Solandra vs. ElasticSearch
Solandra is being used in the 10s of Terabytes range.
Are you saying you want to index a PB of data in solandra or a subset? I think if you want 1 big index with a PB of data you are stretching the limits. but If you want a PB of indexes, then this will scale the same as Cassandra.
How many nodes are you planning to run? how much disk per node?
Have a look into this nice discussion:
http://groups.google.com/a/elasticsearch.com/group/users/browse_thread/thread/3f99e682887f98e4