Summary:
Reorganize the code so that no function call into ReadRangeDelAggregator is needed if there is no tomb range stone.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5202
Differential Revision: D14968155
Pulled By: siying
fbshipit-source-id: 0bd61911293c7a27b4e1b8d57c66d0c4ad6a6a5f
Summary:
This TODO was already addressed, but I forgot to remove it
before landing the PR it came from.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4800
Differential Revision: D13522284
Pulled By: abhimadan
fbshipit-source-id: 7766bc4f5b54e47d355cf26137ef5e86c604472a
Summary:
Now that v2 is fully functional, the v1 aggregator is removed.
The v2 aggregator has been renamed.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4778
Differential Revision: D13495930
Pulled By: abhimadan
fbshipit-source-id: 9d69500a60a283e79b6c4fa938fc68a8aa4d40d6
Summary:
Currently there are two contrun test failures:
* rocksdb-contrun-lite:
> tools/db_bench_tool.cc: In function ‘int rocksdb::db_bench_tool(int, char**)’:
tools/db_bench_tool.cc:5814:5: error: ‘DumpMallocStats’ is not a member of ‘rocksdb’
rocksdb::DumpMallocStats(&stats_string);
^
make: *** [tools/db_bench_tool.o] Error 1
* rocksdb-contrun-unity:
> In file included from unity.cc:44:0:
db/range_tombstone_fragmenter.cc: In member function ‘void rocksdb::FragmentedRangeTombstoneIterator::FragmentTombstones(std::unique_ptr<rocksdb::InternalIteratorBase<rocksdb::Slice> >, rocksdb::SequenceNumber)’:
db/range_tombstone_fragmenter.cc:90:14: error: reference to ‘ParsedInternalKeyComparator’ is ambiguous
auto cmp = ParsedInternalKeyComparator(icmp_);
This PR will fix them
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4587
Differential Revision: D10846554
Pulled By: miasantreble
fbshipit-source-id: 8d3358879e105060197b1379c84aecf51b352b93
Summary:
When there are no range deletions, flush and compaction perform a binary search
on an effectively empty map every time they call ShouldDelete. This PR lazily
initializes each stripe map entry so that the binary search can be elided in
these cases.
After this PR, the total amount of time spent in compactions is 52.541331s, and the total amount of time spent in flush is 5.532608s, the former of which is a significant improvement from the results after #4495.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4497
Differential Revision: D10428610
Pulled By: abhimadan
fbshipit-source-id: 6f7e1ce3698fac3ef86d1197955e6b72e0931a0f
Summary:
Using `./range_del_aggregator_bench --use_collapsed=false
--num_range_tombstones=5000 --num_runs=1000`, here are the results before and
after this change:
Before:
```
=========================
Results:
=========================
AddTombstones: 1822.61 us
ShouldDelete (first): 94.5286 us
```
After:
```
=========================
Results:
=========================
AddTombstones: 199.26 us
ShouldDelete (first): 38.9344 us
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4487
Differential Revision: D10347288
Pulled By: abhimadan
fbshipit-source-id: d44efe3a166d583acfdc3ec1199e0892f34dbfb7
Summary:
To more accurately truncate range tombstones at SST boundaries,
we now represent them in RangeDelAggregator using InternalKeys, which
are end-key-exclusive as they were before this change.
During compaction, "atomic compaction unit boundaries" (the range of
keys contained in neighbouring and overlaping SSTs) are propagated down
to RangeDelAggregator to truncate range tombstones at those boundariies
instead. See https://github.com/facebook/rocksdb/pull/4432#discussion_r221072219 and https://github.com/facebook/rocksdb/pull/4432#discussion_r221138683
for motivating examples.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4432
Differential Revision: D10263952
Pulled By: abhimadan
fbshipit-source-id: 2fe85ff8a02b3a6a2de2edfe708012797a7bd579
Summary:
The CollapsedRangeDelMap was entirely mishandling tombstones at the same
sequence number when the tombstones did not have identical start and end
keys. Such tombstones are common since 90fc40690, which causes
tombstones to be split during compactions.
