I think the key point here is what the definition of a "box" is. Exadata documentation deliberately avoids this term because of its ambiguity. A half rack of Exadata X4-2 contains 4 database servers. Each database server has its own CPUs, memory, etc, so if you configure 4 database servers in a RAC cluster, the cluster keeps running if a single server fails. There is infrastructure that covers the entire rack like power distribution units and InfiniBand switches, but this infrastructure itself is configured redundantly so that the rack will continue functioning even if a single PDU or InfiniBand switch fails.
Thanks Marc for the response.
What i meant by box is the full set of exadata box that consists of all the database servers(considerng a half rac of exadata). When we purchase an exadata box, it is a single piece of box that consisits of database servers inside. My question or concern is , what will happen if something happens with the power supply unit of the exadata box or during a disaster recovery scenario .
I am thinking with respect to a RAC set up. On a traditional 4 node RAC, we have 4 different nodes, if one node goes down, other nodes will work. But in a exadata setup, we have all the 4 nodes residing inside on a single exadata box and in a disaster recovery scenario, how does this help on high availability comparing with a traditional RAC set up.
Let's take your example of power supply unit (PSU) redundancy. Quoting from the X4-2 datasheet (http://www.oracle.com/technetwork/database/exadata/exadata-dbmachine-x4-2-ds-2076448.pdf):
Exadata Database Machine X4-2 Hardware
4 x Database Servers, each
2 x Redundant Hot-Swappable Power Supplies
The key word here is each: every database server has its own set of 2 redundant PSUs, providing equivalent redundancy to 4 separate, commodity servers.