Server virtualisation: How to pick the right model

Which is the fairest of them all?

SGI logo hardware close-up

Virtualisation has become an over-used buzzword.

On mainframes, it has been around for ages. Its introduction to x86 took a concept formerly reserved for Big Tech and let it loose among the masses.

Once a straightforward technology with a limited number of implementation models, virtualisation has been bootstrapped and shoehorned into every crevice of IT imaginable. Even smartphones are getting the treatment.

New capabilities do nothing for refuseniks who eschew the use of virtualisation. Some feel the need to evangelise this choice, while others loudly proclaim the "one true way" to use the technologies involved.

Direct-attached virtualisation versus distributed models is a common ideological battleground.

Direct-attached virtualisation is simple. A server with local storage hosts several virtual machines. These use the virtual switch (vSwitch) provided by the hypervisor to communicate with each other without having to send packets across the network interface card (NIC) and thus out to the rest of the network.

Talk among yourselves

Typically, virtual machines hosted in a direct-attached scenario are capable of communication with servers and clients located outside the host system, but most of the communication occurs among virtual machines residing on that one system.

Distributed virtualisation is very different. The host server is treated as much as possible as an entirely disposable processing unit. Storage is centralised and delivered to multiple hosts over a storage area network (SAN) with communication between virtual machines offloaded to physical switches.

Each model has its quirks.

Direct-attached virtualisation is fast. The maximum theoretical speed that a 10Gb NIC (a standard interface for modern SANs) could provide information is 1280 megabytes per second (MBps). A fairly common PCIe 8x 2.0 RAID card can theoretically provide up to 4000MBps.

Real-world numbers are not so clean. I’ve only ever got a 10Gb network attached storage up to 900MBps, and the best I’ve wrung out of my RAID cards (SSD RAID 10) is 2200MBps. But 2200MBps beats the pants off 900MBps, and handily demonstrates the storage speed advantage that the direct attached model can deliver.

Networking tells a similar tale. A hypervisor’s vSwitch provides each virtual machine with a virtual 10Gb NIC. This allows all the virtual machines located on a single host to chat among themselves at 10Gb, or faster if you feel like attaching multiple virtual NICs to a given virtual machine.

Tight squeeze

When heading off-host to the rest of the network, these virtual machines need to fight for the limited bandwidth provided by the hardware available. Having 30 virtual machines talking merrily away at 10Gb each is a completely different experience from asking those same 30 virtual machines to squeeze through a single 10Gb network card – and back again – to have networking processed by a physical switch.

Were we to consider only the numbers presented so far, distributed virtualisation would seem insane. But it has its advantages, and for many they are worth the cost.

What direct-attached virtualisation can’t do is rapidly move a virtual machine from one host to another. Virtual machines can be quite large, and moving the entire thing across a network can take a long time.

This is not an issue with distributed virtualisation’s centralised storage model. Distributed virtualisation also allows for live migration of running virtual machines between hosts.

High availability is another key selling point for distributed virtualisation.

Direct-attached virtualisation relies on robust, fault-tolerant virtual hosts for high availability. Distributed virtualisation senses when a host has failed and restarts all its virtual machines on other hosts in the cluster. The more hosts you have in play, the more the distributed model makes sense.

I can see you

Another benefit is that despite the speed bottlenecks, forcing all traffic through a physical switch gives network administrators visibility and manageability.

Enterprise-class networks run networking gear with tools providing end-to-end management straight down to the very last port. They can offer encryption between links, traffic isolation, monitoring, quality of service and a bingo card of other tick-box features.

All of that goes away the instant a vSwitch is brought into play. vSwitches don’t speak the same management language as the physical network providers. Instead of being able to control every packet to every system on the network, the closest you can get when using a vSwitch is control to and from the host servers.

Blurred outlines

Until recently, these two models were all we had. You picked the features that were more important to you and lived with your choices. This is unsatisfactory and in the grand IT tradition of nothing ever remaining sacred for long, hybrid virtualisation models have started to appear.

