Some comments

src/Storages/MergeTree/MergeTreeData.cpp

@@ -3666,4 +3666,9 @@ NamesAndTypesList MergeTreeData::getVirtuals() const
     };
 }
 
+size_t MergeTreeData::getTotalMergesWithTTLInMergeList() const
+{
+    return global_context.getMergeList().getExecutingMergesWithTTLCount();
+}
+
 }

src/Storages/MergeTree/MergeTreeData.h

@@ -646,6 +646,11 @@ public:
     /// TTL rule.
     bool isPartInTTLDestination(const TTLDescription & ttl, const IMergeTreeDataPart & part) const;
 
+    /// Get count of total merges with TTL in MergeList (system.merges) for all
+    /// tables (not only current table).
+    /// Method is cheap and doesn't require any locks.
+    size_t getTotalMergesWithTTLInMergeList() const;
+
     using WriteAheadLogPtr = std::shared_ptr<MergeTreeWriteAheadLog>;
     WriteAheadLogPtr getWriteAheadLog();
 

src/Storages/MergeTree/MergeType.h

@@ -5,16 +5,24 @@
 namespace DB
 {
 
+/// Type of Merge. Used to control amount of different merges during merges
+/// assignment. Also allows to apply special logic during merge process
+/// (mergePartsToTemporaryPart). Stored in FutureMergedMutatedPart and
+/// ReplicatedMergeTreeLogEntry.
+///
+/// Order is important, don't try to change it.
 enum class MergeType
 {
-    REGULAR,
-    TTL_DELETE,
+    REGULAR = 1,
+    TTL_DELETE = 2,
 };
 
+/// Check parsed merge_type from raw int and get enum value.
 MergeType checkAndGetMergeType(UInt64 merge_type);
 
 String toString(MergeType merge_type);
 
+/// Check this merge assigned with TTL
 bool isTTLMergeType(MergeType merge_type);
 
 }

src/Storages/MergeTree/ReplicatedMergeTreeQueue.cpp

@@ -1070,7 +1070,34 @@ bool ReplicatedMergeTreeQueue::shouldExecuteLogEntry(
           * because the leader replica does not assign merges of greater size (except OPTIMIZE PARTITION and OPTIMIZE FINAL).
           */
         const auto data_settings = data.getSettings();
-        bool ignore_max_size = (entry.type == LogEntry::MERGE_PARTS) && (max_source_parts_size == data_settings->max_bytes_to_merge_at_max_space_in_pool);
+        bool ignore_max_size = false;
+        if (entry.type == LogEntry::MERGE_PARTS)
+        {
+            ignore_max_size = max_source_parts_size == data_settings->max_bytes_to_merge_at_max_space_in_pool;
+
+            if (isTTLMergeType(entry.merge_type))
+            {
+                if (merger_mutator.ttl_merges_blocker.isCancelled())
+                {
+                    String reason = "Not executing log entry for part " + entry.new_part_name + " because merges with TTL is cancelled now.";
+                    LOG_DEBUG(log, reason);
+                    out_postpone_reason = reason;
+                    return false;
+                }
+                size_t total_merges_with_ttl = data.getTotalMergesWithTTLInMergeList();
+                if (total_merges_with_ttl >= data_settings->max_number_of_merges_with_ttl_in_pool)
+                {
+                    const char * format_str = "Not executing log entry for part {}"
+                        " because {} merges with TTL already executing, maximum {}.";
+                    LOG_DEBUG(log, format_str, entry.new_part_name, total_merges_with_ttl,
+                        data_settings->max_number_of_merges_with_ttl_in_pool);
+
+                    out_postpone_reason = fmt::format(format_str, entry.new_part_name, total_merges_with_ttl,
+                        data_settings->max_number_of_merges_with_ttl_in_pool);
+                    return false;
+                }
+            }
+        }
 
         if (!ignore_max_size && sum_parts_size_in_bytes > max_source_parts_size)
         {

src/Storages/StorageMergeTree.cpp

@@ -633,7 +633,7 @@ bool StorageMergeTree::merge(
 
     /// You must call destructor with unlocked `currently_processing_in_background_mutex`.
     std::optional<CurrentlyMergingPartsTagger> merging_tagger;
-    auto & merge_list = global_context.getMergeList();
+    MergeList::EntryPtr merge_entry;
 
