We need to keep sharding metadata consistent with tablet mapping to
shards in order for node restart to detect that those sstables belong
to a single shard and that resharding is not necessary. Resharding of
sstables based on tablet metadata is not implemented yet and will
abort after this series.
Keeping sharding metadata accurate for tablets is only necessary until
compaction group integration is finished. After that, we can use the
sstable token range to determine the owning tablet and thus the owning
shard. Before that, we can't, because a single sstable may contain
keys from different tablets, and the whole key range may overlap with
keys which belong to other shards.
There are some headers that include tracing/*.hh ones despite all they
need is forward-declared trace_state_ptr
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closes#14155
In that level no io_priority_class-es exist. Instead, all the IO happens
in the context of current sched-group. File API no longer accepts prio
class argument (and makes io_intent arg mandatory to impls).
So the change consists of
- removing all usage of io_priority_class
- patching file_impl's inheritants to updated API
- priority manager goes away altogether
- IO bandwidth update is performed on respective sched group
- tune-up scylla-gdb.py io_queues command
The first change is huge and was made semi-autimatically by:
- grep io_priority_class | default_priority_class
- remove all calls, found methods' args and class' fields
Patching file_impl-s is smaller, but also mechanical:
- replace io_priority_class& argument with io_intent* one
- pass intent to lower file (if applicatble)
Dropping the priority manager is:
- git-rm .cc and .hh
- sed out all the #include-s
- fix configure.py and cmakefile
The scylla-gdb.py update is a bit hairry -- it needs to use task queues
list for IO classes names and shares, but to detect it should it checks
for the "commitlog" group is present.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closes#13963
data_consume_rows keeps an input_stream member that must be closed.
In particular, on the error path, when we destroy it possibly
with readaheads in flight.
Fixes#13836
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closes#13840
Working with the low-level sstable parser and index reader, this
validator also cross-checks the index with the data file, making sure
all partitions are located at the position and in the order the index
describes. Furthermore, if the index also has promoted index, the order
and position of clustering elements is checked against it.
This is above the usual fragment kind order, partition key order and
clustering order checks that we already had with the reader-level
validator.
Currently mp_row_consumer_m creates an alias to data_consumer::proceed.
Code in the rest of the file uses both unqualified name and
mp_row_consumer_m::proceed. Remove the alias and just use
`data_consumer::proceed` directly everywhere, leads to cleaner code.
in C++20, compiler generate operator!=() if the corresponding
operator==() is already defined, the language now understands
that the comparison is symmetric in the new standard.
fortunately, our operator!=() is always equivalent to
`! operator==()`, this matches the behavior of the default
generated operator!=(). so, in this change, all `operator!=`
are removed.
in addition to the defaulted operator!=, C++20 also brings to us
the defaulted operator==() -- it is able to generated the
operator==() if the member-wise lexicographical comparison.
under some circumstances, this is exactly what we need. so,
in this change, if the operator==() is also implemented as
a lexicographical comparison of all memeber variables of the
class/struct in question, it is implemented using the default
generated one by removing its body and mark the function as
`default`. moreover, if the class happen to have other comparison
operators which are implemented using lexicographical comparison,
the default generated `operator<=>` is used in place of
the defaulted `operator==`.
sometimes, we fail to mark the operator== with the `const`
specifier, in this change, to fulfil the need of C++ standard,
and to be more correct, the `const` specifier is added.
also, to generate the defaulted operator==, the operand should
be `const class_name&`, but it is not always the case, in the
class of `version`, we use `version` as the parameter type, to
fulfill the need of the C++ standard, the parameter type is
changed to `const version&` instead. this does not change
the semantic of the comparison operator. and is a more idiomatic
way to pass non-trivial struct as function parameters.
please note, because in C++20, both operator= and operator<=> are
symmetric, some of the operators in `multiprecision` are removed.
they are the symmetric form of the another variant. if they were
not removed, compiler would, for instance, find ambiguous
overloaded operator '=='.
this change is a cleanup to modernize the code base with C++20
features.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#13687
this is a part of a series to migrating from `operator<<(ostream&, ..)`
based formatting to fmtlib based formatting. the goal here is to enable
fmtlib to print following classes without the help of `operator<<`.
