There are several reasons why you should have the journal and ledger disks on separate physical volumes, including performance. But the primary reason is that they serve two different purposes. The journal disk is used for short-term storage of the messages before they are indexed and written to the ledger disk. The journal disk provides data durability guarantees in the event of a failure, the bookie can recover and load the messages from the journal disk. However, if the journal disk fails, Pulsar will continue to operate. So separating them eliminates a single point of failure from the storage layer as well.