docs: add Redis Cluster key prefix documentation

docs/en/reference/how_to_set_up_metadata_engine.md

@@ -31,6 +31,23 @@ When you need to migrate between two types of metadata engines, you can use this
 
 JuiceFS requires Redis version 4.0 and above. Redis Cluster is also supported, but in order to avoid transactions across different Redis instances, JuiceFS puts all metadata for one file system on a single Redis instance.
 
+:::tip Redis Cluster Key Prefix
+When using Redis Cluster, the database number in the URL is used as a **key prefix** rather than for actual database selection (since Redis Cluster only supports database 0). The prefix format is `{N}` (e.g., `{1}`, `{2}`), which uses Redis hash tags to ensure all keys for one volume are routed to the same slot. This allows multiple JuiceFS file systems to share a single Redis Cluster:
+
+```shell
+# Different volumes use different DB numbers as key prefixes
+juicefs format redis://cluster:6379/1 volume1   # keys prefixed with {1}
+juicefs format redis://cluster:6379/2 volume2   # keys prefixed with {2}
+```
+
+You can verify the keys in Redis Cluster using:
+
+```shell
+redis-cli -c -h <host> -p 6379 keys '{1}*'   # list all keys for volume with prefix {1}
+```
+
+:::
+
 To ensure metadata security, JuiceFS requires [`maxmemory-policy noeviction`](https://redis.io/docs/reference/eviction/), otherwise it will try to set it to `noeviction` when starting JuiceFS, and will print a warning log if it fails. Refer to [Redis Best practices](../administration/metadata/redis_best_practices.md) for more.
 
 #### Create a file system

docs/zh_cn/reference/how_to_set_up_metadata_engine.md

@@ -31,6 +31,23 @@ JuiceFS 采用数据和元数据分离的存储架构，元数据可以存储在
 
 JuiceFS 要求使用 4.0 及以上版本的 Redis。JuiceFS 也支持使用 Redis Cluster 作为元数据引擎，但为了避免在 Redis 集群中执行跨节点事务，同一个文件系统的元数据总会坐落于单个 Redis 实例中。
 
+:::tip Redis Cluster 键前缀
+使用 Redis Cluster 时，URL 中的数据库编号会被用作**键前缀**，而不是用于实际的数据库选择（因为 Redis Cluster 仅支持数据库 0）。前缀格式为 `{N}`（例如 `{1}`、`{2}`），使用 Redis 哈希标签（hash tag）确保同一个卷的所有键都被路由到同一个槽（slot）。这使得多个 JuiceFS 文件系统可以共享同一个 Redis Cluster：
+
+```shell
+# 不同的卷使用不同的数据库编号作为键前缀
+juicefs format redis://cluster:6379/1 volume1   # 键前缀为 {1}
+juicefs format redis://cluster:6379/2 volume2   # 键前缀为 {2}
+```
+
+可以使用以下命令在 Redis Cluster 中验证键：
+
+```shell
+redis-cli -c -h <host> -p 6379 keys '{1}*'   # 列出前缀为 {1} 的所有键
+```
+
+:::
+
 为了保证元数据安全，JuiceFS 需要 [`maxmemory-policy noeviction`](https://redis.io/docs/reference/eviction/)，否则在启动 JuiceFS 的时候将会尝试将其设置为 `noeviction`，如果设置失败将会打印告警日志。更多可以参考 [Redis 最佳实践](../administration/metadata/redis_best_practices.md)。
 
 #### 创建文件系统