- partition_key_view
- partition_key
- partition_key::with_schema_wrapper
- key_with_schema
- clustering_key_prefix
- clustering_key_prefix::with_schema_wrapper
the corresponding `operator<<()` are dropped dropped in this change,
as all its callers are now using fmtlib for formatting now. the helper
of `print_key()` is removed, as its only caller is
`operator<<(std::ostream&, const
clustering_key_prefix::with_schema_wrapper&)`.
the reason why all these operators are replaced in one go is that
we have a template function of `key_to_str()` in `db/large_data_handler.cc`.
this template function is actually the caller of operator<< of
`partition_key::with_schema_wrapper` and
`clustering_key_prefix::with_schema_wrapper`.
so, in order to drop either of these two operator<<, we need to remove
both of them, so that we can switch over to `fmt::to_string()` in this
template function.
Refs scylladb#13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
The method needs to create two data sinks -- for Data and for Index
files -- and then wrap it with more stuff (compression, checksums,
streams, etc.). With S3 backend using file-output-stream won't work,
becase S3 storage cannot provide writable file API (it has data_sink
instead).
This patch extracts file_data_sink creation so that it could be
virtualized with storage API later.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
static report:
sstables/mx/reader.cc:1705:58: error: invalid invocation of method 'operator*' on object 'schema' while it is in the 'consumed' state [-Werror,-Wconsumed]
legacy_reverse_slice_to_native_reverse_slice(*schema, slice.get()), pc, std::move(trace_state), fwd, fwd_mr, monitor);
Fixes#13394.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
this series intends to deprecate `::join()`, as it always materializes a range into a concrete string. but what we always want is to print the elements in the given range to stream, or to a seastar logger, which is backed by fmtlib. also, because fmtlib offers exactly the same set of features implemented by to_string.hh, this change would allow us to use fmtlib to replace to_string.hh for better maintainability, and potentially better performance. as fmtlib is lazy evaluated, and claims to be performant under most circumstances.
Closes#13163
* github.com:scylladb/scylladb:
utils: to_string: move join to namespace utils
treewide: use fmt::join() when appropriate
row_cache: pass "const cache_entry" to operator<<
now that fmtlib provides fmt::join(). see
https://fmt.dev/latest/api.html#_CPPv4I0EN3fmt4joinE9join_viewIN6detail10iterator_tI5RangeEEN6detail10sentinel_tI5RangeEEERR5Range11string_view
there is not need to revent the wheel. so in this change, the homebrew
join() is replaced with fmt::join().
as fmt::join() returns an join_view(), this could improve the
performance under certain circumstances where the fully materialized
string is not needed.
please note, the goal of this change is to use fmt::join(), and this
change does not intend to improve the performance of existing
implementation based on "operator<<" unless the new implementation is
much more complicated. we will address the unnecessarily materialized
strings in a follow-up commit.
some noteworthy things related to this change:
* unlike the existing `join()`, `fmt::join()` returns a view. so we
have to materialize the view if what we expect is a `sstring`
* `fmt::format()` does not accept a view, so we cannot pass the
return value of `fmt::join()` to `fmt::format()`
* fmtlib does not format a typed pointer, i.e., it does not format,
for instance, a `const std::string*`. but operator<<() always print
a typed pointer. so if we want to format a typed pointer, we either
need to cast the pointer to `void*` or use `fmt::ptr()`.
* fmtlib is not able to pick up the overload of
`operator<<(std::ostream& os, const column_definition* cd)`, so we
have to use a wrapper class of `maybe_column_definition` for printing
a pointer to `column_definition`. since the overload is only used
by the two overloads of
`statement_restrictions::add_single_column_parition_key_restriction()`,
the operator<< for `const column_definition*` is dropped.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Instead of having callers use get_timeout(), then compare it against the
current time, set up a timeout timer in the permit, which assigned a new
`_ex` member (a `std::exception_ptr`) to the appropriate exception type
when it fires.
