config, sysvar: update descriptions about instance sysvars

character-set-and-collation.md

@@ -141,7 +141,7 @@ For details about the TiDB support of the GBK character set, see [GBK](/characte
 
 In MySQL, the character set `utf8` is limited to a maximum of three bytes. This is sufficient to store characters in the Basic Multilingual Plane (BMP), but not enough to store characters such as emojis. For this, it is recommended to use the character set `utf8mb4` instead.
 
-By default, TiDB provides the same 3-byte limit on `utf8` to ensure that data created in TiDB can still safely be restored in MySQL. This can be disabled by changing the value of `check-mb4-value-in-utf8` to `FALSE` in your TiDB configuration file.
+By default, TiDB provides the same 3-byte limit on `utf8` to ensure that data created in TiDB can still safely be restored in MySQL. This can be disabled by changing the value of the system variable [`tidb_check_mb4_value_in_utf8`](/system-variables.md#tidb_check_mb4_value_in_utf8) to `OFF`.
 
 The following demonstrates the default behavior when inserting a 4-byte emoji character into a table. The `INSERT` statement fails for the `utf8` character set, but succeeds for `utf8mb4`:
 

configure-memory-usage.md

@@ -58,19 +58,19 @@ In this configuration, when the memory usage of a tidb-server instance reaches 3
 
 ## Trigger the alarm of excessive memory usage
 
-In the default configuration, a tidb-server instance prints an alarm log and records associated status files when the machine memory usage reaches 80% of its total memory. You can set the memory usage ratio threshold by configuring `memory-usage-alarm-ratio`. For detailed alarm rules, refer to the description of [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio-new-in-v409).
+In the default configuration, a tidb-server instance prints an alarm log and records associated status files when the machine memory usage reaches 80% of its total memory. You can set the memory usage ratio threshold by configuring the system variable [`tidb_memory_usage_alarm_ratio`](/system-variables.md#tidb_memory_usage_alarm_ratio). For detailed alarm rules, refer to the description of `tidb_memory_usage_alarm_ratio`.
 
 Note that after the alarm is triggered once, it will be triggered again only if the memory usage rate has been below the threshold for more than ten seconds and reaches the threshold again. In addition, to avoid storing excessive status files generated by alarms, currently, TiDB only retains the status files generated during the recent five alarms.
 
 The following example constructs a memory-intensive SQL statement that triggers the alarm:
 
-1. Set `memory-usage-alarm-ratio` to `0.8`:
+1. Set the value of the TiDB configuration item `instance.tidb_memory_usage_alarm_ratio` to `0.8`:
 
     {{< copyable "" >}}
 
     ```toml
-    [performance]
-    memory-usage-alarm-ratio = 0.8
+    [instance]
+    tidb_memory_usage_alarm_ratio = 0.8
     ```
 
 2. Execute `CREATE TABLE t(a int);` and insert 1000 rows of data.
@@ -89,7 +89,7 @@ The following example constructs a memory-intensive SQL statement that triggers
     * `system memory total` indicates the total memory of the current system.
     * `system memory usage` indicates the current system memory usage.
     * `tidb-server memory usage` indicates the memory usage of the tidb-server instance.
-    * `memory-usage-alarm-ratio` indicates the value of [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio-new-in-v409).
+    * `memory-usage-alarm-ratio` indicates the value of [`tidb_memory_usage_alarm_ratio`](/system-variables.md#tidb_memory_usage_alarm_ratio).
     * `record path` indicates the directory of status files.
 
 5. You can see a set of files in the directory of status files (In the above example, the directory is `/tmp/1000_tidb/MC4wLjAuMDo0MDAwLzAuMC4wLjA6MTAwODA=/tmp-storage/record`), including `goroutinue`, `heap`, and `running_sql`. These three files are suffixed with the time when status files are logged. They respectively record goroutine stack information, the usage status of heap memory, and the running SQL information when the alarm is triggered. For the format of log content in `running_sql`, refer to [`expensive-queries`](/identify-expensive-queries.md).
@@ -107,7 +107,7 @@ The following example constructs a memory-intensive SQL statement that triggers
 
 TiDB supports disk spill for execution operators. When the memory usage of a SQL execution exceeds the memory quota, tidb-server can spill the intermediate data of execution operators to the disk to relieve memory pressure. Operators supporting disk spill include Sort, MergeJoin, HashJoin, and HashAgg.
 
