DOCAPI-5203: Added the descriptions for direct I/O settings for MergeTree merge and SELECT queries.

dbms/src/Interpreters/Settings.h

@@ -103,7 +103,7 @@ struct Settings
     \
     M(SettingUInt64, optimize_min_equality_disjunction_chain_length, 3, "The minimum length of the expression `expr = x1 OR ... expr = xN` for optimization ") \
     \
-    M(SettingUInt64, min_bytes_to_use_direct_io, 0, "The minimum number of bytes for input/output operations is bypassing the page cache. 0 - disabled.") \
+    M(SettingUInt64, min_bytes_to_use_direct_io, 0, "The minimum number of bytes for reading the data with O_DIRECT option during SELECT queries execution. 0 - disabled.") \
     \
     M(SettingBool, force_index_by_date, 0, "Throw an exception if there is a partition key in a table, and it is not used.") \
     M(SettingBool, force_primary_key, 0, "Throw an exception if there is primary key in a table, and it is not used.") \

docs/en/operations/server_settings/settings.md

@@ -417,7 +417,7 @@ The value 0 means that you can delete all tables without any restrictions.
 
 ## merge_tree {#server_settings-merge_tree}
 
-Fine tuning for tables in the [ MergeTree](../../operations/table_engines/mergetree.md).
+Fine tuning for tables in the [MergeTree](../../operations/table_engines/mergetree.md).
 
 For more information, see the MergeTreeSettings.h header file.
 

docs/en/operations/settings/settings.md

@@ -175,6 +175,34 @@ Any positive integer.
 
 **Default value**: 1048576.
 
+## min_merge_bytes_to_use_direct_io {#settings-min_merge_bytes_to_use_direct_io}
+
+The threshold for using direct I/O interface of the storage disk.
+
+ClickHouse uses this setting when merging data parts of the [MergeTree](../table_engines/mergetree.md)-family tables. During the merge, ClickHouse calculates summary storage volume of all the data to be merged. If the volume exceeds `min_merge_bytes_to_use_direct_io` bytes, than ClickHouse reads and writes the data using direct I/O interface (`O_DIRECT` option) to the storage disk.
+
+**Possible values**
+
+Positive integer.
+
+0 — The direct I/O is disabled.
+
+**Default value**: `10 * 1024 * 1024 * 1024` bytes.
+
+## min_bytes_to_use_direct_io {#settings-min_bytes_to_use_direct_io}
+
+The threshold for using direct I/O interface of the storage disk.
+
+ClickHouse uses this setting when selecting the data from tables. If summary storage volume of all the data to be read exceeds `min_bytes_to_use_direct_io` bytes, than ClickHouse reads the data from the storage disk directly with `O_DIRECT` option.
+
+**Possible values**
+
+Positive integer.
+
+0 — The direct I/O is disabled.
+
+**Default value**: 0.
+
 ## log_queries
 
 Setting up query logging.