This seems to contradict you.
When a volume containing warm buckets reaches its maxVolumeDataSizeMB, it starts rolling buckets to cold. When a volume containing cold buckets reaches its maxVolumeDataSizeMB, it starts rolling buckets to frozen. If a volume contains both warm and cold buckets (which will happen if an index's homePath and coldPath are both set to the same volume), the oldest bucket will be rolled to frozen.
http://docs.splunk.com/Documentation/Splunk/6.2.2/Indexer/Configureindexstoragesize#Configure_index_size_with_volumes
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