-- The disk spill behavior is jointly controlled by [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query), [`oom-use-tmp-storage`](/tidb-configuration-file.md#oom-use-tmp-storage), [`tmp-storage-path`](/tidb-configuration-file.md#tmp-storage-path), and [`tmp-storage-quota`](/tidb-configuration-file.md#tmp-storage-quota) parameters.
+- The disk spill behavior is jointly controlled by [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query), [`tidb_enable_tmp_storage_on_oom`](/system-variables.md#tidb_enable_tmp_storage_on_oom), [`tmp-storage-path`](/tidb-configuration-file.md#tmp-storage-path), and [`tmp-storage-quota`](/tidb-configuration-file.md#tmp-storage-quota) parameters.
 - When the disk spill is triggered, TiDB outputs a log containing the keywords `memory exceeds quota, spill to disk now` or `memory exceeds quota, set aggregate mode to spill-mode`.
 - Disk spill for the Sort, MergeJoin, and HashJoin operator is introduced in v4.0.0; disk spill for the HashAgg operator is introduced in v5.2.0.
 - When the SQL executions containing Sort, MergeJoin, or HashJoin cause OOM, TiDB triggers disk spill by default. When SQL executions containing HashAgg cause OOM, TiDB does not trigger disk spill by default. You can configure the system variable `tidb_executor_concurrency = 1` to trigger disk spill for HashAgg.

dashboard/dashboard-slow-query.md

@@ -12,7 +12,7 @@ By default, SQL queries with an execution time of more than 300 milliseconds are
 
 > **Note:**
 >
-> If the slow query log is disabled, this feature will be unavailable. The slow query log is enabled by default, and you can enable or disable the slow query log through the [`enable-slow-log`](/tidb-configuration-file.md#enable-slow-log) TiDB configuration item.
+> If the slow query log is disabled, this feature will be unavailable. The slow query log is enabled by default, and you can enable or disable the slow query log through the system variable [`tidb_enable_slow_log`](/system-variables.md#tidb_enable_slow_log).
 
 ## Access the page
 

enable-disk-spill-encrypt.md

@@ -5,7 +5,7 @@ summary: Learn how to enable encryption for disk spill in TiDB.
 
 # Enable Encryption for Disk Spill
 
-When the `oom-use-tmp-storage` configuration item is set to `true`, if the memory usage of a single SQL statement exceeds the limit of the system variable [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query), some operators can save the intermediate results during execution as a temporary file to the disk and delete the file after the query is completed.
+When the system variable [`tidb_enable_tmp_storage_on_oom`](/system-variables.md#tidb_enable_tmp_storage_on_oom) is set to `ON`, if the memory usage of a single SQL statement exceeds the limit of the system variable [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query), some operators can save the intermediate results during execution as a temporary file to the disk and delete the file after the query is completed.
 
 You can enable encryption for disk spill to prevent attackers from accessing data by reading these temporary files.
 

explain-joins.md

@@ -226,7 +226,7 @@ The `operator info` column in the `EXPLAIN` result table also records other info
 
