Summary:
Added append with checksum handoff API to hdfs

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8084

Test Plan: make check

Reviewed By: ajkr

Differential Revision: D27237823

Pulled By: zhichao-cao

fbshipit-source-id: 93b38db23b1811a6daa049afb89240089ec6f67c

Summary:
As title. All core db implementations should stay in db_impl.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8082

Test Plan: make check

Reviewed By: ajkr

Differential Revision: D27211442

Pulled By: riversand963

fbshipit-source-id: e0953fde75064740e899aaff7989ff033b7f5232

Summary:
Fix the following error while running `make crash_test`
```
Traceback (most recent call last):
  File "tools/db_crashtest.py", line 705, in <module>
    main()
  File "tools/db_crashtest.py", line 696, in main
    blackbox_crash_main(args, unknown_args)
  File "tools/db_crashtest.py", line 479, in blackbox_crash_main
    + list({'db': dbname}.items())), unknown_args)
  File "tools/db_crashtest.py", line 414, in gen_cmd
    finalzied_params = finalize_and_sanitize(params)
  File "tools/db_crashtest.py", line 331, in finalize_and_sanitize
    dest_params.get("user_timestamp_size") > 0):
TypeError: '>' not supported between instances of 'NoneType' and 'int'
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8091

Test Plan: make crash_test

Reviewed By: ltamasi

Differential Revision: D27268276

Pulled By: riversand963

fbshipit-source-id: ed2873b9587ecc51e24abc35ef2bd3d91fb1ed1b

Summary:
This is a small fix to what I think is a mistype in two comments in `DBOptionsInterface.java`. If it was not an error, feel free to close.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8086

Reviewed By: ajkr

Differential Revision: D27260488

Pulled By: mrambacher

fbshipit-source-id: 469daadaf6039d5b5187132b8e0c7c3672842f21

Summary:
As title.
Always specify namespace::symbol_name...
Test plan
CircleCI and other CI results

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8090

Reviewed By: ltamasi

Differential Revision: D27256130

Pulled By: riversand963

fbshipit-source-id: b9b9ae2b3a8b4a16f0384292e71c6aecca93c570

Summary:
Add some basic test for user-defined timestamp to db_stress. Currently,
read with timestamp always tries to read using the current timestamp.
Due to the per-key timestamp-sequence ordering constraint, we only add timestamp-
related tests to the `NonBatchedOpsStressTest` since this test serializes accesses
to the same key and uses a file to cross-check data correctness.
The timestamp feature is not supported in a number of components, e.g. Merge, SingleDelete,
DeleteRange, CompactionFilter, Readonly instance, secondary instance, SST file ingestion, transaction,
etc. Therefore, db_stress should exit if user enables both timestamp and these features at the same
time. The (currently) incompatible features can be found in
`CheckAndSetOptionsForUserTimestamp`.

This PR also fixes a bug triggered when timestamp is enabled together with
`index_type=kBinarySearchWithFirstKey`. This bug fix will also be in another separate PR
with more unit tests coverage. Fixing it here because I do not want to exclude the index type
from crash test.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8061

Test Plan: make crash_test_with_ts

Reviewed By: jay-zhuang

Differential Revision: D27056282

Pulled By: riversand963

fbshipit-source-id: c3e00ad1023fdb9ebbdf9601ec18270c5e2925a9

Summary:
Since our stress/crash tests by default generate values of size 8, 16, or 24,
it does not make much sense to set `min_blob_size` to 256. The patch
updates the set of potential `min_blob_size` values in the crash test
