callback function to `LogAndApply`. The callback is responsible for removing the memtables. Therefore, the leader MANIFEST writer can call this callback and remove the memtables before releasing the mutex.
Test plan (devserver)
```
$make merge_test
$./merge_test --gtest_filter=MergeTest.MergeWithCompactionAndFlush
$make check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6069
Reviewed By: cheng-chang
Differential Revision: D18790894
Pulled By: riversand963
fbshipit-source-id: e41bd600c0448b4f4b2deb3f7677f95e3076b4ed
Summary:
Similarly to how https://github.com/facebook/rocksdb/issues/7345
integrated blob file writing into the flush process,
the patch adds support for writing blob files to the compaction logic.
Namely, if `enable_blob_files` is set, large values encountered during
compaction are extracted to blob files and replaced with blob indexes.
The resulting blob files are then logged to the MANIFEST as part of the
compaction job's `VersionEdit` and added to the `Version` alongside any
table files written by the compaction. Any errors during blob file building fail
the compaction job.
There will be a separate follow-up patch to perform blob garbage collection
during compactions.
In addition, the patch continues to chip away at the mess around computing
various compaction related statistics by eliminating some code duplication
and by making the `num_output_files` and `bytes_written` stats more consistent
for flushes, compactions, and recovery.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7573
Test Plan: `make check`
Reviewed By: riversand963
Differential Revision: D24404696
Pulled By: ltamasi
fbshipit-source-id: 21216af3a172ad3ce8f85d11cd30923784ae426c
Summary:
This is intended as the first commit toward a near-optimal alternative to static Bloom filters for SSTs. Stephan Walzer and I have agreed upon the name "Ribbon" for a PHSF based on his linear system construction in "Efficient Gauss Elimination for Near-Quadratic Matrices with One Short Random Block per Row, with Applications" ("SGauss") and my much faster "on the fly" algorithm for gaussian elimination (or for this linear system, "banding"), which can be faster than peeling while also more compact and flexible. See util/ribbon_alg.h for more detailed introduction and background. RIBBON = Rapid Incremental Boolean Banding ON-the-fly
This commit just adds generic (templatized) core algorithms and a basic unit test showing some features, including the ability to construct structures within 2.5% space overhead vs. information theoretic lower bound. (Compare to cache-local Bloom filter's ~50% space overhead -> ~30% reduction anticipated.) This commit does not include the storage scheme necessary to make queries fast, especially for filter queries, nor fractional "result bits", but there is some description already and those implementations will come soon. Nor does this commit add FilterPolicy support, for use in SST files, but that will also come soon.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7491
Reviewed By: jay-zhuang
Differential Revision: D24517954
Pulled By: pdillinger
fbshipit-source-id: 0119ee597e250d7e0edd38ada2ba50d755606fa7
Summary:
Add a threshold timestamp, full_history_ts_low_ of type `std::string*` to
`CompactionIterator`, so that RocksDB can also perform garbage collection during
compaction.
* If full_history_ts_low_ is nullptr, then compaction iterator does not perform
GC, preserving all timestamp history for all keys. Compaction iterator will
treat user key with different timestamps as different user keys.
* If full_history_ts_low_ is not nullptr, then compaction iterator performs
GC. GC will look at keys older than `*full_history_ts_low_` and determine their
eligibility based on factors including snapshots.
Current rules of GC:
* If an internal key is in the same snapshot as a previous counterpart
with the same user key, and this key is eligible for GC, and the key is
not single-delete or merge operand, then this key can be dropped. Note
that the previous internal key cannot be a merge operand either.
* If a tombstone is the most recent one in the earliest snapshot and it
is eligible for GC, and keyNotExistsBeyondLevel() is true, then this
tombstone can be dropped.
* If a tombstone is the most recent one in a snapshot and it is eligible
for GC, and the compaction is at bottommost level, then all other older
internal keys of the same user key must also be eligible for GC, thus
can be dropped
* Single-delete, delete-range and merge are not currently supported.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7556
Test Plan: make check
Reviewed By: ltamasi
Differential Revision: D24507728
Pulled By: riversand963
fbshipit-source-id: 3c09c7301f41eed76dfcf4d1527e68cf6e0a8bb3
Summary:
This PR makes it able to `LogAndApply` `VersionEdit`s related to WALs, and also be able to `Recover` from MANIFEST with WAL related `VersionEdit`s.