 ### Runtime Statistics
 
-If [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query) (default value: 1GB) is exceeded, TiDB will attempt to use temporary storage on condition that the `oom-use-tmp-storage` value is `true` (default). This means that the `Build` operator used as part of the hash join might be created on disk. Runtime statistics such as memory usage are visible in the `execution info` of the `EXPLAIN ANALYZE` result table. The following example shows the output of `EXPLAIN ANALYZE` with a 1GB (default) `tidb_mem_quota_query` quota, and a 500MB quota. At 500MB, disk is used for temporary storage:
+If [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query) (default value: 1 GB) is exceeded, TiDB will attempt to use temporary storage on condition that the [`tidb_enable_tmp_storage_on_oom`](/system-variables.md#tidb_enable_tmp_storage_on_oom) value is `ON` (default). This means that the `Build` operator used as part of the hash join might be created on disk. Runtime statistics such as memory usage are visible in the `execution info` of the `EXPLAIN ANALYZE` result table. The following example shows the output of `EXPLAIN ANALYZE` with a 1 GB (default) `tidb_mem_quota_query` quota, and a 500 MB quota. At 500 MB, disk is used for temporary storage:
 
 ```sql
 EXPLAIN ANALYZE SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;

faq/manage-cluster-faq.md

@@ -226,7 +226,7 @@ The TiClient Region Error indicator describes the error types and metrics that a
 
 ### What's the maximum number of concurrent connections that TiDB supports?
 
-By default, there is no limit on the maximum number of connections per TiDB server. If too large concurrency leads to an increase of response time, it is recommended to increase the capacity by adding TiDB nodes.
+By default, there is no limit on the maximum number of connections per TiDB server. If needed, you can limit the maximum number of connections by setting `instance.max_connections` in the `config.toml` file, or changing the value of the system variable `max_connections`. If too large concurrency leads to an increase of response time, it is recommended to increase the capacity by adding TiDB nodes.
 
 ### How to view the creation time of a table?
 

faq/sql-faq.md

@@ -197,7 +197,7 @@ The display content of TiDB `SHOW PROCESSLIST` is almost the same as that of MyS
 
 ## How to control or change the execution priority of SQL commits?
 
-TiDB supports changing the priority on a [per-session](/system-variables.md#tidb_force_priority), [global](/tidb-configuration-file.md#force-priority) or individual statement basis. Priority has the following meaning:
+TiDB supports changing the priority on a [per-session](/system-variables.md#tidb_force_priority), [global](/system-variables.md#tidb_force_priority) or individual statement basis. Priority has the following meaning:
 
 - `HIGH_PRIORITY`: this statement has a high priority, that is, TiDB gives priority to this statement and executes it first.
 

identify-slow-queries.md

@@ -6,9 +6,9 @@ aliases: ['/docs/dev/identify-slow-queries/','/docs/dev/how-to/maintain/identify
 
 # Identify Slow Queries
 
-To help users identify slow queries, analyze and improve the performance of SQL execution, TiDB outputs the statements whose execution time exceeds [slow-threshold](/tidb-configuration-file.md#slow-threshold) (The default value is 300 milliseconds) to [slow-query-file](/tidb-configuration-file.md#slow-query-file) (The default value is "tidb-slow.log").
+To help users identify slow queries, analyze and improve the performance of SQL execution, TiDB outputs the statements whose execution time exceeds [`tidb_enable_slow_log`](/system-variables.md#tidb_enable_slow_log) (The default value is 300 milliseconds) to [slow-query-file](/tidb-configuration-file.md#slow-query-file) (The default value is "tidb-slow.log").
 
-TiDB enables the slow query log by default. You can enable or disable the feature by modifying the configuration [`enable-slow-log`](/tidb-configuration-file.md#enable-slow-log).
+TiDB enables the slow query log by default. You can enable or disable the feature by modifying the system variable [`tidb_enable_slow_log`](/system-variables.md#tidb_enable_slow_log).
 
 ## Usage example
 

security-compatibility-with-mysql.md

@@ -10,7 +10,7 @@ TiDB supports similar security functionality to MySQL 5.7, with the following ex
 
 - Column level permissions are not supported
 - Password expiry, as well as password last-changed tracking and password lifetime are not supported [#9709](https://github.com/pingcap/tidb/issues/9709)
-- The permission attributes `max_questions`, `max_updated`, `max_connections`, `max_user_connections` are not supported
+- The permission attributes `max_questions`, `max_updated`, `max_user_connections` are not supported
 - Password validation is not currently supported [#9741](https://github.com/pingcap/tidb/issues/9741)
 
 ## Authentication plugin status

system-variables.md

@@ -343,12 +343,12 @@ This variable is an alias for [`last_insert_id`](#last_insert_id).
 ### max_connections
 
 - Scope: GLOBAL
-- Persists to cluster: No
+- Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
 - Type: Integer
 - Default value: `0`
