Where are the remaining two storage strategies?

In `apache-iotdb-1.3.1-all-bin/conf/iotdb-datanode.properties`

```
# multi_dir_strategy
# The strategy is used to choose a directory from data_dirs for the system to store a new tsfile.
# System provides four strategies to choose from, or user can create his own strategy by extending org.apache.iotdb.db.conf.directories.strategy.DirectoryStrategy.
# The info of the four strategies are as follows:
# 1. SequenceStrategy: the system will choose the directory in sequence.
# 2. MaxDiskUsableSpaceFirstStrategy: the system will choose the directory whose disk has the maximum space.
# Set SequenceStrategy or MaxDiskUsableSpaceFirstStrategy to apply the corresponding strategy.
# If this property is unset, system will use SequenceStrategy as default strategy.
# For this property, fully-qualified class name (include package name) and simple class name are both acceptable.
# dn_multi_dir_strategy=SequenceStrategy
```

Two of the four storage strategies mentioned here
# 1. SequenceStrategy: the system will choose the directory in sequence.
# 2. MaxDiskUsableSpaceFirstStrategy: the system will choose the directory whose disk has the maximum space.

Where are the remaining two storage strategies?
And what I need is a strategy that automatically balances multi-directory storage.

Start with the multi_dir_strategy section in apache-iotdb-1.3.1-all-bin/conf/iotdb-datanode.properties and inspect org.apache.iotdb.db.conf.directories.strategy.DirectoryStrategy. Determine which four strategies are actually available and how a strategy that balances storage across data_dirs should be specified; done requires an agreed scope and corresponding documentation or implementation evidence.

Written by the indexing model from the issue text.