I have a Xen-based virtual private server, running Ubuntu. It is running Apache, MySQL, and Drupal.
Usually, it is nice and fast. However, sometimes it slows down for no apparent reason. How can I determine whether the slowdowns are due to resource usage by other VPSes on the same physical hardware, as opposed to slowdowns due to things I can control?
If you can't get this information from your provider then you'll have to figure it out empirically.
Something like this, run bonnie, or a big dd in one screen, then run vmstat or iostat in another, if you have exclusive use of the machines IO then the numbers should stay pretty constant.
You could try running iostat -x on your machine (without load) and look at the service time, unless you are spanking your machine, the service time should stay low. If it starts to head up, then there is contention for the physical IO bandwidth.