Both Solr and ZooKeeper has dataDir. Solr uses dataDir to store indexes while ZooKeeper documentation says its dataDir parameter is used for storing the in-memory database snapshots.
I know ZooKeeper is the service that is used to maintain configuration information and distributed synchronization across Solr. So, what exactly ZooKeeper dataDir is used for, is it stores the indexes of Solr cluster too or not.
ZooKeeper never stores any actual documents from Solr indices. It could store the hash ranges for each shard (thanks to @kellyfj). ZooKeeper in general is agnostic to what kinda of cluster information it maintains, it could be used as a standalone system to create your own distributed system or it with any other system like Hadoop, etc.
Solr dataDir is only related to Solr index and isn't related to ZooKeeper at all. Solr Cloud used ZooKeeper that helps manage the overall structure so that both indexing and search requests can be routed properly.