Callers can now just poll check_abort() which will throw when `_ex`
is not null. This is more natural and allows for more general reasons
for aborting reads in the future.
This prepares the ground for timeouts being managed inside the permit,
instead of by the semaphore. Including timing out while in a wait queue.
It was used in sstables streaming code up until e5be3352 (database,
streaming, messaging: drop streaming memtables) or nearby, then the
whole feature was reworked.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closes#12967
as partition_reversing_data_source_impl has indirectly a member variable which
a member of reference type. this should addres following warning from
Clang:
```
/home/kefu/dev/scylladb/sstables/mx/partition_reversing_data_source.cc:476:43: error: explicitly defaulted move assignment operator is implicitly deleted [-Werror,-Wdefaulted-function-deleted]
partition_reversing_data_source_impl& operator=(partition_reversing_data_source_impl&&) noexcept = default;
^
/home/kefu/dev/scylladb/sstables/mx/partition_reversing_data_source.cc:365:19: note: move assignment operator of 'partition_reversing_data_source_impl' is implicitly deleted because field '_schema' is of reference type 'const schema &'
const schema& _schema;
^
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
because its base class of `writer_impl` has a member variable
`_validator`, which has its copy ctor deleted. let's just
drop the defaulted move ctor, as compiler is not able to
generate one for us.
```
/home/kefu/dev/scylladb/sstables/mx/writer.cc:805:5: error: explicitly defaulted move constructor is implicitly deleted [-Werror,-Wdefaulted-function-deleted]
writer(writer&& o) = default;
^
/home/kefu/dev/scylladb/sstables/mx/writer.cc:528:16: note: move constructor of 'writer' is implicitly deleted because base class 'sstable_writer::writer_impl' has a deleted move constructor
class writer : public sstable_writer::writer_impl {
^
/home/kefu/dev/scylladb/sstables/writer_impl.hh:29:48: note: copy constructor of 'writer_impl' is implicitly deleted because field '_validator' has a deleted copy constructor
mutation_fragment_stream_validating_filter _validator;
^
/home/kefu/dev/scylladb/mutation/mutation_fragment_stream_validator.hh:188:5: note: 'mutation_fragment_stream_validating_filter' has been explicitly marked deleted here
mutation_fragment_stream_validating_filter(const mutation_fragment_stream_validating_filter&) = delete;
^
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#12877
these warnings are found by Clang-17 after removing
`-Wno-unused-lambda-capture` and '-Wno-unused-variable' from
the list of disabled warnings in `configure.py`.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Schema related files are moved there. This excludes schema files that
also interact with mutations, because the mutation module depends on
the schema. Those files will have to go into a separate module.
Closes#12858
Move mutation-related files to a new mutation/ directory. The names
are kept in the global namespace to reduce churn; the names are
unambiguous in any case.
mutation_reader remains in the readers/ module.
mutation_partition_v2.cc was missing from CMakeLists.txt; it's added in this
patch.
This is a step forward towards librarization or modularization of the
source base.
Closes#12788
This is to define the API sstable needs from underlying storage. When implementing object-storage backend it will need to implement those. The API looks like
future<> snapshot(const sstable& sst, sstring dir, absolute_path abs) const;
future<> quarantine(const sstable& sst, delayed_commit_changes* delay);
future<> move(const sstable& sst, sstring new_dir, generation_type generation, delayed_commit_changes* delay);
void open(sstable& sst, const io_priority_class& pc); // runs in async context
future<> wipe(const sstable& sst) noexcept;
future<file> open_component(const sstable& sst, component_type type, open_flags flags, file_open_options options, bool check_integrity);
It doesn't have "list" or alike, because it's not a method of an individual sstable, but rather the one from sstables_manager. It will come as separate PR.
