Prefer local DataNode when LOAD creates DataRegions - #18663

Summary

Add a hidden, hot-reloadable DataNode configuration load_tsfile_prefer_local_node (default true). When enabled, LOAD passes the current DataNodeId to ConfigNode as preferredDataNodeId while creating a database or requesting get-or-create data partitions. ConfigNode then prefers placing newly created DataRegion replicas on the requesting node without modifying the existing region allocation algorithms themselves.

Design / How It Works

1. DataNode configuration

• IoTDBConfig.loadTsFilePreferLocalNode is a volatile boolean, default true.
• IoTDBDescriptor loads it both at startup and during hot reload.
• It is intentionally not added to iotdb-system.properties.template, so it stays hidden from SHOW CONFIGURATION.

2. Database creation path

When LOAD auto-creates a database:

• TreeSchemaAutoCreatorAndVerifier sets preferredDataNodeId on DatabaseSchemaStatement.
• DatabaseSchemaTask.constructDatabaseSchema copies it to TDatabaseSchema.preferredDataNodeId.
• ConfigNode persists the field and RegionBalancer can later read it.

3. Data-partition creation path

This covers the case where the database already exists and LOAD creates devices/tables or requests an unassigned series partition, which triggers DataRegion extension:

• LoadTsFileScheduler.DataPartitionBatchFetcher reads load_tsfile_prefer_local_node.
• It calls the new IPartitionFetcher.getOrCreateDataPartition(..., preferredDataNodeId) overload.
• ClusterPartitionFetcher puts the value into TDataPartitionReq.preferredDataNodeId.
• ConfigNode converts it into GetOrCreateDataPartitionPlan.
• PartitionManager passes the hint down to DataRegion extension.
• RegionBalancer prefers the request-level hint first, then falls back to the database-schema hint.

4. RegionBalancer preference handling

The original allocator is still called first:

regionGroupAllocator.generateOptimalRegionReplicasDistribution(...)

Only after that, preferDataNodeIfPossible adjusts the result:

• If the preferred node is unavailable, do nothing.
• If it is already in the replica set, move it to the first position.
• If it is absent, replace the last replica with the preferred node.

Replication factor, candidate validity, and region identity are preserved.

5. New time partitions

New time partitions do not create a new DataRegion. A new time slot is mapped into an existing DataRegion for the same series-partition slot, so this path needs no preference change.

Validation

mvn -pl iotdb-core/datanode,iotdb-core/confignode -am -DskipTests -Drat.skip=true compile

Result: BUILD SUCCESS.