Describe TiCDC support in TiFlash (#20681) (#20686)

Author: ti-chi-bot

ticdc/ticdc-compatibility.md

@@ -28,6 +28,13 @@ In the physical import mode, TiDB Lightning imports data by inserting SST files
     1. Use the offline import tool provided by your downstream system to import TiDB Lightning's input files.
     2. Create a changefeed to replicate subsequent incremental data written through SQL. For more information, see [Create a replication task](/ticdc/ticdc-manage-changefeed.md#create-a-replication-task).
 
+## Compatibility with TiFlash
+
+Currently, when you use TiCDC to replicate tables to a downstream TiDB cluster, creating TiFlash replicas for the tables is not supported, which means that TiCDC does not support replicating TiFlash-related DDL statements, such as:
+
+* `ALTER TABLE table_name SET TIFLASH REPLICA count;`
+* `ALTER DATABASE db_name SET TIFLASH REPLICA count;`
+
 ## CLI and configuration file compatibility
 
 * In TiCDC v4.0.0, `ignore-txn-commit-ts` is removed and `ignore-txn-start-ts` is added, which uses start_ts to filter transactions.

tiflash/create-tiflash-replicas.md

@@ -53,6 +53,11 @@ ALTER TABLE `tpch50`.`lineitem` SET TIFLASH REPLICA 0;
 
 * In v5.1 and later versions, setting the replicas for the system tables is no longer supported. Before upgrading the cluster, you need to clear the replicas of the relevant system tables. Otherwise, you cannot modify the replica settings of the system tables after you upgrade the cluster to a later version.
 
+* Currently, when you use TiCDC to replicate tables to a downstream TiDB cluster, creating TiFlash replicas for the tables is not supported, which means that TiCDC does not support replicating TiFlash-related DDL statements, such as:
+
+    * `ALTER TABLE table_name SET TIFLASH REPLICA count;`
+    * `ALTER DATABASE db_name SET TIFLASH REPLICA count;`
+
 ### Check replication progress
 
 You can check the status of the TiFlash replicas of a specific table using the following statement. The table is specified using the `WHERE` clause. If you remove the `WHERE` clause, you will check the replica status of all tables.
@@ -103,6 +108,8 @@ Examples:
 >
 > - This statement skips system tables, views, temporary tables, and tables with character sets not supported by TiFlash.
 
+> - You can control the number of tables allowed to remain unavailable during execution by setting the [`tidb_batch_pending_tiflash_count`](/system-variables.md#tidb_batch_pending_tiflash_count-new-in-v60) system variable. Lowering this value helps reduce the pressure on the cluster during replication. Note that this limit is not real-time, so it is still possible for the number of unavailable tables to exceed the limit after the setting is applied.
+
 ### Check replication progress
 
 Similar to creating TiFlash replicas for tables, successful execution of the DDL statement does not mean the completion of replication. You can execute the following SQL statement to check the progress of replication on target tables: