With asymmetric virtualization, the virtualization engine is outside the data path and performs a metadata-style service. The metadata server contains all the mapping and the locking tables while the storage devices contain only data.
In asymmetric virtual storage networks, the data flow, (2) in the Figure 1, is separated from the control flow, (1). A separate network or SAN link is used for control purposes. The metadata server contains all the mapping and locking tables while the storage devices contain only data. Because the flow of control is separated from the flow of data, I/O operations can use the full bandwidth of the SAN. A separate network or SAN link is used for control purposes. However, there are disadvantages to asymmetric virtualization.
- Data is at risk to increased security exposures, and the control network must be protected with a firewall.
- Metadata can become very 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.