     {
         std::unique_lock lock(currently_processing_in_background_mutex);
@@ -653,7 +653,7 @@ bool StorageMergeTree::merge(
         if (partition_id.empty())
         {
             UInt64 max_source_parts_size = merger_mutator.getMaxSourcePartsSizeForMerge();
-            bool merge_with_ttl_allowed = merge_list.getExecutingMergesWithTTLCount() < data_settings->max_number_of_merges_with_ttl_in_pool;
+            bool merge_with_ttl_allowed = getTotalMergesWithTTLInMergeList() < data_settings->max_number_of_merges_with_ttl_in_pool;
 
             /// TTL requirements is much more strict than for regular merge, so
             /// if regular not possible, than merge with ttl is not also not
@@ -716,11 +716,10 @@ bool StorageMergeTree::merge(
         }
 
         merging_tagger.emplace(future_part, MergeTreeDataMergerMutator::estimateNeededDiskSpace(future_part.parts), *this, false);
+        auto table_id = getStorageID();
+        merge_entry = global_context.getMergeList().insert(table_id.database_name, table_id.table_name, future_part);
     }
 
-    auto table_id = getStorageID();
-    MergeList::EntryPtr merge_entry = global_context.getMergeList().insert(table_id.database_name, table_id.table_name, future_part);
-
     /// Logging
     Stopwatch stopwatch;
     MutableDataPartPtr new_part;

src/Storages/StorageReplicatedMergeTree.cpp

@@ -1377,6 +1377,7 @@ bool StorageReplicatedMergeTree::tryExecuteMerge(const LogEntry & entry)
             + backQuote(entry.new_part_name), ErrorCodes::BAD_DATA_PART_NAME);
     }
     future_merged_part.updatePath(*this, reserved_space);
+    future_merged_part.merge_type = entry.merge_type;
 
     auto table_id = getStorageID();
     MergeList::EntryPtr merge_entry = global_context.getMergeList().insert(table_id.database_name, table_id.table_name, future_merged_part);
@@ -2525,13 +2526,12 @@ void StorageReplicatedMergeTree::mergeSelectingTask()
         }
         else
         {
-            const auto & merge_list = global_context.getMergeList();
             UInt64 max_source_parts_size_for_merge = merger_mutator.getMaxSourcePartsSizeForMerge(
                 storage_settings_ptr->max_replicated_merges_in_queue, merges_and_mutations_sum);
             UInt64 max_source_part_size_for_mutation = merger_mutator.getMaxSourcePartSizeForMutation();
 
             bool merge_with_ttl_allowed = merges_and_mutations_queued.merges_with_ttl < storage_settings_ptr->max_replicated_merges_with_ttl_in_queue &&
-                merge_list.getExecutingMergesWithTTLCount() < storage_settings_ptr->max_number_of_merges_with_ttl_in_pool;
+                getTotalMergesWithTTLInMergeList() < storage_settings_ptr->max_number_of_merges_with_ttl_in_pool;
 
             FutureMergedMutatedPart future_merged_part;
             if (max_source_parts_size_for_merge > 0 &&

tests/integration/test_concurrent_ttl_merges/test.py

@@ -30,13 +30,6 @@ def count_ttl_merges_in_queue(node, table):
     return int(result.strip())
 
 
-def count_regular_merges_in_queue(node, table):
-    result = node.query("SELECT count() FROM system.replication_queue WHERE merge_type = 'REGULAR' and table = '{}'".format(table))
-    if not result:
-        return 0
-    return int(result.strip())
-
-
 def count_ttl_merges_in_background_pool(node, table):
     result = node.query("SELECT count() FROM system.merges WHERE merge_type = 'TTL_DELETE' and table = '{}'".format(table))
     if not result:
@@ -84,3 +77,84 @@ def test_no_ttl_merges_in_busy_pool(started_cluster):
         time.sleep(0.5)
 