The `VersionEdit`s related to WAL are treated similarly as those related to column family operations, they are not applied to versions, but can be in a commit group. Mixing WAL related `VersionEdit`s with other types of edits will make logic in `ProcessManifestWrite` more complicated, so `VersionEdit`s related to WAL can either be WAL additions or deletions, like column family add and drop.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7256
Test Plan: a set of unit tests are added in `version_set_test.cc`
Reviewed By: riversand963
Differential Revision: D23123238
Pulled By: cheng-chang
fbshipit-source-id: 246be2ed4744fd03fa2738aba408aaa611d0379c
Summary:
To minimize dependencies for Ribbon filter code in progress,
core part of coding.h for fixed sizes has been moved to coding_lean.h.
Also, generic versions of these functions have been added to math128.h
(since the generic versions are likely only to be used along with
Unsigned128).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7587
Test Plan: Unit tests added for new functions
Reviewed By: jay-zhuang
Differential Revision: D24486718
Pulled By: pdillinger
fbshipit-source-id: a69768f742379689442135fa52237c01dfe2647e
Summary:
In addition to trace block cache access, we want to support trace queries on MySQL. To achieve that StartTrace and EndTrace need to be added to the stackable_db.h
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7585
Reviewed By: zhichao-cao
Differential Revision: D24482306
Pulled By: nmjnmjnmj
fbshipit-source-id: de641b4837c64cd33b44b5bebaeae5d1527c8c31
Summary:
As suggested by pdillinger ,The name of kLogFile is misleading, in some tests, kLogFile is defined as info log. Replace it with kWalFile and move it to public, which will be used in https://github.com/facebook/rocksdb/issues/7523
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7580
Test Plan: make check
Reviewed By: riversand963
Differential Revision: D24485420
Pulled By: zhichao-cao
fbshipit-source-id: 955e3dacc1021bb590fde93b0a568ffe9ad80799
Summary:
This commit makes https://github.com/facebook/rocksdb/issues/6262's code change tidier and easier to understand by:
1. Wrapping parallel compression initialization and termination into
common methods;
2. Wrapping BlockRep initialization, push/pop into common methods;
3. Wrapping file size estimation into common methods;
4. Fixing function declarations that use non-const reference;
5. Fixing some uninitialized variables;
6. Fixing first_block data race;
7. Making BlockRep::status check in BlockBasedTableBuilder::Finish only present
if ok();
8. Making assert(ok()) in BlockBasedTableBuilder::CompressAndVerifyBlock only
present in non-parallel compression mode. In parallel compression mode,
compression will abort if status is not OK;
9. Eliminating potential data race caused by BlockBasedTableBuilder::GetStatus()
and BlockBasedTableBuilder::GetIOStatus() by returning status copy instead of
unprotected reference.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6888
Reviewed By: ajkr
Differential Revision: D21957110
Pulled By: jay-zhuang
fbshipit-source-id: 3a29892f249209513f030349756cecd7736eae80
Summary:
Remove function calling in assert statement as assert is a no
op in opt build and that function might not be called. This causes hang
in closing RocksDB when refit level is set.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7581
Test Plan: make check -j64
Reviewed By: riversand963
Differential Revision: D24466420
Pulled By: akankshamahajan15
fbshipit-source-id: 97db4ec5a95ae693c3290e176a3c12a9b1ad2f6d
Summary:
When `ASSERT_STATUS_CHECKED` is enabled, `transaction_test` does not pass without this PR.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7572
Test Plan: `ASSERT_STATUS_CHECKED=1 make -j32 transaction_test && ./transaction_test`
Reviewed By: zhichao-cao
Differential Revision: D24404319
Pulled By: cheng-chang
fbshipit-source-id: 13689035995366ab06d8eada3ea404e45fef8bc5
Summary:
Further refinement of the earlier PR. Now the Status is NotFound with a subcode of PathNotFound. Also the existing functions for options parsing/loading are reverted to return InvalidArgument no matter in which way the user-provided arguments are deemed invalid.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7563
Reviewed By: zhichao-cao
Differential Revision: D24422491
Pulled By: ajkr
fbshipit-source-id: ba6b237cd0584d3f925c5ba0d349aeb8c250af67
Summary:
Ignore read error in 'FilePrefetchBuffer::TryReadFromCache' as status is ignored
and bool value is returned. Return error if prefetch fails in
'PrefetchTail' as we have planned to return Prefetch failures to users.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7574
Test Plan:
make check -j64,
python -u tools/db_crashtest.py blackbox
Reviewed By: zhichao-cao
Differential Revision: D24408825
Pulled By: akankshamahajan15
fbshipit-source-id: feebda771415998253fbe54632f13e6e75b7a243
Summary:
This PR adds support for writing a location identifier of the DB host to SST files as a table property. By default, the hostname is used, but can be overridden by the user. There have been some recent corruptions in files written by ```SstFileWriter``` before checksumming, so this property can be used to trace it back to the writing host and checking the host for hardware isues.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7479
Test Plan: Add new unit tests
Reviewed By: pdillinger
Differential Revision: D24340671
Pulled By: anand1976
fbshipit-source-id: 2038949fd8d160c0633ccb4f9da77740f19fa2a2
Summary:
In order to be able to introduce more locking protocols, we need to abstract out the locking subsystem in TransactionDB into a set of interfaces.