For example, if the tombstone [a, c) @ 1 lies across a compaction
boundary at b, it will be split into [a, b) @ 1 and [b, c) @ 1. Without
this patch, the collapsed range deletion map would look like this:
a -> 1
b -> 1
c -> 0
Notice how the b -> 1 entry is redundant. When the tombstones overlap,
the problem is even worse. Consider tombstones [a, c) @ 1 and [b, d) @
1, which produces this map without this patch:
a -> 1
b -> 1
c -> 0
d -> 0
This map is corrupt, as a map can never contain adjacent sentinel (zero)
entries. When the iterator advances from b to c, it will notice that c
is a sentinel enty and skip to d--but d is also a sentinel entry! Asking
what tombstone this iterator points to will trigger an assertion, as it
is not pointing to a valid tombstone.
/cc ajkr
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4424
Differential Revision: D10039248
Pulled By: abhimadan
fbshipit-source-id: 6d737c1e88d60e80cf27286726627ba44463e7f4
Summary:
Add a unit test for range collapsing when non-default comparator is used. This exposes the bug fixed in #4386.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4388
Differential Revision: D9918252
Pulled By: ajkr
fbshipit-source-id: 99501b96b251eab41791a7e33b27055ee36c5c39
Summary:
The Comparator passed to CollapsedRangeDelMap was not used for
operator less of the std::map `rep_` object contained in
CollapsedRangeDelMap. So the map was always sorted using the
default ByteWiseComparator, which seems wrong.
Passing the specified Comparator through for usage in that map
object fixes actual problems we were seeing with RangeDelete operations
that do not delete keys as expected when using a custom Comparator.
I found that the tests in current master crash when I run them locally,
both with and without my patch, at the very same location. I therefore
don't know if the patch breaks something else, but it seems to fix
RangeDeletion issues in our product that uses RocksDB.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4386
Differential Revision: D9916506
Pulled By: ajkr
fbshipit-source-id: 27bff8c775831f089dde8c5289df7343d88b2d66
Summary:
`RangeDelAggregator::AddTombstones` contained an assertion which stated that, if a range tombstone extended past the largest key in the sstable, then `FileMetaData::largest` must have a sentinel sequence number of `kMaxSequenceNumber`, which implies that the tombstone's end key is safe to truncate. However, `largest` will not be a sentinel key when the next sstable in the level's smallest key is equal to the current sstable's largest key, which caused the assertion to fail.
The assertion must hold for the truncation to be safe, so it has been moved to an additional check on end-key truncation.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4356
Differential Revision: D9760891
Pulled By: abhimadan
fbshipit-source-id: 7c20c3885cd919dcd14f291f88fd27aa33defebc
Summary:
In C++ 11, the order of argument and move evaluation in a statement such
as below is unspecified -
foo(a.b).bar(std::move(a))
The compiler is free to evaluate std::move(a) first, and then a.b is unspecified.
In C++ 17, this will be safe if a draft proposal around function
chaining rules is accepted.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4348
Differential Revision: D9688810
Pulled By: anand1976
fbshipit-source-id: e4651d0ca03dcf007e50371a0fc72c0d1e710fb4
Summary:
This is a followup to #4311. Checking `!RangeDelAggregator::IsEmpty()` before opening a dedicated range tombstone SST did not properly prevent empty SSTs from being generated. That's because it relies on `CollapsedRangeDelMap::Size`, which had an underflow bug when the map was empty. This PR fixes that underflow bug.
Also fixed an uninitialized variable in db_stress.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4336
Differential Revision: D9600080
Pulled By: ajkr
fbshipit-source-id: bc6980ca79d2cd01b825ebc9dbccd51c1a70cfc7
Summary:
We want to sample the file I/O issued by RocksDB and report the function calls. This requires us to include the file paths otherwise it's hard to tell what has been going on.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4039
Differential Revision: D8670178
Pulled By: riversand963
fbshipit-source-id: 97ee806d1c583a2983e28e213ee764dc6ac28f7a
Summary:
We add two subcommands `write_extern_sst` and `ingest_extern_sst` to ldb. This PR avoids changing existing code because we hope to cherry-pick to earlier releases to support compatibility check for external SST file ingestion.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4205
Differential Revision: D9112711
Pulled By: riversand963
fbshipit-source-id: 7cae88380d4de86da8440230e87eca66755648e4
Summary:
`CollapsedRangeDelMap` internally uses seqno zero as a sentinel value to
denote a gap between range tombstones or the end of range tombstones. It
therefore expects to never have consecutive sentinel tombstones.
