Therefore, the virtualization engine is a central point of control for device and advanced function management. Symmetric virtualization enables you to build a firewall in the storage network. Only the virtualization engine can grant access through the firewall. 

Symmetric virtualization can have disadvantages. The main disadvantage that is associated with symmetric virtualization is scalability. Scalability can cause poor performance because all input/output (I/O) must flow through the virtualization engine. To solve this problem, you can use an n -way cluster of virtualization engines that has failover capacity. 

You can scale the additional processor power, cache memory, and adapter bandwidth to achieve the level of performance that you want. Additional memory and processing power are needed to run advanced services, such as Copy Services and caching. The SVC uses symmetric virtualization. Single virtualization engines, which are known as nodes , are combined to create clusters. Each cluster can contain 2 - 8 nodes. 

## GLYPH<SM590000> Asymmetric: Out-of-band or controller-based 

With asymmetric virtualization, the virtualization engine is outside the data path and performs a metadata-style service. The metadata server contains all of the mapping and the locking tables, and the storage devices contain only data. In asymmetric virtual storage networks, the data flow is separated from the control flow. 

A separate network or SAN link is used for control purposes. Because the control flow is separated from the data flow, I/O operations can use the full bandwidth of the SAN. A separate network or SAN link is used for control purposes. 

Asymmetric virtualization can have the following disadvantages: 

– Data is at risk to increased security exposures, and the control network must be protected with a firewall. 

– Metadata can become complicated when files are distributed across several devices. 

– Each host that accesses the SAN must  know how to access and interpret the metadata. Specific device drivers or agent software must therefore be running on each of these hosts. 

– The metadata server cannot run advanced functions, such as caching or Copy Services, because it only “knows” about the metadata and not about the data itself. 

