The Storage Network Reality

First it’s important to admit that any storage network, regardless of protocol, becomes more complex as it scales and as the organization becomes dependent upon it. Special considerations have to be made to optimize performance and availability. Second, as mentioned above, it is important to realize that despite its seemingly bad press, fibre channel has maintained its dominant share of the storage infrastructure used in virtualized environments. This is true despite the fact that fibre channel SANs have not enjoyed the ‘only option’ status for a few years in VMware, which now supports a variety of protocols, including iSCSI and NAS (NFS).

Part One - A Case for Fibre Channel

There are several reasons that fibre channel storage has maintained its position as the preferred technology. The primary one may be just the sheer number of fibre channel solutions supporting server virtualization and the number of installations already in production. The advantage of that position is that when a problem arises it’s more than likely been seen many times before. And, consequently, finding a solution becomes relatively straight forward.

Fibre channel is also a mature and virtualization-aware protocol. Using NPIV and switched based QoS via storage buffer credits from vendors like Brocade, priorities can be established on a per-VM basis. This allows storage and server virtualization administrators to assure uneasy application owners that their applications will get the resources they need, even in a virtualized environment. This capability leads to greater virtual machine density which reduces the potential cost delta between fibre channel and other protocols.

Another reason that fibre channel remains a popular consideration is that VMware does development of new features on VMFS (block based storage) first. VMFS is a very robust, shared, cluster-able file system. It was written by VMware to handle their requirements for shared storage and, almost without exception, fibre channel is the first protocol in line to gain access to any new developments. Site Recovery Manager by VMware is a good example. This capability to manage a DR workflow became available first on fibre channel and was only available on fibre channel for almost two years after its release.

Potentially, the most important consideration is the performance advantage that fibre channel enjoys over the iSCSI and NAS protocols. These two IP-based solutions must deal with IP overhead on every storage I/O transaction, read or write. While for some environments, the storage I/O penalty of IP is acceptable, for larger enterprise environments it’s not. In fact, a recent white paper measuring performance of the three major protocols on vSphere found that the host server resource cost-per-storage I/O was significantly greater on iSCSI and NFS.

This resource cost is an important consideration, because it diminishes the economics of building very VM-dense server hosts while it lowers the comfort level for IT administration. The impact of less dense virtual machine populations are increased costs for additional physical hosts and the connections those additional hosts will require for network and storage resources. Other than the cost of the additional physical servers, Storage Switzerland finds that the typical connectivity cost of a physical host is about $5,000 per server.

A final consideration is that not all server virtualization projects will be based on VMware; as other hypervisor options come out they will have their own list of supported storage protocols. For example, don't expect Microsoft's HyperV to support NAS-attached virtual images via HyperV. It today supports iSCSI and of course fibre. Regardless of hypervisor, the initial support will almost always include fibre channel.

The problem is that fibre channel does have its shortcomings. Most of it centers around the complexity of system setup and operation. This however is not really a fault of the infrastructure, fibre itself, it is more the fault of the systems that run on fibre channel infrastructures. Companies like 3PAR are addressing these shortcomings and bring the ease of management often associated with NAS systems to the block fibre channel world.

George Crump, Senior Analyst

3PAR is a client of Storage Switzerland