However, since `DeleteRange` is now supported in `SstFileWriter`, an
ingested file may contain range tombstones, and that ingested file may
be assigned global seqno zero. When such tombstones are added to the
collapsed map, they resemble sentinel tombstones due to having seqno
zero. Then, the invariant mentioned above about never having consecutive
sentinel tombstones can be violated.
The symptom of this violation was dereferencing the `end()` iterator
(#4204). The fix in this PR is to not add range tombstones with seqno
zero to the collapsed map. They're not needed anyways since they can't
possibly cover anything (in case of a key and a range tombstone with the
same seqno, the key is visible).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4216
Differential Revision: D9121716
Pulled By: ajkr
fbshipit-source-id: f5b78a70bea9527354603ea7ac8542a7e2b6a210
Summary:
Fixes#3391.
This change adds a `DeleteRange` method to `SstFileWriter` and adds
support for ingesting SSTs with range deletion tombstones. This is
important for applications that need to atomically ingest SSTs while
clearing out any existing keys in a given key range.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/3778
Differential Revision: D8821836
Pulled By: anand1976
fbshipit-source-id: ca7786c1947ff129afa703dab011d524c7883844
Summary:
Do not consider the range tombstone sentinel key as causing 2 adjacent
sstables in a level to overlap. When a range tombstone's end key is the
largest key in an sstable, the sstable's end key is so to a "sentinel"
value that is the smallest key in the next sstable with a sequence
number of kMaxSequenceNumber. This "sentinel" is guaranteed to not
overlap in internal-key space with the next sstable. Unfortunately,
GetOverlappingFiles uses user-keys to determine overlap and was thus
considering 2 adjacent sstables in a level to overlap if they were
separated by this sentinel key. This in turn would cause compactions to
be larger than necessary.
Note that this conflicts with
https://github.com/facebook/rocksdb/pull/2769 and cases
`DBRangeDelTest.CompactionTreatsSplitInputLevelDeletionAtomically` to
fail.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4050
Differential Revision: D8844423
Pulled By: ajkr
fbshipit-source-id: df3f9f1db8f4cff2bff77376b98b83c2ae1d155b
Summary:
This fixes the same performance issue that #3992 fixes but with much more invasive cleanup.
I'm more excited about this PR because it paves the way for fixing another problem we uncovered at Cockroach where range deletion tombstones can cause massive compactions. For example, suppose L4 contains deletions from [a, c) and [x, z) and no other keys, and L5 is entirely empty. L6, however, is full of data. When compacting L4 -> L5, we'll end up with one file that spans, massively, from [a, z). When we go to compact L5 -> L6, we'll have to rewrite all of L6! If, instead of range deletions in L4, we had keys a, b, x, y, and z, RocksDB would have been smart enough to create two files in L5: one for a and b and another for x, y, and z.
With the changes in this PR, it will be possible to adjust the compaction logic to split tombstones/start new output files when they would span too many files in the grandparent level.
ajkr please take a look when you have a minute!
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4014
Differential Revision: D8773253
Pulled By: ajkr
fbshipit-source-id: ec62fa85f648fdebe1380b83ed997f9baec35677
Summary:
`RangeDelAggregator` holds the pointers returned by `BlockIter::key()` and `BlockIter::value()` so requires the data to which they point is pinned. `BlockIter::key()` points into block memory and is guaranteed to be pinned if and only if prefix encoding is disabled (or, equivalently, restart interval is set to one). I think `BlockIter::value()` is always pinned. Added an assert for these and removed the wrong TODO about increasing restart interval, which would enable key prefix encoding and break the assertion.
Closes https://github.com/facebook/rocksdb/pull/3875
Differential Revision: D8063667
Pulled By: ajkr
fbshipit-source-id: 60b5ebcc0cdd610dd6aad9e74a23378793672c41
Summary:
The origin commit #3635 will hurt performance for users who aren't using range deletions， because unneeded std::set operations, so it was reverted by commit 44653c7b7a. (see #3672)
To fix this, move the set to and add a check in , i.e., file will be added only if is non-nullptr.
