Solution provider takeaway: Storage solution providers have four main storage networking protocols to consider for their VMware server virtualization customers.
In the world of server virtualization platforms, VMware has long been the dominant choice. So while your customers might not have to think too hard about the platform itself, they do need to give careful consideration to the storage networking protocol they implement. And it's likely going to be your responsibility to walk them through the various options and help them choose a protocol.
There are a number of storage networking protocol options for VMware, but these four are the most important: Fibre Channel, iSCSI, NFS and the newest method, internally shared storage via a virtual storage appliance. (Most of the
Fibre Channel is by far the protocol with the longest history. VMware and its virtualization competitors supported Fibre Channel before the other protocols. This longevity is really Fibre Channel's biggest advantage over the other protocols. It's a known entity.
Fibre Channel SANs, however, have been characterized as expensive and difficult to manage; most of this characterization is based on old history. Companies like Brocade, Emulex and QLogic have gone a long way toward making Fibre Channel less expensive and easier to use.
And, while Fibre Channel also has the advantage of being the fastest option available in just about every test case, it's important to consider whether your customer actually needs such speed. The answer is always "it depends" -- depends on what's in place in your customer's data center, what their performance requirements are and what they're most comfortable with. Also keep in mind that virtualization changes the workload profile. Before virtualization, the workload profile was on a single server, a single application to a typically dedicated LUN. With virtualization, that workload is one of many on a single host, accessing an often-shared LUN. Storage IO performance becomes very important as the number of virtual machines increases. (An upcoming article on SearchStorage.com discusses how 8GB Fibre Channel is being adopted more rapidly than 4GB Fibre Channel was, propelled primarily by server virtualization.)
The first alternative storage networking protocol to come on the server virtualization scene was iSCSI, and for many reasons it has captured a measurable share of the market. The performance of iSCSI, while not at Fibre Channel's level, is acceptable for many environments.
iSCSI has the advantage of running on a protocol that many of your customers are already familiar with: IP. Also, because it can use standard 1Gigabit Ethernet NICs, it can cost significantly less to deploy initially. And the iSCSI storage options available from Dell EqualLogic and LeftHand Networks, for example, are less expensive than traditional Fibre Channel SANs.
Caution should be used, however, as your customer begins to scale out an iSCSI environment, adding more virtual machines and physical hosts. While performance can be tweaked on iSCSI by adding iSCSI NICs or trunking 1GB segments, this quickly adds to the cost and complexity and may make Fibre Channel a more attractive offering.
NFS is one of the least understood protocol options for virtualized server environments. Virtual machine disk images, or VMDKs, can be booted from a NAS appliance like those from NetApp, EMC or OnStor, which can serve files up via NFS.
For many customers NFS is simpler than even iSCSI, because it really is IP. With Fibre Channel and iSCSI, VMDKs are stored on a shared SAN file system. With VMware, the shared SAN file system is a special system called VMFS; it stores the VMDKs. NFS, on the other hand, is inherently a shared file system, and it can simply manage the VMDKs as files.
In addition to its ease of use, NAS also enables management of the VMDK files just like any other file; this, for example, allows backup and recovery operations to be performed on a backup device directly attached to the NAS.
Performance is always questioned when discussing NFS as a protocol option. But in VMware's own testing, NFS performed nearly identical to iSCSI and was exceeded only by Fibre Channel.
Shared storage via virtual storage appliances
Finally, there is the option of using a virtual storage appliance to share the storage within the physical hosts. This solution is ideal for smaller customers that want server virtualization but cannot justify the entire infrastructure.
Companies like DataCore and FalconStor provide virtual storage appliances. With these appliances, the internal disk inside one of the VMware cluster hosts is designated as the primary storage, and storage inside another host is the backup storage. This gives the customer the cost advantage of using internal storage while eliminating the need to build a storage infrastructure.
How to pick the right storage networking protocol option
Protocol selection for your customers should driven by several factors. The first factor is what the customer already has in place. If they already have an infrastructure that's suitable for expansion, the easiest course of action is to simply build onto their infrastructure. Be aware, however, that server virtualization changes everything in measuring performance. As I said above, the move from relatively predictable workloads on a single machine to multiple workloads on a single machine with very random IO is going to stress storage IO requirements rapidly.
The second factor in determining the right protocol is the amount of the virtualization project budget that has been allocated for storage. If the budget is tight, make sure that the customer understands the ramifications of and potential risks to stability and performance from a slower system.
The third factor to consider in the decision is flexibility; in virtualization especially, no one can predict how the system will evolve and change as the implementation grows and matures. Flexibility in protocol is critical. Always suggest a solution that can adapt; avoid recommending a single-protocol solution. Some of the NAS appliances on the market have the ability to support either Fibre Channel or iSCSI or both. This allows you to get the customer started with the relatively simple NAS/NFS protocol and then add either Fibre Channel or iSCSI as performance demands justify the need. And your customer could develop tiers of service within their infrastructure; modest-IO virtual machines can access their images from the NFS-mounted storage, and more demanding systems can pull their images from Fibre Channel.
Another factor to consider is whether your customer needs a specific storage add-on that VMware supports only on a particular protocol. Make sure that the storage networking protocol you recommend is supported by any of the advanced features your customer is interested in. Also, even if it does not come up in the conversation, let them know what your protocol of choice does and does not support. If they're looking for the widest range of support, Fibre Channel is the safest bet because it is usually the first protocol supported by any new feature.
Once all these factors have been weighed, there are two other key things to remember: First, you really need to know how all the protocols work in a virtualization environment since otherwise you won't be able to make the best recommendation in every case. There are variables involved in every server virtualization project that will have a bearing on the choice of storage networking protocol. And second, make sure that you have a thorough understanding of whatever protocol you recommend, especially how it behaves in a VMware environment. Not having that knowledge could be embarrassing during the implementation phase.
About the author
George Crump is chief steward of Storage Switzerland, an IT analyst firm focused on the storage and virtualization segments. With 25 years of experience designing storage solutions for data centers across the United States, he has seen the birth of such technologies as RAID, NAS and SAN. Prior to founding Storage Switzerland, George was chief technology officer at one of the nation's largest storage integrators, where he was in charge of technology testing, integration and product selection. Find Storage Switzerland's disclosure statement here.
This was first published in January 2009