script and in `db_stress` where it might be set dynamically using
`SetOptions`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8085

Test Plan: Ran `make check` and tried the crash test script.

Reviewed By: riversand963

Differential Revision: D27238620

Pulled By: ltamasi

fbshipit-source-id: 4a96f9944b1ed9220d3045c5ab0b34c49009aeee

Summary:
The implementation of TransactionDB::WrapDB() and
TransactionDB::WrapStackableDB() are almost identical, except for the
type of the first argument `db`. This PR adds a new template function in
anonymous namespace, and calls it in the above two functions.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8079

Test Plan: make check

Reviewed By: lth

Differential Revision: D27184575

Pulled By: riversand963

fbshipit-source-id: f2855a6db3a7e897d0d611f7050ca4b696c56a7a

Summary:
Update release version to 6.19

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8083

Test Plan: no code change

Reviewed By: riversand963

Differential Revision: D27222083

Pulled By: zhichao-cao

fbshipit-source-id: 94b49997019347e6e6a9e341837f4f9d3149428c

Summary:
This does not add any new public APIs or published
functionality, but adds the ability to read and use (and in tests,
write) backups with a new meta file schema, based on the old schema
but not forward-compatible (before this change). The new schema enables
some capabilities not in the old:

* Explicit versioning, so that users get clean error messages the next
time we want to break forward compatibility.
* Ignoring unrecognized fields (with warning), so that new non-critical
features can be added without breaking forward compatibility.
* Rejecting future "non-ignorable" fields, so that new features critical
to some use-cases could potentially be added outside of linear schema
versions, with broken forward compatibility.
* Fields at the end of the meta file, such as for checksum of the meta
file's contents (up to that point)
* New optional 'size' field for each file, which is checked when present
* Optionally omitting 'crc32' field, so that we aren't required to have
a crc32c checksum for files to take a backup. (E.g. to support backup
via hard links and to better support file custom checksums.)

Because we do not have a JSON parser and to share code, the new schema
is simply derived from the old schema.

BackupEngine code is updated to allow missing checksums in some places,
and to make that easier, `has_checksum` and `verify_checksum_after_work`
are eliminated. Empty `checksum_hex` indicates checksum is unknown. I'm
not too afraid of regressing on data integrity, because
(a) we have pretty good test coverage of corruption detection in backups, and
(b) we are increasingly relying on the DB itself for data integrity rather than
it being an exclusive feature of backups.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8069

Test Plan:
new unit tests, added to crash test (some local run with
boosted backup probability)

Reviewed By: ajkr

Differential Revision: D27139824

Pulled By: pdillinger

fbshipit-source-id: 9e0e4decfb42bb84783d64d2d246456d97e8e8c5

Summary:
Remove unused variable `Slice blob_to_write` in `db/blob/blob_file_cache_test.cc`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8067

Reviewed By: zhichao-cao

Differential Revision: D27107693

Pulled By: riversand963

fbshipit-source-id: 9bfd4d296a6a1714ad5c1fa5bb231a0c52dbd56d

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/8068

Reviewed By: zhichao-cao

Differential Revision: D27147685

Pulled By: riversand963

fbshipit-source-id: 5428b1c0142ecae17c977fba31a6d49b52983d1c

Summary:
switch may not cover all branch in `db/c.cc`:

