SAN (Storage Area Network) storage is used to provide block-level storage to client hosts using a remote connection, which is typically iSCSI over ethernet, but in on-premises environments may also be over a fiber channel network. This decouples the compute layer from the storage layer, allowing each to be managed independently.
Accessing block-level storage from compute instances in the cloud involves connecting to remote disks over a network connection, which is the hallmark of all SAN storage solutions. Using this technique ensures that applications can always access as much storage as they need, while at the same time providing transparent data redundancy. Amazon EBS, for example, creates a redundant copy of disk data within an Availability Zone in order to remove single points of failure at the storage level.