The db_bench command which find the performance regression:
> ./db_bench --benchmarks=fillrandom,seekrandomwhilewriting --threads=1 --num=1000000 --reads=150000 --key_size=66 > --value_size=1262 --statistics=0 --compression_ratio=0.5 --histogram=1 --seek_nexts=1 --stats_per_interval=1 > --stats_interval_seconds=600 --max_background_flushes=4 --num_multi_db=1 --max_background_compactions=16 --seed=1522388277 > -write_buffer_size=1048576 --level0_file_num_compaction_trigger=10000 --compression_type=none
Before and after the modification, I re-run this command on the machine, the results of are as follows：
**fillrandom**
Table | P50 | P75 | P99 | P99.9 | P99.99 |
---- | --- | --- | --- | ----- | ------ |
before commit | 5.92 | 8.57 | 19.63 | 980.97 | 12196.00 |
after commit | 5.91 | 8.55 | 19.34 | 965.56 | 13513.56 |
**seekrandomwhilewriting**
Table | P50 | P75 | P99 | P99.9 | P99.99 |
---- | --- | --- | --- | ----- | ------ |
before commit | 1418.62 | 1867.01 | 3823.28 | 4980.99 | 9240.00 |
after commit | 1450.54 | 1880.61 | 3962.87 | 5429.60 | 7542.86 |
Closes https://github.com/facebook/rocksdb/pull/3800
Differential Revision: D7874245
Pulled By: ajkr
fbshipit-source-id: 2e8bec781b3f7399246babd66395c88619534a17
Summary:
RangeDelAggregator will remember the files whose range tombstones have been added,
so the caller can check whether the file has been added before call AddTombstones.
Closes https://github.com/facebook/rocksdb/pull/3635
Differential Revision: D7354604
Pulled By: ajkr
fbshipit-source-id: 9b9f7ec130556028df417e650711554b46d8d107
Summary:
Before we were checking every file in the level which was unnecessary. We can piggyback onto the code for checking point-key overlap, which already opens all the files that could possibly contain overlapping range deletions. This PR makes us check just the range deletions from those files, so no extra ones will be opened.
Closes https://github.com/facebook/rocksdb/pull/3179
Differential Revision: D6358125
Pulled By: ajkr
fbshipit-source-id: 00e200770fdb8f3cc6b1b2da232b755e4ba36279
Summary:
Move uncommon code paths in RangeDelAggregator::ShouldDelete() and IterKey::EnlargeBufferIfNeeded() to a separate function, so that the inlined strcuture can be more optimized.
Optimize it because these places show up in CPU profiling, though minimum. The performance is really hard measure. I ran db_bench with readseq benchmark against in-memory DB many times. The variation is big, but it seems to show 1% improvements.
Closes https://github.com/facebook/rocksdb/pull/2877
Differential Revision: D5828123
Pulled By: siying
fbshipit-source-id: 41a49e229f91e9f8409f85cc6f0dc70e31334e4b
Summary:
* db/range_del_aggregator.cc (AddTombstone): Avoid a potential
use-after-move bug. The original code would both use and move
`tombstone` in a context where the order of those operations is
not specified. The fix is to perform the use on a new, preceding
statement.
Author: meyering
Closes https://github.com/facebook/rocksdb/pull/2796
Differential Revision: D5721163
Pulled By: ajkr
fbshipit-source-id: a1d328d6a77a17c6425e8069860a202e615e2f48
Summary:
when writing RangeDelAggregator::AddToBuilder, I forgot that there are sentinel tombstones in the middle of the interval map since gaps between real tombstones are represented with sentinels.
blame: #1614
Closes https://github.com/facebook/rocksdb/pull/1804
Differential Revision: D4460426
Pulled By: ajkr
fbshipit-source-id: 69444b5
Summary:
When deletion-collapsing mode is enabled (i.e., for DBIter/CompactionIterator), we maintain position in the tombstone maps across calls to ShouldDelete(). Since iterators often access keys sequentially (or reverse-sequentially), scanning forward/backward from the last position can be faster than binary-searching the map for every key.