```c++
void rocksdb_options_set_access_hint_on_compaction_start(
    rocksdb_options_t* opt, int v) {
  switch(v) {
    case 0:
      opt->rep.access_hint_on_compaction_start =
          ROCKSDB_NAMESPACE::Options::NONE;
      break;
    case 1:
      opt->rep.access_hint_on_compaction_start =
          ROCKSDB_NAMESPACE::Options::NORMAL;
      break;
    case 2:
      opt->rep.access_hint_on_compaction_start =
          ROCKSDB_NAMESPACE::Options::SEQUENTIAL;
      break;
    case 3:
      opt->rep.access_hint_on_compaction_start =
          ROCKSDB_NAMESPACE::Options::WILLNEED;
      break;
  }
}
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8065

Reviewed By: riversand963

Differential Revision: D27102892

Pulled By: zhichao-cao

fbshipit-source-id: ad1d20d192712878e61597311ba75b55df0066d7

Summary:
Add the new Append and PositionedAppend API to env WritableFile. User is able to benefit from the write checksum handoff API when using the legacy Env classes. FileSystem already implemented the checksum handoff API.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8071

Test Plan: make check, added new unit test.

Reviewed By: anand1976

Differential Revision: D27177043

Pulled By: zhichao-cao

fbshipit-source-id: 430c8331fc81099fa6d00f4fff703b68b9e8080e

Summary:
As title.
Test plan
ASSERT_STATUS_CHECKED=1 make -j20 backupable_db_test error_handler_fs_test
./backupable_db_test
./error_handler_fs_test

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8075

Reviewed By: zhichao-cao

Differential Revision: D27173832

Pulled By: riversand963

fbshipit-source-id: 37dac50f7c89127804ff2572abddd4174642de30

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/8077

Test Plan: Travis CI

Reviewed By: zhichao-cao

Differential Revision: D27178276

Pulled By: riversand963

fbshipit-source-id: 17911dcc2d5790eb396efcd7f90dea76a127cf15

Summary:
Currently, a few ldb commands do not check the execution result of
database operations. This PR checks the execution results and tries to
improve the error reporting.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8072

Test Plan:
```
make check
```
and
```
ASSERT_STATUS_CHECKED=1 make -j20 ldb
python tools/ldb_test.py
```

Reviewed By: zhichao-cao

Differential Revision: D27152466

Pulled By: riversand963

fbshipit-source-id: b94220496a4b3591b61c1d350f665860a6579f30

Summary:
In previous codebase, if WAL is used, all the retryable IO Error will be treated as hard error. So write is stalled. In this PR, the retryable IO error from WAL sync is separated from SST file flush io error. If WAL Sync is ok and retryable IO Error only happens during SST flush, the error is mapped to soft error. So user can continue insert to Memtable and append to WAL.

Resolve the bug that if WAL sync fails, the memtable status does not roll back due to calling PickMemtable early than calling and checking SyncClosedLog.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8049

Test Plan: added new unit test, make check

Reviewed By: anand1976

Differential Revision: D26965529

Pulled By: zhichao-cao

fbshipit-source-id: f5fecb66602212523c92ee49d7edcb6065982410

Summary:
WriteController had a number of issues:
* It could introduce a delay of 1ms even if the write rate never exceeded the
configured delayed_write_rate.
* The DB-wide delayed_write_rate could be exceeded in a number of ways
with multiple column families:
  * Wiping all pending delay "debts" when another column family joins
  the delay with GetDelayToken().
  * Resetting last_refill_time_ to (now + sleep amount) means each
  column family can write with delayed_write_rate for large writes.
  * Updating bytes_left_ for a partial refill without updating
  last_refill_time_ would essentially give out random bonuses,
  especially to medium-sized writes.

Now the code is much simpler, with these issues fixed. See comments in
the new code and new (replacement) tests.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8064

Test Plan: new tests, better than old tests

Reviewed By: mrambacher

Differential Revision: D27064936

Pulled By: pdillinger

fbshipit-source-id: 497c23fe6819340b8f3d440bd634d8a2bc47323f

Summary:
Add statistics and info log for error handler: counters for bg error, bg io error, bg retryable io error, auto resume, auto resume total retry, and auto resume sucess; Histogram for auto resume retry count in each recovery call.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8050

Test Plan: make check and add test to error_handler_fs_test

Reviewed By: anand1976

Differential Revision: D26990565

Pulled By: zhichao-cao

fbshipit-source-id: 49f71e8ea4e9db8b189943976404205b56ab883f

Summary:
Extend support to track blob files in SST File manager.
 This PR notifies SstFileManager whenever a new blob file is created,
 via OnAddFile and  an obsolete blob file deleted via OnDeleteFile
 and delete file via ScheduleFileDeletion.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8037

Test Plan: Add new unit tests

Reviewed By: ltamasi

Differential Revision: D26891237

Pulled By: akankshamahajan15

fbshipit-source-id: 04c69ccfda2a73782fd5c51982dae58dd11979b6

Summary:
support getUsage and getPinnedUsage in JavaAPI for Cache
also fix a typo in LRUCacheTest.java that the highPriPoolRatio is not valid(set 5, I guess it means 0.05)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7925

