Frequently-asked questions about virtual machines
What’s different or special about User-mode Linux servers compared to a normal Linux server?
As far as the user and system administrator is concerned, nothing: the same binaries, filesystems and conventions apply, so any Linux application will run, and our distributions do not need any special treatment.
Our kernels do not allow modules to be inserted, but standard kernel tools such as iptables (for packet filtering), tc (for bandwidth throttling and QoS) will work, as will any tweaks you want to make to kernel parameters in /proc.
Once I’ve signed up for a plan, can I easily change upgrade my memory, add IP addresses or disc space?
Yes, upgrading only takes a few minutes and only needs a reboot
As a pre-requisite for an, upgrade we may need to move your virtual machine to another host that has the resources you require, but this takes only 30-60 minutes of down time which can be scheduled at your convenience.
Can I host more than one domain on your servers? Do I need an IP address for each one?
There is no technical reason to stop you hosting as many domains as you like on your Virtual Machine, and we do not limit this artificially either. The only reason you may need another IP to host more sites is if you need more than one SSL-enabled web site. Otherwise the usual web and mail servers can be configured with as many domains as you need.
What performance can I expect from your Virtual Machines? Will it run my favourite application in 150MB of memory?
Performance on a virtual machine can vary, even while it is idle, because the hardware is being shared between (typically) 40-50 users. We try to keep the loads on our host below about 4.0 which normally gives excellent response times and CPU availability, but there is no option for “more CPU time”, only more memory.
The more memory your system has, the less the system has to read from its discs, and so the faster your application goes. In general we find that Rails, Zope, Java and other “heavyweight” application frameworks tend to demand enough that a 150MB machine could only be used for prototyping rather than much production work – however common PHP, CGI scripts and email applications will sit absolutely fine in a 150MB server, even at higher loads.
If your VM isn’t performing well for a particular task, we can move your Virtual Machine to a host where it is less loaded. Since There’s no technical solution to guaranteeing processor time, we prefer to take a human approach to the problem to make sure everyone on a particular host is happy.
Will a Virtual Machine be able to handle a “Slashdotting”?
(referring to the effect of a very popular web site publishing a link to your site, causing a sudden spike in load to the site in question as thousands of visitors click the same link at once)
If your software is written with some efficiency in mind, or if you’re just serving static pages and pictures. If you’ve written a set of PHP pages which open a database connection for every page fetch, no server on earth will stand up to the load generated by inefficient programming!
How many Virtual Machines do you run on each host?
Around 20-25 per CPU ensures the performance and doesn’t overload the RAM in each server. We have some servers with one processor, and some with two, so you may be sharing a server with up to 60 people. However we always monitor the load on each host machine and take action if we see the performance drop overall.
Can my Virtual Machine be “swapped out”?
No, the RAM you pay for is the RAM you get! It is never swapped out to disc. Your VM does of course have swap of its own, but this is under your control.
Why should I expect better performance from a VM with more memory?
The main point of contention on our host machines is not generally the processor, but the speed of disc access. If you have more memory, your kernel can cache more and run more quickly without having to use its swap space or read from the disc as often. If you’re unsure as to whether an upgrade would help you solve a performance problem on your VM we are always happy to let you try a higher-spec machine for free for a few days.
What hardware do you use to run Virtual Machines?
The CPUs that we use vary as time goes by. We started using Pentium 4 chips with 2GB of RAM , and have gone via dual Xeon boxes to dual Opteron boxes with 4G. Hotswap RAID arrays are common to all our machines—we use 3Ware/AMCC RAID cards to provide reliable hot-swap capability for high uptimes.
Can I upgrade just my memory and not pay for the extra disc/bandwidth?
Sorry, no. The memory is the main component of the cost.
Anything else…?
If you can’t find the answer to your question here, feel free to contact us