When looking for a VPS to buy, the sheer number of product characteristics and options may look daunting. This guide is written to a potential customer looking for advice on what VPS plan to go with. You may also wish to fill in the VPS Finder survey to narrow down your number of choices.
System Resources
Different types of websites requires different applications. Different applications require different amounts of system resources. System resources cost money and unless you like wasting money, you need to figure out how much resources you'll need.
If you are short on cash, keep in mind that it always is much smarter to buy a to small VPS, than a to large one. Most suppliers will be delighted to upgrade your hosting plan to a more expensive one, but not so happy when downgrading you to a cheaper one.
RAM
The most scarce system resource of any VPS is RAM memory. It is likely also the most important contributor to your VPS performance. Most operating systems perform page caching so that data read from the disk is temporarily stored in memory which significantly reduces the number of times the disk is accessed. Because the disk access time is likely to be your bottleneck, extra RAM can have a huge impact on performance. It may also pay off to ensure that your vps' disks are of as high quality as possible.
For a small WordPress blog, a mere 128 MB RAM may be enough. Somewhat more if you are using Drupal. One gigabyte or more if you are deploying a large J2EE web site.
RAM is the most expensive resource, so get ready to pay premium prices if you need lots of it.
Bandwidth
Bandwidth is the next most important parameter to take into account. Hosting providers denote their bandwidth ratios as a number of gigabytes transfer per month. If a VPS crosses the limit, it risks being limited or even disabled.
Usually, your VPS will have more than enough bandwidth unless you are streaming video, hosting large files or are doing something very abusive. For example, a ratio as low as 50 GB per month would be good for over 17 thousand page hits per day if a page hit averages at 100 kB.
Be vary of providers advertising "unlimited bandwidth." Most of the time, there is still some limit hidden in there, or the provider may just plain not accept that your VPS crosses some unknown bandwidth threshold.
Disk Space
As with bandwidth, disk space is almost never a problem. The exception is if you are hosting large files. But performance may wary depending on what type of disks the hosting provider is using.
It is much harder to compare "disk quality" among hosts because it is not just a simple number such as X giga or megabytes as with RAM memory, you just have to know which disk systems are fast and which are slower. Good providers usually use some form of RAID setups which increases the bandwidth and robustness of the system.
Location
Generally, you want your VPS to be physically located as close to your target market as possible. The closer the VPS are, the less time it will take to load a web page from it. The difference may be up to a few hundred milliseconds if the client is located in Europe and the server in North America or vice versa.
Although price gap is closing, VPS plans are still quite a bit cheaper in the United States than in other parts of the world.
Virtualization Technologies
When talking about virtualization, the term host operating system is used to describe what operating system is running on the server that implements the VPS. On the host operating system, virtualization software is executing which enables multiple guest operating systems to be running concurrently.
The performance characteristic of a VPS can vary widely depending on what type of Virtualization Technology the hosting provider is using for implementing resource sharing. Each technology has its advantages and disadvantages and people who prefer one over the other.
The following lists the most common VPS hosting software:
KVM
KVM stands for Kernel-based Virtual Machine and is a Linux-specific virtualization technology. It is fairly new technology and very uncommon among VPS providers.
OpenVZ
OpenVZ is a virtualization technology working on the operating system level. Therefore it is not a real virtualization technology and VPS instances aren't as "separate" as with other technologies. In that respect it is more similar to FreeBSD jails. Because of that, VPS instances in OpenVZ are limited to Linux distributions, so running a Windows VPS won't work, for instance.
Because the instances aren't separate virtual machines, OpenVZ may be faster, CPU wise than other alternatives. OpenVZ installations often also allow their users to use "burst memory" which means that a VPS may use more memory than what it is guaranteed for short amounts of time if other VPS:es on the same server uses less than their full allocation.
But there are also disadvantages with the shared approach that OpenVZ uses. For example, if one VPS instance is accessing the disk to much, other instances on the same server may suffer slowdowns. You are also limited in how you can modify the VPS environment. For example, there are certain kernel parameters that the hosting provider has set that you cannot reconfigure.
OpenVZ is open source but there is also Parallels Virtuozzo Containers which is a commercial virtualization solution offered by Parallels built on OpenVZ. Parallels is also sponsoring the work on OpenVZ.
Due to the way OpenVZ keeps track of system resources, memory usage for applications on OpenVZ are slightly larger than in other systems. Mainly because OpenVZ counts allocated memory in addition to used memory.
Virtuozzo
Parallels Virtuozzo Containers is the commercial variant of OpenVZ. The differences from a VPS users point of view are negligible.
Xen
Xen is a virtual machine monitor which can be used to construct virtual environments (VE:s). It enables several guest operating systems to run on the same computer hardware concurrently. In contrast to KVM, Xen is not limited to Linux as the client and it can run any operating system unmodified as a client. However, a Unix-like operating system has to be used as the host.
In constrast to OpenVZ, Xen shares almost no system resources, so OpenVZ-specific VPS features such as "burst memory" are not available and it has a significantly higher CPU overhead. On the other hand, it is much harder for Xen guest operating systems to disturb each other than in OpenVZ.
For more information about Xen see xen.org.
References
Other Considerations
Miscallenious things to consider when purchasing a VPS plan.
Site Content
Many hosting companies refuse to host sites containing pornographic content, gambling, drug paraphernalia other types of content they deem inappropriate. Such restrictions may or may not be documented in the providers Terms of Service. Therefore, to save yourself much grief, it is always better to ask upfront whether your site is acceptable than having it disabled at a later time.
Backup
All sites need some form of backup. It can be handled either by the hosting provider or the VPS operator. If the plan includes backup, it means that the provider periodically takes a snapshot of your VPS state information so that you can rollback to an earlier version in case of a failure.
Even if a plan does not come with backup included, you can often buy it as an addon-service from the operator.