[improvememt](file-cache) increase virtual node number to make file cache more even (#24143)

The origin virtual number is Math.max(Math.min(512 / backends.size(), 32), 2);, which is too small,
causing uneven cache distribution when enabling file cache.

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -2157,4 +2157,12 @@ public class Config extends ConfigBase {
             "是否禁止LocalDeployManager删除节点",
             "Whether to disable LocalDeployManager drop node"})
     public static boolean disable_local_deploy_manager_drop_node = true;
+
+    @ConfField(mutable = true, description = {
+            "开启 file cache 后，一致性哈希算法中，每个节点的虚拟节点数。"
+                    + "该值越大，哈希算法的分布越均匀，但是会增加内存开销。",
+            "When file cache is enabled, the number of virtual nodes of each node in the consistent hash algorithm. "
+                    + "The larger the value, the more uniform the distribution of the hash algorithm, "
+                    + "but it will increase the memory overhead."})
+    public static int virtual_node_number = 2048;
 }

fe/fe-core/src/main/java/org/apache/doris/planner/external/FederationBackendPolicy.java

@@ -96,9 +96,8 @@ public class FederationBackendPolicy {
             throw new UserException("No available backends");
         }
         backendMap.putAll(backends.stream().collect(Collectors.groupingBy(Backend::getHost)));
-        int virtualNumber = Math.max(Math.min(512 / backends.size(), 32), 2);
         consistentHash = new ConsistentHash<>(Hashing.murmur3_128(), new ScanRangeHash(),
-                new BackendHash(), backends, virtualNumber);
+                new BackendHash(), backends, Config.virtual_node_number);
     }
 
     public Backend getNextBe() {