- When Next() is invoked on an iterator, we use kForwardTraversal to scan forwards, if needed, until arriving at the range deletion containing the next key.
- Similarly for Prev(), we use kBackwardTraversal to scan backwards in the range deletion map.
- When the iterator seeks, we use kBinarySearch for repositioning
- After tombstones are added or before the first ShouldDelete() invocation, the current position is set to invalid, which forces kBinarySearch to be used.
- Non-iterator users (i.e., Get()) use kFullScan, which has the same behavior as before---scan the whole map for every key passed to ShouldDelete().
Closes https://github.com/facebook/rocksdb/pull/1701
Differential Revision: D4350318
Pulled By: ajkr
fbshipit-source-id: 5129b76
Summary:
Added a tombstone-collapsing mode to RangeDelAggregator, which eliminates overlap in the TombstoneMap. In this mode, we can check whether a tombstone covers a user key using upper_bound() (i.e., binary search). However, the tradeoff is the overhead to add tombstones is now higher, so at first I've only enabled it for range scans (compaction/flush/user iterators), where we expect a high number of calls to ShouldDelete() for the same tombstones. Point queries like Get() will still use the linear scan approach.
Also in this diff I changed RangeDelAggregator's TombstoneMap to use multimap with user keys instead of map with internal keys. Callers sometimes provided ParsedInternalKey directly, from which it would've required string copying to derive an internal key Slice with which we could search the map.
Closes https://github.com/facebook/rocksdb/pull/1614
Differential Revision: D4270397
Pulled By: ajkr
fbshipit-source-id: 93092c7
Summary:
- "rocksdb.compaction.key.drop.range_del" - number of keys dropped during compaction due to a range tombstone covering them
- "rocksdb.compaction.range_del.drop.obsolete" - number of range tombstones dropped due to compaction to bottom level and no snapshot saving them
- s/CompactionIteratorStats/CompactionIterationStats/g since this class is no longer specific to CompactionIterator -- it's also updated for range tombstone iteration during compaction
- Move the above class into a separate .h file to avoid circular dependency.
Closes https://github.com/facebook/rocksdb/pull/1520
Differential Revision: D4187179
Pulled By: ajkr
fbshipit-source-id: 10c2103
Summary:
The Arena construction/destruction introduced significant overhead to read-heavy workload just by creating empty vectors for its blocks, so avoid it in RangeDelAggregator.
Closes https://github.com/facebook/rocksdb/pull/1547
Differential Revision: D4207781
Pulled By: ajkr
fbshipit-source-id: 9d1c130
Summary:
Since a RangeDelAggregator is created for each read request, these heap-allocating member variables were consuming significant CPU (~3% total) which slowed down request throughput. The map and pinning manager are only necessary when range deletions exist, so we can defer their initialization until the first range deletion is encountered. Currently lazy initialization is done for reads only since reads pass us a single snapshot, which is easier to store on the stack for later insertion into the map than the vector passed to us by flush or compaction.
Note the Arena member variable is still expensive, I will figure out what to do with it in a subsequent diff. It cannot be lazily initialized because we currently use this arena even to allocate empty iterators, which is necessary even when no range deletions exist.
Closes https://github.com/facebook/rocksdb/pull/1539
Differential Revision: D4203488
Pulled By: ajkr
fbshipit-source-id: 3b36279
Summary:
Adjusted AddToBuilder() to take lower_bound and upper_bound, which serve two purposes: (1) only range deletions overlapping with the interval [lower_bound, upper_bound) will be added to the output file, and (2) the output file's boundaries will not be extended before lower_bound or after upper_bound. Our computation of lower_bound/upper_bound consider both subcompaction boundaries and previous/next files within the subcompaction.
Test cases are here (level subcompactions: https://gist.github.com/ajkr/63c7eae3e9667c5ebdc0a7efb74ac332, and universal subcompactions: https://gist.github.com/ajkr/5a62af77c4ebe4052a1955c496d51fdb) but can't be included in this diff as they depend on committing the API first. They fail before this change and pass after.