Reviewed By: mrambacher

Differential Revision: D26900241

Pulled By: ajkr

fbshipit-source-id: 735d1e40a16fa8919c89c7c7154ba7f81208ec33

Summary:
The new options are:
* compact0 - compact L0 into L1 using one thread
* compact1 - compact L1 into L2 using one thread
* flush - flush memtable
* waitforcompaction - wait for compaction to finish

These are useful for reproducible benchmarks to help get the LSM tree shape
into a deterministic state. I wrote about this at:
http://smalldatum.blogspot.com/2021/02/read-only-benchmarks-with-lsm-are.html

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8027

Reviewed By: riversand963

Differential Revision: D27053861

Pulled By: ajkr

fbshipit-source-id: 1646f35584a3db03740fbeb47d91c3f00fb35d6e

Summary:
Fixes 3 minor Javadoc copy-paste errors in the `RocksDB#newIterator()` and `Transaction#getIterator()` variants that take a column family handle but are talking about iterating over "the database" or "the default column family".

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8034

Reviewed By: jay-zhuang

Differential Revision: D26877667

Pulled By: mrambacher

fbshipit-source-id: 95dd95b667c496e389f221acc9a91b340e4b63bf

Summary:
These classes were wraps of Env that provided only extensions to the FileSystem functionality.  Changed the classes to be FileSystems and the wraps to be of the CompositeEnvWrapper.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7968

Reviewed By: anand1976

Differential Revision: D26900253

Pulled By: mrambacher

fbshipit-source-id: 94001d8024a3c54a1c11adadca2bac66c3af2a77

Summary:
When timestamp is enabled, key comparison should take this into account.
In `BlockBasedTableReader::Get()`, `BlockBasedTableReader::MultiGet()`,
assume the target key is `key`, and the timestamp upper bound is `ts`.
The highest key in current block is (key, ts1), while the lowest key in next
block is (key, ts2).
If
```
ts1 > ts > ts2
```
then
```
(key, ts1) < (key, ts) < (key, ts2)
```
It can be shown that if `Compare()` is used, then we will mistakenly skip the next
block. Instead, we should use `CompareWithoutTimestamp()`.

The majority of this PR makes some existing tests in `db_with_timestamp_basic_test.cc`
parameterized so that different index types can be tested. A new unit test is
also added for more coverage.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8062

Test Plan: make check

Reviewed By: ltamasi

Differential Revision: D27057557

Pulled By: riversand963

fbshipit-source-id: c1062fa7c159ed600a1ad7e461531d52265021f1

Summary:
As title.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8054

Test Plan: make check

Reviewed By: mrambacher

Differential Revision: D27017955

Pulled By: riversand963

fbshipit-source-id: 829497d507bc89afbe982f8a8cf3555e52fd7098

Summary:
For performance purposes, the lower level routines were changed to use a SystemClock* instead of a std::shared_ptr<SystemClock>.  The shared ptr has some performance degradation on certain hardware classes.

For most of the system, there is no risk of the pointer being deleted/invalid because the shared_ptr will be stored elsewhere.  For example, the ImmutableDBOptions stores the Env which has a std::shared_ptr<SystemClock> in it.  The SystemClock* within the ImmutableDBOptions is essentially a "short cut" to gain access to this constant resource.

There were a few classes (PeriodicWorkScheduler?) where the "short cut" property did not hold.  In those cases, the shared pointer was preserved.

Using db_bench readrandom perf_level=3 on my EC2 box, this change performed as well or better than 6.17:

6.17: readrandom   :      28.046 micros/op 854902 ops/sec;   61.3 MB/s (355999 of 355999 found)
6.18: readrandom   :      32.615 micros/op 735306 ops/sec;   52.7 MB/s (290999 of 290999 found)
PR: readrandom   :      27.500 micros/op 871909 ops/sec;   62.5 MB/s (367999 of 367999 found)

(Note that the times for 6.18 are prior to revert of the SystemClock).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8033

Reviewed By: pdillinger

Differential Revision: D27014563

Pulled By: mrambacher

fbshipit-source-id: ad0459eba03182e454391b5926bf5cdd45657b67

Summary:
CompactionDeletionTriggerReopen was observed to be flaky recently:
https://app.circleci.com/pipelines/github/facebook/rocksdb/6030/workflows/787af4f3-b9f7-4645-8e8d-1fb0ebf05539/jobs/101451.

I went through it and the related tests and arrived at different
conclusions on what constraints we can expect on DB size. Some
constraints got looser and some got tighter. The particular constraint
that flaked got a lot looser so at least the flake linked above would have been prevented.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8036

Reviewed By: riversand963

Differential Revision: D26862566

Pulled By: ajkr

fbshipit-source-id: 3512b86b4fb41aeecae32e1c7382c03916d88d88

Summary:
`DBTest.GetLiveBlobFiles` and `ObsoleteFilesTest.BlobFiles` both modify the
current `Version` in their setup phase, implicitly assuming that no other
threads would touch the `Version` while this is happening. The periodic
stats dumper thread violates this assumption; the patch fixes this by
disabling it in the affected test cases. (Note: the data race is
harmless in the sense that it only affects test code.)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8055

Test Plan:
```
COMPILE_WITH_TSAN=1 make db_test -j24
gtest-parallel --repeat=10000 ./db_test --gtest_filter="*GetLiveBlobFiles"
COMPILE_WITH_TSAN=1 make obsolete_files_test -j24
gtest-parallel --repeat=10000 ./obsolete_files_test --gtest_filter="*BlobFiles"
```

Reviewed By: riversand963

Differential Revision: D27022715

Pulled By: ltamasi

fbshipit-source-id: b6cc77ed63d8bc1cbe0603522ff1a572182fc9ab

Summary:
This is for cases that do not meet the Facebook criteria for
SKIP (see new comments). Also made ROCKSDB_GTEST_{SKIP,BYPASS} print the
message because gtest doesn't ever seem to.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8048

Test Plan: manual inspection of ./ribbon_test output, CI

Reviewed By: mrambacher

Differential Revision: D26953688

Pulled By: pdillinger

fbshipit-source-id: c914eaffe7d419db6ab90a193d474531e23582e5

Summary:
a trial gtest upgrade discovered some parameterized tests missing instantiation. By some miracle, they still pass.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8051

Test Plan: thisisthetest

Reviewed By: mrambacher

Differential Revision: D27003684

Pulled By: pdillinger

fbshipit-source-id: cde1cab1551fb282f67d462d46574bd30bd5e61f

Summary:
This API can be used for things like determining how much space
can be freed up by deleting a particular backup, etc.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8042

Test Plan:
validation of the API added to many existing backup unit
tests

Reviewed By: mrambacher

Differential Revision: D26936577

Pulled By: pdillinger

fbshipit-source-id: f0bbd90f0917b9781a6837652fb4616d9247816a

Summary:
This PR does the following:

- Enable backward iteration for keys with user-defined timestamp. Note that merge, single delete, range delete are not supported yet.
- Introduces a new helper API `Comparator::EqualWithoutTimestamp()`.
- Fix a typo in `SetTimestamp()`.
- Add/update unit tests

Run db_bench (built with DEBUG_LEVEL=0) to demonstrate that no overhead is introduced for CPU-intensive workloads with a lot of `Prev()`. Also provided results of iterating keys with timestamps.

