Risks

There are many risk factors associated with the implementation of virtualisation which is why customers are looking to work with ViFX as a credible, knowledgeable and experienced company.

Backing out of a failed implementation

Once the abstraction layer is in place, only the virtualiser knows where the data actually resides on the physical medium. Backing out of a virtual storage environment therefore requires the reconstruction of the logical disks as contiguous disks that can be used in a traditional manner.
Most implementations will provide some form of back-out procedure and with the data migration services it is at least possible, but time consuming.

Interoperability and vendor support

Interoperability is a key enabler to any virtualisation software or device. It applies to the actual physical storage controllers and the hosts, their operating systems, multi-pathing software and connectivity hardware.
Interoperability requirements differ based on the implementation chosen. For example virtualisation implemented within a storage controller adds no extra overhead to host-based interoperability, but will require additional support of other storage controllers if they are to be virtualised by the same software.
Switch based virtualisation may not require specific host interoperability — if it uses packet-cracking techniques to redirect the I/O. Network-based appliances have the highest level of interoperability requirements as they have to interoperate with all devices, storage and hosts.

Complexity affects several areas

Management of environment Although a virtual storage infrastructure benefits from a single point of logical disk and replication service management, the physical storage must still be managed. Problem determination and fault isolation can also become complex, due to the abstraction layer.
Infrastructure design Traditional design ethics may no longer apply, virtualisation brings a whole range of new ideas and concepts to think about (as detailed here)

The software or device itself Some implementations are more complex to design and code - network based, especially in-band (symmetric) designs in particular — these implementations actually handle the I/O requests and so latency becomes an issue.

Meta-data management

Information is one of the most valuable assets in today's business environments and when virtualised, the meta-data are the glue in the middle. If the meta-data are lost so is all the actual data as it would be virtually impossible to reconstruct the logical drives without the mapping information.

Any implementation must ensure its protection with appropriate levels of back-ups and replicas. It is important to be able to reconstruct the meta-data in the event of a catastrophic failure.

The meta-data management also has implications on performance. Any virtualisation software or device must be able to keep all the copies of the meta-data atomic and quickly updateable. Some implementations restrict the ability to provide certain fast update functions, such as point-in-time copies and caching where super fast updates are required to ensure minimal latency to the actual I/O being performed.

Performance and scalability

In some implementations the performance of the physical storage can actually be improved, mainly due to caching. Caching however requires the visibility of the data contained within the I/O request and so is limited to in-band and symmetric virtualization software and devices. However these implementations also directly influence the latency of an I/O request (cache miss), due to the I/O having to flow through the software or device. Assuming the software or device is efficiently designed this impact should be minimal when compared with the latency associated with physical disk accesses.

Due to the nature of virtualisation, the mapping of logical to physical requires some processing power and lookup tables. Therefore every implementation will add some small amount of latency. In addition to response time concerns, throughput has to be considered. The bandwidth into and out of the meta-data lookup software directly impacts the available system bandwidth. In asymmetric implementations, where the meta-data lookup occurs before the information is read or written, bandwidth is less of a concern as the meta-data are a tiny fraction of the actual I/O size.

In-band, symmetric flow through designs are directly limited by their processing power and connectivity bandwidths.

Most implementations provide some form of scale-out model, where the inclusion of additional software or device instances provides increased scalability and potentially increased bandwidth. The performance and scalability characteristics are directly influenced by the chosen implementation.