Closes#12217
* github.com:scylladb/scylladb:
sstable, storage: Mark dir/temp_dir private
sstable: Remove get_dir() (well, almost)
sstable: Add quarantine() method to storage
sstable: Use absolute/relative path marking for snapshot()
sstable: Remove temp_... stuff from sstable
sstable: Move open_component() on storage
sstable: Mark rename_new_sstable_component_file() const
sstable: Print filename(type) on open-component error
sstable: Reorganize new_sstable_component_file()
sstable: Mark filename() private
sstable: Introduce index_filename()
tests: Disclosure private filename() calls
sstable: Move wipe_storage() on storage
sstable: Remove temp dir in wipe_storage()
sstable: Move unlink parts into wipe_storage
sstable: Remove get_temp_dir()
sstable: Move write_toc() to storage
sstable: Shuffle open_sstable()
sstable: Move touch_temp_dir() to storage
sstable: Move move() to storage
sstable: Move create_links() to storage
sstable: Move seal_sstable() to storage
sstable: Tossing internals of seal_sstable()
sstable: Move remove_temp_dir() to storage
sstable: Move create_links_common() to storage
sstable: Move check_create_links_replay() to storage
sstable: Remove one of create_links() overloads
sstable: Remove create_links_and_mark_for_removal()
sstable: Indentation fix after prevuous patch
sstable: Coroutinize create_links_common()
sstable: Rename create_links_common()'s "dir" argument
sstable: Make mark_for_removal bool_class
sstable, table: Add sstable::snapshot() and use in table::take_snapshot
sstable: Move _dir and _temp_dir on filesystem_storage
sstable: Use sync_directory() method
test, sstable: Use component_basename in test
sstables: Move read_{digest|checksum} on sstable
This PR fixes several bugs related to handling of non-full
clustering keys.
One is in trim_clustering_row_ranges_to(), which is broken for non-full keys in reverse
mode. It will trim the range to position_in_partition_view::after_key(full_key) instead of
position_in_partition_view::before_key(key), hence it will include the
key in the resulting range rather than exclude it.
Fixes#12180
after_key() was creating a position which is after all keys prefixed
by a non-full key, rather than a position which is right after that
key.
This will issue will be caught by cql_query_test::test_compact_storage
in debug mode when mutation_partition_v2 merging starts inserting
sentinels at position after_key() on preemption.
It probably already causes problems for such keys as after_key() is used
in various parts in the read path.
Refs #1446Closes#12234
* github.com:scylladb/scylladb:
position_in_partition: Make after_key() work with non-full keys
position_in_partition: Introduce before_key(position_in_partition_view)
db: Fix trim_clustering_row_ranges_to() for non-full keys and reverse order
types: Fix comparison of frozen sets with empty values
When an sstable is prepared to be written on disk the .write_toc() is
called on it which created temporary toc file. Prior to this, the writer
code calls generate_toc() to collect components on the sstable.
This patch adds the .open_sstable() API call that does both. This
prepares the write_toc() part to be moved to storage, because it's not
just "write data into TOC file", it's the first step in transaction
implemeted on top of rename()s.
The test need care -- there's rewrite_toc_without_scylla_component()
thing in utils that doesn't want the generate_toc() part to be called.
It's not patched here and continues calling .write_toc().
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
This fixes a long standing bug related to handling of non-full
clustering keys, issue #1446.
after_key() was creating a position which is after all keys prefixed
by a non-full key, rather than a position which is right after that
key.
This will issue will be caught by cql_query_test::test_compact_storage
in debug mode when mutation_partition_v2 merging starts inserting
sentinels at position after_key() on preemption.
It probably already causes problems for such keys.
This change removes sstables.hh from some other headers replacing it
with version.hh and shared_sstable.hh. Also this drops
sstables_manager.hh from some more headers, because this header
propagates sstables.hh via self. That change is pretty straightforward,
but has a recochet in database.hh that needs disk-error-handler.hh.
Without the patch touch sstables/sstable.hh results in 409 targets
recompillation, with the patch -- 299 targets.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closes#12222
Since `_partition_size_entry` and `_rows_in_partition_entry`
are accessed at the same time when updated, and similarly
`_cell_size_entry` and `_elements_in_collection_entry`,
place the member pairs closely together to improve data
cache locality.