A new generation of NICs is starting to blur the lines, employing leading-edge standards such as 802.1Qbg, also known as Edge Virtual Bridging or Virtual Ethernet Port Aggregation (VEPA).

VEPA NICs are switches in their own right. When in use, virtual machines on a host bypass the vSwitch and talk directly to the switch integrated into the NIC. The NIC can talk to the management software, and now we have all the advantages of distributed networking without the bottleneck caused by having to send all virtual machine traffic out to the physical switch.

The competing approach to VEPA is 802.1Qbh, also known as Bridge Port Extension or VN-Tag. It is backed almost exclusively by Cisco, and requires an extension to the Ethernet specification, thus lots of new hardware.

This is a stark contrast to VEPA, which doesn't require you to rip up and replace your network estate, and yet provides a viable solution to end-port management issues in virtual environments.

Configurations making use of both direct-attached storage and distributed storage in a single host are also beginning to appear. I have recently finished a deployment in which all hosts have a large amount of local storage to facilitate backups.

Each host has a virtual backup appliance (VBA) that takes live image-based backups of the virtual machines assigned to that host and stores them on the local buffer drive. This makes for very fast backups.

A central VBA reads the backups from all hosts and writes them out to tape during the day. The tape drive is mapped directly through from the host to the VBA rather than being a network-attached device.

This hybrid approach was not found in a whitepaper but born out of the necessity to make the best use of existing equipment. It has worked so well that, with refinements, I will re-use it in future deployments.

Perpetual movement

The continual introduction of new technologies into the mix will ensure that no virtualisation model stays static for long. IOMMU is the latest greatest, and promises to allow individual virtual machines direct access to system devices such as graphics cards.

Virtual machines will have the ability to tap into the full power of GPGPU computing, and will need to be fed data far faster than distributed technologies such as fibre channel can provide.

Advances in fault-tolerant hardware promise to make the individual host more reliable while new networking technologies push to 40Gb, 100Gb and beyond.

We have come full circle. Virtualisation started on the mainframe, and virtualisation is driving x86 to adopt technologies that bring it closer behaving like a mainframe.

Regardless of the similarities, there remains a fundamental difference between a mainframe and a cluster of x86 virtual hosts.

The mainframe is designed to be a single entity. Rack after rack, node after node, everything from the operating system to the interconnects binding individual nodes together, treats the mainframe as a single gigantic computer that is then sliced up for individual tasks.

x86 virtualisation, on the other hand, is a kludge

An x86 virtual cluster is very different. Whether direct attached, distributed or hybrid, each processing node is very much a distinct unit. Each node matters: it must be configured, licensed and designed separately as well as with consideration to the whole.

A mainframe is an expensive computer that you custom-design software for: a high-performance system worth high-quality development. The x86 virtual cluster is a collection of cheap systems that you wrap around existing software.

A mainframe is what you build when you are running a financial system where milliseconds of latency can mean millions of dollars. It shines when you feed it applications that can break work down into small chunks and lots of small tasks in parallel.

x86 virtualisation, on the other hand, is a kludge.

It is our way of compensating for the fact that we are dragging around decades worth of software that is remarkably single-threaded, not very environmentally aware, and which needs to be insulated from other programs running on the same system.

x86 virtualisation models will continue to evolve because of this need to accommodate the sheer diversity of x86 applications.

There are many options available today to accomplish large amounts of computing efficiently. You can buy a mainframe or maintain a fleet of x86 systems with applications installed on the bare metal.

You can venture into x86 virtualisation and explore all the myriad different possibilities it presents. You could even lash together a few thousand cell phones into an incredibly awkward Beowulf cluster if you so chose.

There is no "one true way" to get the job done. The needs of your software and the capabilities of the hardware available to you will determine the implementation paths you can choose. ®

Trevor Pott is a sysadmin from Edmonton, Canada.

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