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## 3.1.2 Single storage architecture 

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. 

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. 

Figure 3-2 shows such a layout from a logical point of view. 

<img src='content_image/976145.jpg'>

## 3.1.3 Stretched cluster 

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. 