PR https://github.com/facebook/rocksdb/pull/7013 introduces interface `LockTracker`. This PR is a follow up to take the first step to abstract out a `LockManager` interface.
Further modifications to the interface may be needed when introducing the first implementation of range lock. But the idea here is to put the range lock implementation based on range tree under the `utilities/transactions/lock/range/range_tree`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7532
Test Plan: point_lock_manager_test
Reviewed By: ajkr
Differential Revision: D24238731
Pulled By: cheng-chang
fbshipit-source-id: 2a9458cd8b3fb008d9529dbc4d3b28c24631f463
Summary:
The patch adds blob file support to the `Get` API by extending `Version` so that
whenever a blob reference is read from a file, the blob is retrieved from the corresponding
blob file and passed back to the caller. (This is assuming the blob reference is valid
and the blob file is actually part of the given `Version`.) It also introduces a cache
of `BlobFileReader`s called `BlobFileCache` that enables sharing `BlobFileReader`s
between callers. `BlobFileCache` uses the same backing cache as `TableCache`, so
`max_open_files` (if specified) limits the total number of open (table + blob) files.
TODO: proactively open/cache blob files and pin the cache handles of the readers in the
metadata objects similarly to what `VersionBuilder::LoadTableHandlers` does for
table files.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7540
Test Plan: `make check`
Reviewed By: riversand963
Differential Revision: D24260219
Pulled By: ltamasi
fbshipit-source-id: a8a2a4f11d3d04d6082201b52184bc4d7b0857ba
Summary:
Make LoadLatestOptions return PathNotFound if the options file does not exist. Added tests for the LoadOptions related methods.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7554
Reviewed By: akankshamahajan15
Differential Revision: D24298985
Pulled By: zhichao-cao
fbshipit-source-id: c9ae3cb12fc4a5bbef07743e1c1300f98a2441b3
Summary:
- Takes the burden off developer to close ColumnFamilyHandle instances before closing RocksDB instance
- The change is backward-compatible
----
Previously the pattern for working with Column Families was:
```java
try (final ColumnFamilyOptions cfOpts = new ColumnFamilyOptions().optimizeUniversalStyleCompaction()) {
// list of column family descriptors, first entry must always be default column family
final List<ColumnFamilyDescriptor> cfDescriptors = Arrays.asList(
new ColumnFamilyDescriptor(RocksDB.DEFAULT_COLUMN_FAMILY, cfOpts),
new ColumnFamilyDescriptor("my-first-columnfamily".getBytes(), cfOpts)
);
// a list which will hold the handles for the column families once the db is opened
final List<ColumnFamilyHandle> columnFamilyHandleList =
new ArrayList<>();
try (final DBOptions options = new DBOptions()
.setCreateIfMissing(true)
.setCreateMissingColumnFamilies(true);
final RocksDB db = RocksDB.open(options,
"path/to/do", cfDescriptors,
columnFamilyHandleList)) {
try {
// do something
} finally {
// NOTE user must explicitly frees the column family handles before freeing the db
for (final ColumnFamilyHandle columnFamilyHandle :
columnFamilyHandleList) {
columnFamilyHandle.close();
}
} // frees the column family options
}
} // frees the db and the db options
```
With the changes in this PR, the Java user no longer has to worry about manually closing the Column Families, which allows them to write simpler symmetrical create/free oriented code like this:
```java
try (final ColumnFamilyOptions cfOpts = new ColumnFamilyOptions().optimizeUniversalStyleCompaction()) {
// list of column family descriptors, first entry must always be default column family
final List<ColumnFamilyDescriptor> cfDescriptors = Arrays.asList(
new ColumnFamilyDescriptor(RocksDB.DEFAULT_COLUMN_FAMILY, cfOpts),
new ColumnFamilyDescriptor("my-first-columnfamily".getBytes(), cfOpts)
);
// a list which will hold the handles for the column families once the db is opened
final List<ColumnFamilyHandle> columnFamilyHandleList =
new ArrayList<>();
try (final DBOptions options = new DBOptions()
.setCreateIfMissing(true)
.setCreateMissingColumnFamilies(true);
final RocksDB db = RocksDB.open(options,
"path/to/do", cfDescriptors,
columnFamilyHandleList)) {
// do something
} // frees the column family options, then frees the db and the db options
}
}
```
**NOTE**: The changes in this PR are backwards API compatible, which means existing code using the original approach will also continue to function correctly.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7428
Reviewed By: cheng-chang
Differential Revision: D24063348
Pulled By: jay-zhuang
fbshipit-source-id: 648d7526669923128c863ead94516bf4d50ac658
Summary:
fix for clang_analyzer build failure in table_test because of
potential memory leak of memtable in case of ASSERT failure.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7553
Test Plan:
USE_CLANG=1 make analyze;
make check -j64
Reviewed By: jay-zhuang
Differential Revision: D24295042
Pulled By: akankshamahajan15
fbshipit-source-id: e9ea184367970fff3b520e33f3ceebf28d66ac8d
Summary:
Allows adding event listeners in RocksJava.