Follow the same order when preparing the
`scylla_metadata::large_data_stats` map.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
To save the map lookup on the hot write path,
keep each large data stats entry as a member in the writer
object and build a map for storing the disk_hash in the
scylla metadata only when finalizing it in consume_end_of_stream.
Fixes#11686
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Detect large_collections when the number of collection_elements
is above the configured threshold.
Next step would be to record the number of collection_elements
in the system.large_cells table, when the respective
cluster feature is enabled.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
And update the sstable elements_in_collection
stats entry.
Next step would be to forward it to
large_data_handler().maybe_record_large_cells().
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Add a new large_data_stats type and entry for keeping
the collection_elements_count_threshold and the maximum value
of collection_elements.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Long-term index caching in the global cache, as introduced in 4.6, is a major
pessimization for workloads where accesses to the index are (spacially) sparse.
We want to have a way to disable it for the affected workloads.
There is already infrastructure in place for disabling it for BYPASS CACHE
queries. One way of solving the issue is hijacking that infrastructure.
This patch adds a global flag (and a corresponding CLI option) which controls
index caching. Setting the flag to `false` causes all index reads to behave
like they would in BYPASS CACHE queries.
Consequences of this choice:
- The per-SSTable partition_index_cache is unused. Every index_reader has
its own, and they die together. Independent reads can no longer reuse the
work of other reads which hit the same index pages. This is not crucial,
since partition accesses have no (natural) spatial locality. Note that
the original reason for partition_index_cache -- the ability to share
reads for the lower and upper bound of the query -- is unaffected.
- The per-SSTable cached_file is unused. Every index_reader has its own
(uncached) input stream from the index file, and every
bsearch_clustered_cursor has its own cached_file, which dies together with
the cursor. Note that the cursor still can perform its binary search with
caching. However, it won't be able to reuse the file pages read by
index_reader. In particular, if the promoted index is small, and fits inside
the same file page as its index_entry, that page will be re-read.
It can also happen that index_reader will read the same index file page
multiple times. When the summary is so dense that multiple index pages fit in
one index file page, advancing the upper bound, which reads the next index
page, will read the same index file page. Since summary:disk ratio is 1:2000,
this is expected to happen for partitions with size greater than 2000
partition keys.
Fixes#11202
Adds support for splitting large partitions during compaction.
Large partitions introduce many problems, like memory overhead and
breaks incremental compaction promise. We want to split large
partitions across fixed-size fragments. We'll allow a partition
to exceed size limit by 10%, as we don't want to unnecessarily split
partitions that just crossed the limit boundary.
To avoid having to open a minimal of 2 fragments in a read, partition
tombstone will be replicated to every fragment storing the
partition.
The splitting isn't enabled by default, and can be used by
strategies that are run aware like ICS. LCS still cannot support
it as it's still using physical level metadata, not run id.
An incremental reader for sstable runs will follow soon.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Said method currently emits a partition-end. This method is only called
when the last fragment in the stream is a range tombstone change with a
position after all clustered rows. The problem is that
consume_partition_end() is also called unconditionally, resulting in two
partition-end fragments being emitted. The fix is simple: make this
method a no-op, there is nothing to do there.
Also add two tests: one targeted to this bug and another one testing the
crawling reader with random mutations generated for random schema.