 - Range: `[0, 100000]`
-- The maximum number of connections permitted for a single TiDB instance.
-- The value of `0` means no limit.
+- The maximum number of connections permitted for a single TiDB instance simultaneously. This variable can be used for resources control.
+- The default value of `0` means no limit. When the value of this variable is larger than `0`, and the number of connections reaches the value, TiDB server rejects new connections from clients.
 
 ### max_execution_time
 
@@ -1194,6 +1194,14 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
     - `ON` indicates that primary keys are created as clustered indexes by default.
     - `INT_ONLY` indicates that the behavior is controlled by the configuration item `alter-primary-key`. If `alter-primary-key` is set to `true`, all primary keys are created as non-clustered indexes by default. If it is set to `false`, only the primary keys which consist of an integer column are created as clustered indexes.
 
+### tidb_enable_ddl
+
+- Scope: GLOBAL
+- Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to. 
+- Default value: `ON`
+- Possible values: `OFF`, `ON`
+- This variable controls whether the corresponding TiDB server can run DDL statements or not.
+
 ### tidb_enable_collect_execution_info
 
 <CustomContent platform="tidb-cloud">
@@ -1547,7 +1555,7 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 
 <CustomContent platform="tidb">
 
-- When the operator that reads data has only one thread left and the memory usage of a single SQL statement continues to exceed [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query), this SQL statement triggers other memory control behaviors, such as [spilling data to disk](/tidb-configuration-file.md#oom-use-tmp-storage).
+- When the operator that reads data has only one thread left and the memory usage of a single SQL statement continues to exceed [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query), this SQL statement triggers other memory control behaviors, such as [spilling data to disk](/system-variables.md#tidb_enable_tmp_storage_on_oom).
 - This variable controls memory usage effectively when an SQL statement only reads data. If computing operations (such as join or aggregation operations) are required, memory usage might not be under the control of `tidb_mem_quota_query`, which increases the risk of OOM.
 
 </CustomContent>
@@ -1574,6 +1582,15 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 - Default value: `ON`
 - This variable is used to control whether to enable the slow log feature.
 
+### tidb_enable_tmp_storage_on_oom
+
+- Scope: GLOBAL
+- Persists to cluster: Yes
+- Default value: `ON`
+- Value options: `OFF`, `ON`
+- Controls whether to enable the temporary storage for some operators when a single SQL statement exceeds the memory quota specified by the system variable [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query).
+- Before v6.3.0, this feature can be enabled or disabled using the TiDB configuration item `oom-use-tmp-storage`. After upgrading the cluster to v6.3.0 or a later version, the TiDB cluster will initialize this variable using the value of `oom-use-tmp-storage` automatically. After that, changing the value of `oom-use-tmp-storage` does NOT take effect anymore.
+
 ### tidb_enable_stmt_summary <span class="version-mark">New in v3.0.4</span>
 
 - Scope: GLOBAL
@@ -2359,14 +2376,14 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 
 <CustomContent platform="tidb">
 
-- TiDB triggers an alarm when the percentage of the memory it takes exceeds a certain threshold. For the detailed usage description of this feature, see [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio-new-in-v409).
-- You can set the initial value of this variable by configuring [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio-new-in-v409).
+- TiDB triggers an alarm when the percentage of the memory it takes exceeds a certain threshold. For the detailed usage description of this feature, see [`tidb_memory_usage_alarm_ratio`](/tidb-configuration-file.md#tidb_memory_usage_alarm_ratio).
+- You can set the initial value of this variable by configuring [`tidb_memory_usage_alarm_ratio`](/tidb-configuration-file.md#tidb_memory_usage_alarm_ratio).
 
 </CustomContent>
 
 <CustomContent platform="tidb-cloud">
 
-- TiDB triggers an alarm when the percentage of the memory it takes exceeds a certain threshold. For the detailed usage description of this feature, see [`memory-usage-alarm-ratio`](https://docs.pingcap.com/tidb/stable/tidb-configuration-file#memory-usage-alarm-ratio-new-in-v409).
+- TiDB triggers an alarm when the percentage of the memory it takes exceeds a certain threshold. For the detailed usage description of this feature, see [`tidb_memory_usage_alarm_ratio`](https://docs.pingcap.com/tidb/stable/tidb-configuration-file#tidb_memory_usage_alarm_ratio).
 
 </CustomContent>
 
@@ -2905,7 +2922,6 @@ explain select * from t where age=5;
 ### tidb_read_consistency <span class="version-mark">New in v5.4.0</span>
 
 - Scope: SESSION
-- Persists to cluster: No
 - Type: String
 - Default value: `strict`
 - This variable is used to control the read consistency for an auto-commit read statement.