HashMap<PartitionKey, OrderedSet<ClusterKey>>
So you can lookup data by the ParititionKey, then perform range queries on the ClusterKey to filter data belonging to that partition.
If your access patterns look like that, Cassandra could be a great fit.
For anything else, you probably want to consider a different data store.
Though HashMap should be just "Map" and the value of the top level map is not a set, it's another map:
Map<PartitionKey, SortedMap<ClusterKey, Record>>
C* is essentially scaled/automated MySQL Sharding and blob storage.. except you pay a huge cost for any server side filtering.
The Partition Key would be the equivalent of your MySQL shard id. ClusterKey is your primary key. if you treat everything like PUT $paritionKey, $primaryKey, $data and GET $partitionKey, $primaryKey you get a slow and massively scaling redis. If you do anything else with it you'll likely start regretting your choices and looking for a replacement database.