Archive for August, 2010

I had a problem installing the tun/tap adapter for openVPN on my windows 7 machines. Windows complains about the driver not being digitally signed. I don’t care about this so I switched it off by doing this:

Hit your windows key and type cmd in the ‘Search Programs and Files Box’, now instead of just hitting ENTER, use CTRL+SHIFT ENTER to run as Administrator. Type the following into the cmd prompt and reboot.

bcdedit.exe -set loadoptions DDISABLE_INTEGRITY_CHECKS
bcdedit.exe -set TESTSIGNING ON



Performing many tasks can take some time, and we know that XenServer can deal with a few of these operations at once. I had some issues, but finally have a one-liner that is suitable (using xargs again :-))

xe vm-list is-control-domain=false power-state=running --minimal | 
    tr -d [:cntrl:] | 
    xargs -d, -n1 -P5 -I '{}' xe vm-param-list uuid='{}'

You notice that I strip all control characters out with tr. This is to get rid of a strange line break that xargs will process even when running -r.

Everybody wants servers now!
Now that servers are virtual, everybody knows you can click a button and give them what they want!
Before you know it, you’ll have tripled your server count!

How can you make sure that things won’t fail in a massive way?
Physically plan your virtual setup!

Think about this, you’ve got sixteen machines to create a pool of hypervisors. They’ve all got two, quad-core processors and thirty-two gigabytes of RAM.

You only have one switch per rack, and you plan to use four racks (four hypervisors in each rack). This gives you the possibility to recover from a rack failure (most likely switch failure) as long as you keep the pool on seventy-five percent loaded on CPU, RAM, Network and Storage.


You must also think about maximum VM sizing. If you size a single VM over twenty-five percent of the capacity of one hypervisor, then you risk not being able to migrate all machines from the four hypervisors that are allowed to fail.