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. 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. Chapter 2. System overview  9 GLYPH<SM590000> Asymmetric: Out-of-band or controller-based