Update follower read and learner read descriptions (#21470) (#21813)

Author: ti-chi-bot

follower-read.md

@@ -31,8 +31,8 @@ Default: leader
 
 This variable is used to set the expected data read mode.
 
-- When the value of `tidb_replica_read` is set to `leader` or an empty string, TiDB maintains its default behavior and sends all read operations to the leader replica to perform.
-- When the value of `tidb_replica_read` is set to `follower`, TiDB selects a follower replica of the Region to perform all read operations.
+- When you set the value of `tidb_replica_read` to `leader` or an empty string, TiDB maintains its default behavior and sends all read operations to the leader replica to perform.
+- When you set the value of `tidb_replica_read` to `follower`, TiDB selects a follower replica of the Region to perform read operations. If the Region has learner replicas, TiDB also considers them for reads with the same priority. If no available follower or learner replicas exist for the current Region, TiDB reads from the leader replica.
 - When the value of `tidb_replica_read` is set to `leader-and-follower`, TiDB can select any replicas to perform read operations. In this mode, read requests are load balanced between the leader and follower.
 - When the value of `tidb_replica_read` is set to `prefer-leader`, TiDB prefers to select the leader replica to perform read operations. If the leader replica is obviously slow in processing read operations (such as caused by disk or network performance jitter), TiDB will select other available follower replicas to perform read operations.
 - When the value of `tidb_replica_read` is set to `closest-replicas`, TiDB prefers to select a replica in the same availability zone to perform read operations, which can be a leader or a follower. If there is no replica in the same availability zone, TiDB reads from the leader replica.
@@ -41,13 +41,18 @@ This variable is used to set the expected data read mode.
     - If the estimated result of a read request is greater than or equal to the value of [`tidb_adaptive_closest_read_threshold`](/system-variables.md#tidb_adaptive_closest_read_threshold-new-in-v630), TiDB prefers to select a replica in the same availability zone for read operations. To avoid unbalanced distribution of read traffic across availability zones, TiDB dynamically detects the distribution of availability zones for all online TiDB and TiKV nodes. In each availability zone, the number of TiDB nodes whose `closest-adaptive` configuration takes effect is limited, which is always the same as the number of TiDB nodes in the availability zone with the fewest TiDB nodes, and the other TiDB nodes automatically read from the leader replica. For example, if TiDB nodes are distributed across 3 availability zones (A, B, and C), where A and B each contains 3 TiDB nodes and C contains only 2 TiDB nodes, the number of TiDB nodes whose `closest-adaptive` configuration takes effect in each availability zone is 2, and the other TiDB node in each of the A and B availability zones automatically selects the leader replica for read operations.
     - If the estimated result of a read request is less than the value of [`tidb_adaptive_closest_read_threshold`](/system-variables.md#tidb_adaptive_closest_read_threshold-new-in-v630), TiDB can only select the leader replica for read operations.
 
-- When the value of `tidb_replica_read` is set to `learner`, TiDB reads data from the learner replica. If there is no learner replica in the Region, TiDB returns an error.
+- When you set the value of `tidb_replica_read` to `learner`, TiDB reads data from the learner replica. If no learner replica is available for the current Region, TiDB reads from an available leader or follower replica.
 
 <CustomContent platform="tidb">
 
 > **Note:**
 >
-> When the value of `tidb_replica_read` is set to `closest-replicas` or `closest-adaptive`, you need to configure the cluster to ensure that replicas are distributed across availability zones according to the specified configuration. To configure `location-labels` for PD and set the correct `labels` for TiDB and TiKV, refer to [Schedule replicas by topology labels](/schedule-replicas-by-topology-labels.md). TiDB depends on the `zone` label to match TiKV nodes in the same availability zone, so you need to make sure that the `zone` label is included in the `location-labels` of PD and `zone` is included in the configuration of each TiDB and TiKV node. If your cluster is deployed using TiDB Operator, refer to [High availability of data](https://docs.pingcap.com/tidb-in-kubernetes/v1.4/configure-a-tidb-cluster#high-availability-of-data).
+> When you set `tidb_replica_read` to `closest-replicas` or `closest-adaptive`, to ensure that replicas are distributed across availability zones according to the specified configuration, you need to configure `location-labels` for PD and set the correct `labels` for TiDB and TiKV according to [Schedule replicas by topology labels](/schedule-replicas-by-topology-labels.md). TiDB depends on the `zone` label to match TiKV nodes in the same availability zone, so you need to make sure that the `zone` label is included in the `location-labels` of PD and `zone` is included in the configuration of each TiDB and TiKV node. If your cluster is deployed using TiDB Operator, refer to [High availability of data](https://docs.pingcap.com/tidb-in-kubernetes/stable/configure-a-tidb-cluster#high-availability-of-data).
+>
+> For TiDB v7.5.0 and earlier versions:
+>
+> - If you set `tidb_replica_read` to `follower` and no follower or learner replicas are available, TiDB returns an error.
+> - If you set `tidb_replica_read` to `learner` and no learner replicas are available, TiDB returns an error.
 
 </CustomContent>