1. Disable timestamp, run:
```
./db_bench -db=/dev/shm/rocksdb -disable_wal=1 -benchmarks=fillseq,seekrandom[-W1-X6] -reverse_iterator=1 -seek_nexts=5
```
Results:
> Baseline
> - seekrandom [AVG    6 runs] : 96115 ops/sec;   53.2 MB/sec
> - seekrandom [MEDIAN 6 runs] : 98075 ops/sec;   54.2 MB/sec
>
> This PR
> - seekrandom [AVG    6 runs] : 95521 ops/sec;   52.8 MB/sec
> - seekrandom [MEDIAN 6 runs] : 96338 ops/sec;   53.3 MB/sec

2. Enable timestamp, run:
```
./db_bench -user_timestamp_size=8  -db=/dev/shm/rocksdb -disable_wal=1 -benchmarks=fillseq,seekrandom[-W1-X6] -reverse_iterator=1 -seek_nexts=5
```
Result:
> Baseline: not supported
>
> This PR
> - seekrandom [AVG    6 runs] : 90514 ops/sec;   50.1 MB/sec
> - seekrandom [MEDIAN 6 runs] : 90834 ops/sec;   50.2 MB/sec

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8035

Reviewed By: ltamasi

Differential Revision: D26926668

Pulled By: riversand963

fbshipit-source-id: 95330cc2242397c03e09d29e5417dfb0adc98ef5

Summary:
This PR

- adds a class `ManifestTailer` that inherits from `VersionEditHandlerPointInTime`. `ManifestTailer::Iterate()` can be called multiple times to tail the primary instance's MANIFEST and apply the changes to the secondary,
- updates the implementation of `ReactiveVersionSet::ReadAndApply` to use this class,
- removes unused code in version_set.cc,
- updates existing tests, e.g. removing deleted sync points from unit tests,
- adds a new test to address the bug in https://github.com/facebook/rocksdb/issues/7815.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7998

Test Plan:
make check
Existing and newly-added tests in version_set_test.cc and db_secondary_test.cc

Reviewed By: jay-zhuang

Differential Revision: D26926641

Pulled By: riversand963

fbshipit-source-id: 8d4dd15db0ba863c213f743e33b5a207e948c980

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7893

Reviewed By: ajkr

Differential Revision: D26050966

Pulled By: jay-zhuang

fbshipit-source-id: 9df2bb65d82defd7fad49d5369979b03e22d39c2

Summary:
The variable `byteCompressionType` is only assigned values of primitive type and is never 'null', but it is declared with the boxed type 'Byte'.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7981

Reviewed By: ajkr

Differential Revision: D26546600

Pulled By: jay-zhuang

fbshipit-source-id: 07b579cdfcfc2262a448ca3626e216416fd05892

Summary:
OS: Ubuntu 14.04
Compiler: GCC 4.9.4
Compile error:
```
db/forward_iterator.cc:996:62: error: declaration of ‘key’ shadows a member of 'this' [-Werror=shadow]
   auto cmp = [&](const FileMetaData* f, const Slice& key) -> bool {
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7908

Reviewed By: jay-zhuang

Differential Revision: D26899986

Pulled By: ajkr

fbshipit-source-id: 66b0b97aefd0f13a085e063491f8207366a9f848

Summary:
This PR adds support for custom file systems to ldb and sst_dump by adding command line options for specifying --fs_uri and --backup_fs uri (for ldb backup/restore commands). fs_uri is already supported in db_bench and db_stress, and there is already support in ldb and db stress for specifying customized envs.

The PR also fixes what looks like a bug in the ldb backup/restore commands. As it is right now, backups can only be made from and to the same environment/file system which does not seem to be the intended behavior. This PR makes it possible to do/restore backups between different envs/file systems.

Example:
`./ldb backup --fs_uri=zenfs://dev:nvme2n1 --backup_fs_uri=posix:// --backup_dir=/tmp/my_rocksdb_backup  --db=rocksdbtest/dbbench
`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8010

Reviewed By: jay-zhuang

Differential Revision: D26904654

Pulled By: ajkr

fbshipit-source-id: 9b695ed8b944fcc6b27c4daaa9f52e87ee2c1fb4

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7994

Reviewed By: jay-zhuang

Differential Revision: D26904603

Pulled By: ajkr

fbshipit-source-id: 0af92a51de895b40c7faaa4f0870b3f63279fe21