* Adds listeners getter and setter in `Options` and `DBOptions` classes.
* Adds `EventListener` Java interface and base class for implementing custom event listener callbacks - `AbstractEventListener`, which has an underlying native callback class implementing C++ `EventListener` class.
* `AbstractEventListener` class has mechanism for selectively enabling its callback methods in order to prevent invoking Java method if it is not implemented. This decreases performance cost in case only subset of event listener callback methods is needed - the JNI code for remaining "no-op" callbacks is not executed.
* The code is covered by unit tests in `EventListenerTest.java`, there are also tests added for setting/getting listeners field in `OptionsTest.java` and `DBOptionsTest.java`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7425
Reviewed By: pdillinger
Differential Revision: D24063390
Pulled By: jay-zhuang
fbshipit-source-id: 508c359538983d6b765e70d9989c351794a944ee
Summary:
Right now all I/O failures under
PartitionFilterBlock::CacheDependencies() is swallowed. Return error in
case prefetch fails.
On returning error in PartitionedFilterBlockReader::CacheDependencies was causing stress test failure because PrefetchBuffer is initialized with enable_ = true, as result when PosixMmapReadableFile::Read is called from Prefetch, scratch is ignored causing buffer to fill with garbage values. Initializing prefetch buffer by CreatePrefetchBuffer that sets enable_ with !ioptions.allow_mmap_reads fixed the problem as it returns without prefetching data if allow_mmap_reads is set.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7543
Test Plan:
make check -j64;
python -u tools/db_crashtest.py --simple blackbox
Reviewed By: anand1976
Differential Revision: D24284596
Pulled By: akankshamahajan15
fbshipit-source-id: f3f0fd44b59dcf60645730436f28564a07884868
Summary:
Update IOTrace operations in stackabledb.h and also trace few
other IO operations.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7514
Test Plan: make check -j64
Reviewed By: anand1976
Differential Revision: D24151202
Pulled By: akankshamahajan15
fbshipit-source-id: 112cd3d2041f8c6398b7b0ba1a783b8c93224d4a
Summary:
These notes existed on the release branches where they were backported, but were never added on master branch. Added them now and mentioned what minor release the fix originally appeared.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7545
Reviewed By: riversand963
Differential Revision: D24281759
Pulled By: ajkr
fbshipit-source-id: 7422e984b667793d6260dd32a7492afcb2ff1c4b
Summary:
Added unit tests that have paranoid_check = true that perform range deletions. At the moment, the deleted ranges do not appear to be checked as part of the paranoid checks.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7521
Reviewed By: zhichao-cao
Differential Revision: D24262175
Pulled By: ajkr
fbshipit-source-id: 1035e968f7ab8ccaa7af086b835a4e72c7e56743
Summary:
If crash test fails, don't delete the `expected_values_file` for later
debug. More details: https://github.com/facebook/rocksdb/issues/7530
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7534
Test Plan: local host
Reviewed By: ajkr
Differential Revision: D24239655
Pulled By: jay-zhuang
fbshipit-source-id: 3566f91a30aae1e27d2f51d910cddd08edb7d4cf
Summary:
as title
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7536
Test Plan: see the new `build-macos` tests pass in circleci
Reviewed By: jay-zhuang
Differential Revision: D24243218
Pulled By: cheng-chang
fbshipit-source-id: 9b5f8a859e54c99a9ebe7efff6f336458a5d42de
Summary:
The old flag-based APIs (`BlockBasedTableOptions::pin_l0_filter_and_index_blocks_in_cache` and `BlockBasedTableOptions::pin_top_level_index_and_filter`) were insufficient for our needs. For example, it was impossible to pin only unpartitioned meta-blocks, which could prevent block cache contention when turning on dictionary compression or during a migration to partitioned indexes/filters. It was also impossible to pin all meta-blocks in memory while having predictable memory usage via block cache. If we had continued adding flags to address these scenarios, they would have had significant overlap causing confusion. Instead, this PR deprecates the flags and starts a new API with non-overlapping options.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7520
