br: add description for --granularity

br/br-snapshot-guide.md

@@ -71,6 +71,8 @@ tiup br restore full --pd "${PD_IP}:2379" \
 --storage "s3://backup-101/snapshot-202209081330?access-key=${access-key}&secret-access-key=${secret-access-key}"
 ```
 
+To address the potential bottleneck issue during the restore when there are too many regions, starting from v7.6.0, BR supports a coarse-grained region shuffling approach. You can enable this method by specifying `--granularity=coarse-grained`.
+
 During restore, a progress bar is displayed in the terminal as shown below. When the progress bar advances to 100%, the restore task is completed and statistics such as total restore time, average restore speed, and total data size are displayed.
 
 ```shell
@@ -200,6 +202,19 @@ The impact of backup on cluster performance can be reduced by limiting the backu
 
 - During data restore, TiDB tries to fully utilize the TiKV CPU, disk IO, and network bandwidth resources. Therefore, it is recommended to restore the backup data on an empty cluster to avoid affecting the running applications.
 - The speed of restoring backup data is much related with the cluster configuration, deployment, and running applications. In internal tests, the restore speed of a single TiKV node can reach 100 MiB/s. The performance and impact of snapshot restore are varied in different user scenarios and should be tested in actual environments.
+- Starting from v7.6.0, BR introduces an experimental feature that allows the use of a coarse-grained region shuffling algorithm by specifying the command-line parameter `--granularity=coarse-grained`. Additionally, you can leverage the `--tikv-max-restore-concurrency` option to set the concurrency of download tasks for a single TiKV node, making full use of all resources on each TiKV node to achieve parallel and rapid restore. In practical scenarios, in the case of a large-scale region, the cluster snapshot restore speed can be increased by approximately 10 times. An example of usage is as follows:
+
+```bash
+     br restore full \
+     --pd "${PDIP}:2379" \
+     --storage "s3://${Bucket}/${Folder}" \
+     --s3.region "${region}" \
+     --granularity "coarse-grained" \
+     --tikv-max-restore-concurrency 128 \
+     --send-credentials-to-tikv=true \
+     --log-file restorefull.log
+```
+
 
 ## See also