Fixes: #11421Closes#11422
- Introduce a simpler substitute for `flat_mutation_reader`-resulting-from-a-downgrade that is adequate for the remaining uses but is _not_ a full-fledged reader (does not redirect all logic to an `::impl`, does not buffer, does not really have `::peek()`), so hopefully carries a smaller performance overhead. The name `mutation_fragment_v1_stream` is kind of a mouthful but it's the best I have
- (not tests) Use the above instead of `downgrade_to_v1()`
- Plug it in as another option in `mutation_source`, in and out
- (tests) Substitute deliberate uses of `downgrade_to_v1()` with `mutation_fragment_v1_stream()`
- (tests) Replace all the previously-overlooked occurrences of `mutation_source::make_reader()` with `mutation_source::make_reader_v2()`, or with `mutation_source::make_fragment_v1_stream()` where deliberate or still required (see below)
- (tests) This series still leaves some tests with `mutation_fragment_v1_stream` (i.e. at v1) where not called for by the test logic per se, because another missing piece of work is figuring out how to properly feed `mutation_fragment_v2` (i.e. range tombstone changes) to `mutation_partition`. While that is not done (and I think it's better to punt on it in this PR), we have to produce `mutation_fragment` instances in tests that `apply()` them to `mutation_partition`, thus we still use downgraded readers in those tests
- Remove the `flat_mutation_reader` class and things downstream of it
Fixes#10586Closes#10654
* github.com:scylladb/scylla:
fix "ninja dev-headers"
flat_mutation_reader ist tot
tests: downgrade_to_v1() -> mutation_fragment_v1_stream()
tests: flat_reader_assertions: refactor out match_compacted_mutation()
tests: ms.make_reader() -> ms.make_fragment_v1_stream()
repair/row_level: mutation_fragment_v1_stream() instead of downgrade_to_v1()
stream_transfer_task: mutation_fragment_v1_stream() instead of downgrade_to_v1()
sstables_loader: mutation_fragment_v1_stream() instead of downgrade_to_v1()
mutation_source: add ::make_fragment_v1_stream()
introduce mutation_fragment_v1_stream
tests: ms.make_reader() -> ms.make_reader_v2()
tests: remove test_downgrade_to_v1_clear_buffer()
mutation_source_test: fix indentation
tests: remove some redundant calls to downgrade_to_v1()
tests: remove some to-become-pointless ms.make_reader()-using tests
tests: remove some to-become-pointless reader downgrade tests
To avoid a discrepancy about underlying generation type once something other than integer is allowed for the sstable generation.
Also simplifies one generic writer interface for sealing sstable statistics.
Closes#10703
* github.com:scylladb/scylla:
sstables: Use generation_type for compaction ancestors
sstables: Make compaction ancestors optional when sealing statistics
Compaction ancestors is only available in versions older than mx,
therefore we can make it optional in seal_statistics(). The motivation
is that mx writer will no longer call sstable::compaction_ancestors()
which return type will be soon changed to type generation_type, so the
returned value can be something other than an integer, e.g. uuid.
We could kill compaction_ancestors in seal_statistics interface, but
given that most generic write functions still work for older versions,
if there were still a writer for them, I decided to not do it now.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
The change
- adds a test which exposes a problem of a peculiar setup of
tombstones that trigger a mutation fragment stream validation exception
- fixes the problem
Applying tombstones in the order:
range_tombstone_change pos(ck1), after_all_prefixed, tombstone_timestamp=1
range_tombstone_change pos(ck2), before_all_prefixed, tombstone=NONE
range_tombstone_change pos(NONE), after_all_prefixed, tombstone=NONE
Leads to swapping the order of mutations when written and read from
disk via sstable writer. This is caused by conversion of
range_tombstone_change (in memory representation) to range tombstone
marker (on disk representation) and back.
When this mutation stream is written to disk, the range tombstone
markers type is calculated based on the relationship between
range_tombstone_changes. The RTC series as above produces markers
(start, end, start). When the last marker is loaded from disk, it's kind
gets incorrectly loaded as before_all_prefixed instead of
after_all_prefixed. This leads to incorrect order of mutations.
The solution is to skip writing a new range_tombstone_change with empty
tombstone if the last range_tombstone_change already has empty
tombstone. This is redundant information and can be safely removed,
while the logic of encoding RTCs as markers doesn't handle such
redundancy well.
Closes#10643
Counts the number of promoted index auto-scale events.
A large number of those, relative to `partition_writes`,
indicates that `column_index_size_in_kb` should be increased.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Add column_index_auto_scale_threshold_in_kb to the configuration
(defaults to 10MB).
When the promoted index (serialized) size gets to this
threshold, it's halved by merging each two adjacent blocks
into one and doubling the desired_block_size.
Fixes#4217
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