     assert_eq_with_retry(node1, "SELECT COUNT() FROM test_ttl", "0")
+
+
+def test_limited_ttl_merges_in_empty_pool(started_cluster):
+    node1.query("CREATE TABLE test_ttl_v2 (d DateTime, key UInt64, data UInt64) ENGINE = MergeTree() ORDER BY tuple() PARTITION BY key TTL d + INTERVAL 1 MONTH SETTINGS merge_with_ttl_timeout = 0")
+
+    node1.query("SYSTEM STOP TTL MERGES")
+
+    for i in range(100):
+        node1.query("INSERT INTO test_ttl_v2 SELECT now() - INTERVAL 1 MONTH, {}, number FROM numbers(1)".format(i))
+
+    assert node1.query("SELECT COUNT() FROM test_ttl_v2") == "100\n"
+
+    node1.query("SYSTEM START TTL MERGES")
+
+    merges_with_ttl_count = set({})
+    while True:
+        merges_with_ttl_count.add(count_ttl_merges_in_background_pool(node1, "test_ttl_v2"))
+        time.sleep(0.01)
+        if node1.query("SELECT COUNT() FROM test_ttl_v2") == "0\n":
+            break
+
+    assert max(merges_with_ttl_count) <= 2
+
+
+def test_limited_ttl_merges_in_empty_pool_replicated(started_cluster):
+    node1.query("CREATE TABLE replicated_ttl (d DateTime, key UInt64, data UInt64) ENGINE = ReplicatedMergeTree('/test/t', '1') ORDER BY tuple() PARTITION BY key TTL d + INTERVAL 1 MONTH SETTINGS merge_with_ttl_timeout = 0")
+
+    node1.query("SYSTEM STOP TTL MERGES")
+
+    for i in range(100):
+        node1.query("INSERT INTO replicated_ttl SELECT now() - INTERVAL 1 MONTH, {}, number FROM numbers(1)".format(i))
+
+    assert node1.query("SELECT COUNT() FROM replicated_ttl") == "100\n"
+
+    node1.query("SYSTEM START TTL MERGES")
+
+    merges_with_ttl_count = set({})
+    entries_with_ttl_count = set({})
+    while True:
+        merges_with_ttl_count.add(count_ttl_merges_in_background_pool(node1, "replicated_ttl"))
+        entries_with_ttl_count.add(count_ttl_merges_in_queue(node1, "replicated_ttl"))
+        time.sleep(0.01)
+        if node1.query("SELECT COUNT() FROM replicated_ttl") == "0\n":
+            break
+
+    assert max(merges_with_ttl_count) <= 2
+    assert max(entries_with_ttl_count) <= 1
+
+
+def test_limited_ttl_merges_two_replicas(started_cluster):
+    # Actually this test quite fast and often we cannot catch any merges.
+    # To check for sure just add some sleeps in mergePartsToTemporaryPart
+    node1.query("CREATE TABLE replicated_ttl_2 (d DateTime, key UInt64, data UInt64) ENGINE = ReplicatedMergeTree('/test/t2', '1') ORDER BY tuple() PARTITION BY key TTL d + INTERVAL 1 MONTH SETTINGS merge_with_ttl_timeout = 0")
+    node2.query("CREATE TABLE replicated_ttl_2 (d DateTime, key UInt64, data UInt64) ENGINE = ReplicatedMergeTree('/test/t2', '2') ORDER BY tuple() PARTITION BY key TTL d + INTERVAL 1 MONTH SETTINGS merge_with_ttl_timeout = 0")
+
+    node1.query("SYSTEM STOP TTL MERGES")
+    node2.query("SYSTEM STOP TTL MERGES")
+
+    for i in range(100):
+        node1.query("INSERT INTO replicated_ttl_2 SELECT now() - INTERVAL 1 MONTH, {}, number FROM numbers(10000)".format(i))
+
+    node2.query("SYSTEM SYNC REPLICA replicated_ttl_2", timeout=10)
+    assert node1.query("SELECT COUNT() FROM replicated_ttl_2") == "1000000\n"
+    assert node2.query("SELECT COUNT() FROM replicated_ttl_2") == "1000000\n"
+
+    node1.query("SYSTEM START TTL MERGES")
+    node2.query("SYSTEM START TTL MERGES")
+
+    merges_with_ttl_count_node1 = set({})
+    merges_with_ttl_count_node2 = set({})
+    while True:
+        merges_with_ttl_count_node1.add(count_ttl_merges_in_background_pool(node1, "replicated_ttl_2"))
+        merges_with_ttl_count_node2.add(count_ttl_merges_in_background_pool(node2, "replicated_ttl_2"))
+        if node1.query("SELECT COUNT() FROM replicated_ttl_2") == "0\n" and node2.query("SELECT COUNT() FROM replicated_ttl_2") == "0\n":
+            break
+
+    # Both replicas can assign merges with TTL. If one will perform better than
+    # the other slow replica may have several merges in queue, so we don't
+    # check them
+    assert max(merges_with_ttl_count_node1) <= 2
+    assert max(merges_with_ttl_count_node2) <= 2