add doc for tidb_txn_entry_size_limit (#15714)

system-variables.md

@@ -5611,6 +5611,32 @@ For details, see [Identify Slow Queries](/identify-slow-queries.md).
 
 </CustomContent>
 
+### tidb_txn_entry_size_limit <span class="version-mark">New in v7.6.0</span>
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
+- Type: Integer
+- Default value: `0`
+- Range: `[0, 125829120]`
+- Unit: Bytes
+
+<CustomContent platform="tidb">
+
+- This variable is used to dynamically modify the TiDB configuration item [`performance.txn-entry-size-limit`](/tidb-configuration-file.md#txn-entry-size-limit-new-in-v50). It limits the size of a single row of data in TiDB, which is equivalent to the configuration item. The default value of this variable is `0`, which means that TiDB uses the value of the configuration item `txn-entry-size-limit` by default. When this variable is set to a non-zero value, `txn-entry-size-limit` is also set to the same value.
+
+</CustomContent>
+
+<CustomContent platform="tidb-cloud">
+
+- This variable is used to dynamically modify the TiDB configuration item [`performance.txn-entry-size-limit`](https://docs.pingcap.com/tidb/stable/tidb-configuration-file#txn-entry-size-limit-new-in-v50). It limits the size of a single row of data in TiDB, which is equivalent to the configuration item. The default value of this variable is `0`, which means that TiDB uses the value of the configuration item `txn-entry-size-limit` by default. When this variable is set to a non-zero value, `txn-entry-size-limit` is also set to the same value.
+
+</CustomContent>
+
+> **Note:**
+>
+> Modifying this variable with the SESSION scope only affects the current user session, not the internal TiDB session. This might lead to transaction failure if the entry size of an internal TiDB transaction exceeds the limit of the configuration item. Therefore, to dynamically increase the limit, it is recommended that you modify the variable with the GLOBAL scope.
+
 ### tidb_txn_mode
 
 > **Note:**

tidb-configuration-file.md

@@ -495,6 +495,7 @@ Configuration items related to performance.
 - The size limit of a single row of data in TiDB.
 - Default value: `6291456` (in bytes)
 - The size limit of a single key-value record in a transaction. If the size limit is exceeded, TiDB returns the `entry too large` error. The maximum value of this configuration item does not exceed `125829120` (120 MB).
+- Starting from v7.6.0, you can use the system variable [`tidb_txn_entry_size_limit`](/system-variables.md#tidb_txn_entry_size_limit-new-in-v760) to dynamically modify the value of this configuration item.
 - Note that TiKV has a similar limit. If the data size of a single write request exceeds [`raft-entry-max-size`](/tikv-configuration-file.md#raft-entry-max-size), which is 8 MB by default, TiKV refuses to process this request. When a table has a row of large size, you need to modify both configurations at the same time.
 - The default value of [`max_allowed_packet`](/system-variables.md#max_allowed_packet-new-in-v610) (the maximum size of a packet for the MySQL protocol) is 67108864 (64 MiB). If a row is larger than `max_allowed_packet`, the row gets truncated.
 - The default value of [`txn-total-size-limit`](#txn-total-size-limit) (the size limit of a single transaction in TiDB) is 100 MiB. If you increase the `txn-entry-size-limit` value to be over 100 MiB, you need to increase the `txn-total-size-limit` value accordingly.