Closes https://github.com/facebook/rocksdb/pull/1501
Reviewed By: yhchiang
Differential Revision: D4171685
Pulled By: ajkr
fbshipit-source-id: ee99db8
Summary:
This makes it easier to implement future optimizations like range collapsing.
Closes https://github.com/facebook/rocksdb/pull/1504
Differential Revision: D4172214
Pulled By: ajkr
fbshipit-source-id: ac4942f
Summary:
This fixes a correctness issue where ranges with same begin key would overwrite each other.
This diff uses InternalKey as TombstoneMap's key such that all tombstones have unique keys even when their start keys overlap. We also update TombstoneMap to use an internal key comparator.
End-to-end tests pass and are here (https://gist.github.com/ajkr/851ffe4c1b8a15a68d33025be190a7d9) but cannot be included yet since the DeleteRange() API is yet to be checked in. Note both tests failed before this fix.
Closes https://github.com/facebook/rocksdb/pull/1484
Differential Revision: D4155248
Pulled By: ajkr
fbshipit-source-id: 304b4b9
Summary:
Note: reviewed in https://reviews.facebook.net/D65115
- DBIter maintains a range tombstone accumulator. We don't cleanup obsolete tombstones yet, so if the user seeks back and forth, the same tombstones would be added to the accumulator multiple times.
- DBImpl::NewInternalIterator() (used to make DBIter's underlying iterator) adds memtable/L0 range tombstones, L1+ range tombstones are added on-demand during NewSecondaryIterator() (see D62205)
- DBIter uses ShouldDelete() when advancing to check whether keys are covered by range tombstones
Closes https://github.com/facebook/rocksdb/pull/1464
Differential Revision: D4131753
Pulled By: ajkr
fbshipit-source-id: be86559
Summary:
During Get()/MultiGet(), build up a RangeDelAggregator with range
tombstones as we search through live memtable, immutable memtables, and
SST files. This aggregator is then used by memtable.cc's SaveValue() and
GetContext::SaveValue() to check whether keys are covered.
added tests for Get on memtables/files; end-to-end tests mainly in https://reviews.facebook.net/D64761
Closes https://github.com/facebook/rocksdb/pull/1456
Differential Revision: D4111271
Pulled By: ajkr
fbshipit-source-id: 6e388d4
Summary:
This diff introduces RangeDelAggregator, which takes ownership of iterators
provided to it via AddTombstones(). The tombstones are organized in a two-level
map (snapshot stripe -> begin key -> tombstone). Tombstone creation avoids data
copy by holding Slices returned by the iterator, which remain valid thanks to pinning.
For compaction, we create a hierarchical range tombstone iterator with structure
matching the iterator over compaction input data. An aggregator based on that
iterator is used by CompactionIterator to determine which keys are covered by
range tombstones. In case of merge operand, the same aggregator is used by
MergeHelper. Upon finishing each file in the compaction, relevant range tombstones
are added to the output file's range tombstone metablock and file boundaries are
updated accordingly.
To check whether a key is covered by range tombstone, RangeDelAggregator::ShouldDelete()
considers tombstones in the key's snapshot stripe. When this function is used outside of
compaction, it also checks newer stripes, which can contain covering tombstones. Currently
the intra-stripe check involves a linear scan; however, in the future we plan to collapse ranges
within a stripe such that binary search can be used.
RangeDelAggregator::AddToBuilder() adds all range tombstones in the table's key-range
to a new table's range tombstone meta-block. Since range tombstones may fall in the gap
between files, we may need to extend some files' key-ranges. The strategy is (1) first file
extends as far left as possible and other files do not extend left, (2) all files extend right
until either the start of the next file or the end of the last range tombstone in the gap,
whichever comes first.
One other notable change is adding release/move semantics to ScopedArenaIterator
such that it can be used to transfer ownership of an arena-allocated iterator, similar to
how unique_ptr is used for malloc'd data.
Depends on D61473
Test Plan: compaction_iterator_test, mock_table, end-to-end tests in D63927
Reviewers: sdong, IslamAbdelRahman, wanning, yhchiang, lightmark
Reviewed By: lightmark
Subscribers: andrewkr, dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D62205
