1 Vital questions you must ask before you commit your company to Cloud Servers Turing House, Archway, Manchester, M15 5RL tel: fax: e:
2 First, the benefits Cloud Servers and cloud computing are changing the way we think about hosting high-performance and high-availability applications. The technology uses virtualisation techniques to abstract the underlying hardware from the end-product, i.e. the server itself. In theory this means: Higher availability as there is no single point of failure (SPF) associated with any particular hardware; Lower costs, as an entire physical node is not dedicated to one server task any more, i.e. multiple Cloud Servers can run concurrently on one physical node; Lower energy consumption per server, for the same reasons as above; Quicker lead times on server upgrades as virtual hardware can be scaled up or down instantly or near-instantly. How is a Cloud Server different from shared hosting? It may seem that Cloud Servers are an evolution of low-end shared hosting, where multiple websites live on the same server. However this couldn t be further from the truth. True virtualisation enables each virtual machine to live concurrently on the same physical node, with its own operating system, allocated memory and disk space, and share of CPU resources. Cloud Servers co-existing on the same hardware are completely autonomous and private. So what are the dangers? As ever, with this new technology, the devil is in the detail. We ve identified five major dangers you need to check before choosing your Cloud Server hosting service. 1. The danger of hardware failure One of the benefits of virtualisation should be an improvement of service availability, so it s important that the topology of the cluster is well thought out, such that a failure of a major central component such as a network switch, or even just the physical node (server) that your Cloud Server is running on, doesn t cause your Cloud Server to go offline. Use SAN Storage The best solution uses centralised SAN storage, so that if the physical node (server) you re running on fails, your Cloud Server will automatically re-start on a spare node. An added benefit of this is that your hosting service can take nodes out of service for maintenance without affecting your Cloud Server. That means you re truly available for the maximum amount of time possible. Look for an option of fast disks (i.e. 15,000rpm SAS disks) to keep the I/O throughput of your Cloud Server tip-top.
3 Use Hardware Resilience Nodes should be dual-uplinked through two switches, to both the frontend (i.e. the internet) and also the backend (i.e. to the storage) such that the failure of a network switch or uplink cable (or just someone accidentally un-plugging it) does not stop your Cloud Server from running, or being accessible. The centralised SAN storage should be built with resilient/failover power supplies and controllers, such that the failure of any component within the SAN does not take offline any Cloud Server. A decent service provider will offer the option of offsite backup space to be mounted to your Cloud Server to give you a self-standing off-site copy of your important data. 2. The danger of interference from other customers Cloud Servers There are several ways in which one Cloud Server can interfere with another, when they re running on the same node. However, when security is properly implemented, this risk is completely mitigated, and the solution is just as secure as a dedicated server. A responsible host will have thought out these issues and put safeguards in place to ensure that one Cloud Server cannot impact upon another. Here are some questions to ask your potential Cloud Server provider: Will I have my own VLAN or are all Cloud Servers on the same network segment? This is important, as Cloud Servers on the same VLAN are on the same broadcast domain. This means if someone enters your IP address in their network configuration incorrectly, they could take your Cloud Server offline. It also means you d receive their broadcast traffic. Finally, it means that there is complete lack of firewalling between your Cloud Server and others, unless a software firewall is put in-place (see the next point for more on this). Will I have the protection of a hardware firewall? With a separate VLAN, it s possible to protect a Cloud Server in the same way as you would a dedicated server. This means that not only traffic from the outside world, but also to other neighbouring customers, is allowed through only if it meets the firewall rules you ve set. Are the kernels shared with the host operating system? Slightly more complex this one. Basically some virtualisation technologies share the host node s base operating system kernel with the Cloud Servers for various reasons usually so the provider can put more Cloud Servers on a node. However this means that a compromised kernel on a Cloud Server could effectively take down the host node, which basically means all Cloud Servers running on the node will be taken down too. 3. The danger of not having a Service Level Agreement (SLA) It s one thing to say that a service is reliable, but quite another to put a money-back guarantee or Service Level Agreement (SLA) on it. Obviously what you want is a working service, not a money-back guarantee, but it certainly puts an emphasis on the provider having a standard to work towards. Many providers say SLAs are a waste of time, and marketing hype. If they truly believed this, surely they d offer an SLA anyway as they have nothing to lose?
4 Some Cloud Server products come without an SLA; some even charge you extra for an SLA. In short, any decent high-availability Cloud Server offering should have an SLA on both the network connectivity, but more importantly the availability of the virtual hardware of your Cloud Server. A decent SLA should provide a realistic and achievable service level (100% SLAs are usually vanity only, or exclude so much that they re almost pointless) and a penalty for the host if the SLA is not achieved. It should also be clear what the SLA covers. For example, a lot of network/connectivity SLAs cover only the provider s own network in their data centre, and not their upstream providers. That means your server may be inaccessible but if it was due to their provider and not their own internal network, you re not covered. 4. The danger of over-contention There s a potential for over-selling with Cloud Servers, as the provider may not tell you how many virtual machines they intend running on one physical node, whether memory and CPU time are contended, or how fast the physical node is uplinked to the network. The whole point is to put limitations in-place to stop one Cloud Server from hogging the resources at a performance cost to neighbouring Cloud Servers. A responsible provider uses a virtualisation technology that does not allow memory or disk space to be contended, and has fixed parameters in place to ensure CPU and network resources are fairly shared out. This includes setting an upper limit to the number of virtual machines that can run on a physical node, and also ensuring the physical nodes are uplinked to the net at a suitable speed such that every server gets a decent sized connection. For example a gigabit connection shared between 30 virtual machines gives approximately 30Mbps to each virtual machine. 5. The danger of not having tools to help yourself in the case of emergency. You would generally want KVM over IP and remote reboot facilities on a dedicated server, to give you sat in front of the machine access in the case of a major OS failure, or to correct network settings when the machine is otherwise inaccessible. The same should apply to a Cloud Server. This technology can be a life-saver when you need to work on a Cloud Server at 2am without waiting for your provider to respond to help you out. It gives you complete self-sufficiency.
5 Conclusion There s plenty of scope for Cloud Server providers to cut corners. That said, if you research your provider well, using the above questions as part of your decision-making process, you re likely to find a service that s a high-availability and high-performance alternative to dedicated servers. To sum up, make sure that any Cloud Server provider can meet the following criteria and that you ve done your utmost to mitigate the dangers we ve described: 1. Uses SAN Storage. 2. SAN and host servers have redundant critical components. 3. Nodes dual-uplinked through two switches (front-end and backend). 4. Provides you with your own VLAN. 5. Provides a hardware firewall. 6. Does not practice kernel sharing between host node and virtual machines. 7. Has a reasonable (and realistic) SLA. 8. Provides console access with reboot facility. 9. Has support that is responsive and knowledgeable. 10. Knows how to look after customers. We re different. In a really over-crowded industry, we feel that what sets Melbourne apart from our competitors, is the fact we care about our customers, we look after our staff, and we provide an incredibly high level of service at an affordable price. Hear it for yourself. We ve commissioned some short videos featuring some of our customers talking about their experiences with Melbourne. Watch them now at To learn more, give us a call on: Turing House, Archway, Manchester, M15 5RL tel: fax: e:
It s like a dedicated server. But different. Turing House, Archway, Manchester, M15 5RL tel: 0161 232 0001 fax: 0161 232 9125 e: email@example.com www.melbourne.co.uk Meet UltraVM. It s quite different.
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