3.1.2 Single storage architecture 3.1.3 Stretched cluster Figure 3-2 Cluster with single storage subsystem  A stretched cluster involves separating the cluster nodes into  sites . A site can be in a different  building within a campus or separated by a few kilometers in terms of distance. In this  configuration, there is a storage area network (SAN) that spans the sites, and storage can be  presented across sites. As with any multi-site cluster, TCP/IP communications are essential. Multiple links and routes  are suggested such that a single network component or path failure can be incurred and  communications between sites still be maintained. Figure 3-2 shows such a layout from a logical point of view. When it is possible to use a storage-based mirroring feature such as IBM SAN Volume  Controller or a SAN Volume Controller stretched cluster, the layout look, from a physical point  of view, is identical or nearly identical to the mirrored architecture that is described in 3.1.1,  “Mirrored architecture” on page 52. However, from an AIX and cluster point of view, it is a  single storage architecture because it is aware of only a single set of LUNs. For more  information about the layout in a SAN Volume Controller stretched cluster, see 3.1.3,  “Stretched cluster” on page 53. In a single storage architecture, the storage is shared by both the primary and backup logical  partition (LPAR). This solution can be used when there are lower availability requirements for  the data, and is not uncommon when the LPARs are in the same location. Chapter 3. Planning considerations  53