Symmetric Storage virtualization

In symmetric storage virtualisation the data and control flow go down the same path (Figure).

This means that the abstraction from physical to logical storage necessaryfor virtualisation must take place within the data flow.

As a result, the metadata controller is positioned precisely in the data flow between server and storage devices, which is why symmetric virtualisation is also called in-band virtualisation.

Advantages of symmetric virtualisation are:

The application servers can easily be provided with data access both on block and file level, regardless of the underlying physical storage devices.

The administrator has complete control over which storage resources are available to which servers at a central point. This increases security and eases the administration.

Assuming that the appropriate protocols are supported, symmetric virtualisation does not place any limit on specific operating system platforms. It can thus also be used in heterogeneous environments.

The performance of existing storage networks can be improved by the use of caching and clustering in the metadata controllers.

The use of a metadata controller means that techniques such as snapshots or mirroring can be implemented in a simple manner, since they control the storage access directly. They can also be used on storage devices such as JBODs or simple RAID arrays that do not provide to these techniques themselves.

The disadvantages of a symmetric virtualisation are:

Each individual metadata controller must be administered. If several metadata controllers are used in a cluster arrangement, then the administration is relatively complex and time-consuming particularly due to the cross-computer data access layer. This disadvantage can, however, be reduced by the use of a central administration console for the metadata controller.

Several controllers plus cluster technology are indispensable to guarantee thefault-tolerance of data access.

As an additional element in the data path, the controller can lead to performance problems, which makes the use of caching or load distribution over several controllers indispensable.

It can sometimes be difficult to move the data between storage devices if this is managed by different metadata controllers.

Asymmetric Storage virtualization

In contrast to symmetric virtualization, in asymmetric virtualization the data flow is separated from the control flow. This is achieved by moving all mapping operations from logical to physical drives to a metadata controller outside the data path

The metadata controller now only has to look after the administrative and control tasks of virtualization, the flow of data takes place directly from the application servers to the storage devices. As a result, this approach is also called out-band virtualization

The following advantages of asymmetric virtualization can be established:

• Complete control of storage resources by an absolutely centralized management on the metadata controller.

• Maximum throughput between servers and storage devices by the separation of the control flow from the data flow, thus avoiding additional devices in the data path.

• In comparison to the development and administration of a fully functional volume manager on every server, the porting of the agent software is associated with a low cost.

• As in the symmetric approach, advanced storage functions such as snapshots or mirroring can be used on storage devices that do not themselves support these functions.

• To improve fault-tolerance, several metadata controllers can be brought together to form a cluster. This is easier than in the symmetric approach, since no physical connection from the servers to the metadata controllers is necessary for the data flow.

The disadvantages of asymmetric virtualization are:

A special agent software is required on the servers or the host bus adapters. This can make it more difficult to use this approach in heterogeneous environments, since such software or a suitable host bus adapter must be present for every platform. Incompatibilities between the agent software and existing applications may sometimes make the use of asymmetric virtualization impossible.

The development cost increases further if the agent software and the metadata controller are also to permit access on file level in addition to access on block level.

A performance bottleneck can arise as a result of the frequent communication between agent software and metadata controller. These performance bottlenecks can be remedied by the caching of the physical storage information.

Caching to increase performance requires an ingenious distributed caching algorithm to avoid data inconsistencies. A further option would be the installation of a dedicated cache server in the storage network.