Test Plan:
- new unit test
- added new options to stress/crash test and ran for a while: `$ python tools/db_crashtest.py blackbox --simple --max_key=1000000 -write_buffer_size=1048576 -target_file_size_base=1048576 -max_bytes_for_level_base=4194304 --interval=10 -value_size_mult=33 -column_families=1 -reopen=0`
Reviewed By: pdillinger
Differential Revision: D24200034
Pulled By: ajkr
fbshipit-source-id: 3fa7cfc71e7960f7a867511dd6ae5834dd73b13e
Summary:
This option determines whether WALs will be tracked in MANIFEST and verified on recovery.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7275
Test Plan:
db_options_test
options_test
Reviewed By: pdillinger
Differential Revision: D23181418
Pulled By: cheng-chang
fbshipit-source-id: 5dd1cdc166f3dfc1c93c094df4a2f7734e3b4547
Summary:
Cover paranoid_file_checks in crash test.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7489
Test Plan: Run crash tests for hours and didn't see any failure.
Reviewed By: ajkr
Differential Revision: D24063868
fbshipit-source-id: 7b48b110e66ce78ae5d0c99a9f32af86edd34c1e
Summary:
The `std::pair(const T1& x, const T2& y);` constructor is `constexpr`
only starting from C++14; relying on this breaks compilation on certain
compilers/platforms we need to support.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7519
Test Plan: `make check`
Reviewed By: ajkr
Differential Revision: D24195747
Pulled By: ltamasi
fbshipit-source-id: 665e8fbc9747675bb49c5d895aad3dcf2714750f
Summary:
The patch does some cleanup in and around the legacy `BlobLogReader` class:
* It renames the class to `BlobLogSequentialReader` to emphasize that it is for
sequentially iterating through blobs in a blob file, as opposed to doing random
point reads using `BlobIndex`es (which is `BlobFileReader`'s jurisdiction).
* It removes some dead code from the old BlobDB implementation that references
`BlobLogReader` (namely the method `BlobFile::OpenRandomAccessReader`).
* It cleans up some `#include`s and forward declarations.
* It fixes some incorrect/outdated comments related to the reader class.
* It adds a few assertions to the `Read` methods of the class.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7517
Test Plan: `make check`
Reviewed By: riversand963
Differential Revision: D24172611
Pulled By: ltamasi
fbshipit-source-id: 43e2ae1eba5c3dd30c1070cb00f217edc45bd64f
Summary:
The patch adds a class called `BlobFileReader` that can be used to retrieve blobs
using the information available in blob references (e.g. blob file number, offset, and
size). This will come in handy when implementing blob support for `Get`, `MultiGet`,
and iterators, and also for compaction/garbage collection.
When a `BlobFileReader` object is created (using the factory method `Create`),
it first checks whether the specified file is potentially valid by comparing the file
size against the combined size of the blob file header and footer (files smaller than
the threshold are considered malformed). Then, it opens the file, and reads and verifies
the header and footer. The verification involves magic number/CRC checks
as well as checking for unexpected header/footer fields, e.g. incorrect column family ID
or TTL blob files.
Blobs can be retrieved using `GetBlob`. `GetBlob` validates the offset and compression
type passed by the caller (because of the presence of the header and footer, the
specified offset cannot be too close to the start/end of the file; also, the compression type
has to match the one in the blob file header), and retrieves and potentially verifies and
uncompresses the blob. In particular, when `ReadOptions::verify_checksums` is set,
`BlobFileReader` reads the blob record header as well (as opposed to just the blob itself)
and verifies the key/value size, the key itself, as well as the CRC of the blob record header
and the key/value pair.
In addition, the patch exposes the compression type from `BlobIndex` (both using an
accessor and via `DebugString`), and adds a blob file read latency histogram to
`InternalStats` that can be used with `BlobFileReader`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7461
Test Plan: `make check`
Reviewed By: riversand963
Differential Revision: D23999219
Pulled By: ltamasi
fbshipit-source-id: deb6b1160d251258b308d5156e2ec063c3e12e5e
Yanqin Jin