br: update restriction of ratelimit and concurrency (#15239) (#17841)

Author: ti-chi-bot

Diff for: br/br-auto-tune.md

@@ -13,7 +13,7 @@ To reduce the impact of backup tasks on the cluster, TiDB v5.4.0 introduces the
 
 If you want to reduce the impact of backup tasks on the cluster, you can enable the auto-tune feature. With this feature enabled, TiDB performs backup tasks as fast as possible without excessively affecting the cluster.
 
-Alternatively, you can limit the backup speed by using the TiKV configuration item [`backup.num-threads`](/tikv-configuration-file.md#num-threads-1) or using the parameter `--ratelimit`.
+Alternatively, you can limit the backup speed by using the TiKV configuration item [`backup.num-threads`](/tikv-configuration-file.md#num-threads-1) or using the parameter `--ratelimit`. When `--ratelimit` is set, to avoid too many tasks causing the speed limit to fail, the `concurrency` parameter of br is automatically adjusted to `1`.
 
 ## Use auto-tune
 

Diff for: br/br-snapshot-guide.md

@@ -165,7 +165,7 @@ To illustrate the impact of backup, this document lists the test conclusions of
 
 You can use the following methods to manually control the impact of backup tasks on cluster performance. However, these two methods also reduce the speed of backup tasks while reducing the impact of backup tasks on the cluster.
 
-- Use the `--ratelimit` parameter to limit the speed of backup tasks. Note that this parameter limits the speed of **saving backup files to external storage**. When calculating the total size of backup files, use the `backup data size(after compressed)` as a benchmark.
+- Use the `--ratelimit` parameter to limit the speed of backup tasks. Note that this parameter limits the speed of **saving backup files to external storage**. When calculating the total size of backup files, use the `backup data size(after compressed)` as a benchmark. When `--ratelimit` is set, to avoid too many tasks causing the speed limit to fail, the `concurrency` parameter of br is automatically adjusted to `1`.
 - Adjust the TiKV configuration item [`backup.num-threads`](/tikv-configuration-file.md#num-threads-1) to limit the number of threads used by backup tasks. According to internal tests, when BR uses no more than `8` threads for backup tasks, and the total CPU utilization of the cluster does not exceed 60%, the backup tasks have little impact on the cluster, regardless of the read and write workload.
 
 The impact of backup on cluster performance can be reduced by limiting the backup threads number, but this affects the backup performance. The preceding tests show that the backup speed is proportional to the number of backup threads. When the number of threads is small, the backup speed is about 20 MiB/thread. For example, 5 backup threads on a single TiKV node can reach a backup speed of 100 MiB/s.