Subversion: Setting the repository path for svn+ssh://server/repos

For some reason, it doesn’t seem to be possible to set up a path for your repository when using subversion as: svn+ssh://server/repos out of the box. So you end up having to specify svn+ssh://server/some/path/repos.

But it really isn’t that hard to do. Here is how I do it. Beware that you’re going to be making system-wide changes to how svnserve operates. The basic idea is to replace svnserve with our own version, that calls the original one with a -r parameter.

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Classical Mechanics: Why does Power depend on frame of reference?

A rocket

Imagine we’re in a rocket.

We have an imaginary engine, that provides a constant amount of thrust. How much power does such an engine provide? In essence the power depends linearly on the velocity:

And the velocity depends (by definition) on the frame of reference.

So imagine that we started from a planet a while back. Our thrust is constant (remember?) so our acceleration is constant, and so velocity grows linearly.

Does that make sense?

If this was a classical rocket engine (and we can somehow neglect that our rocket looses mass), then the Power would continue to grow. At some point, the power will exceed the power present in the fuel (power = energy/time). That doesn’t make sense to me.

Also, lets say we hook up with another rocket that has the same velocity as us. After having drinks and dinner together, we start up our engine and head off. Depending on our frame of reference, we either have enormous power (if we keep our original frame of reference – the planet we started from) or very little power (if we use the other rocket as a frame of reference). That also makes no sense.

Our assumption was that we have an imaginary engine that provides a constant amount of thrust. When faced with conclusions that make no sense, it is customary to question the assumptions. Our imaginary engine.

So, have I then proved that it is impossible to create such an imaginary engine? Really?

Related to the above, we have the definition of kinetic energy:

Here again, we have that the energy depends on the frame of reference. Energy seems such a crucial cornerstone of physics, that I don’t understand how it can depend on the frame of reference. When e.g. the energy content of fuel or batteries or whatever are absolutes values.

I’ve been toying with this for years, never taking it seriously, but I am perplexed. I was just trying to explore the characteristics that such an imaginary engine would have and I don’t know what to make of it. Do you?

Deep Packet Inspection? Yup, bring it on!

Deep packet inspection could be useful to ISPs who oversell limited bandwidth to (too) many customers. With Deep Packet Inspection, my ISP could inspect the contents and details of the high-bandwith traffic going to and from my neighbor:

Our ISP could then determine most of it to be peer-2-peer traffic, and then limit exactly that peer-2-peer traffic so that my neighbor ( 🙂 ) doesn’t hog all the bandwidth that we all have to share. On the surface, that sounds like a great idea? Why should my neighbor ruin the internet experience for the rest of us? There’s a catch…

Interestingly, ISPs aren’t going to tell customers exactly what traffic will be limited / throttled, they’ll just go ahead and do it. Today peer-2-peer will be throttled. Tomorrow it’ll be large HTTP downloads and by the way, your ISP might decide that since they also sell their own Voice over IP solution, they’ll throttle Skype so it isn’t usable, and you’ll have to buy their Voice over IP solution. And so on. This is Net Neutrality – we don’t want ISPs selectively throttling certain traffic, because we don’t trust that they’ll have our interests at heart when they do it.

If ISPs do want to do it, they can always do it based on what servers we communicate with. We can’t stop them doing that.

They can also base it on what TCP and UDP ports we use. That is useful, but can be circumvented by using non-standard ports. And some protocols don’t even use standard ports. Deep Packet Inspection would come into play if the ISP wanted to identify even though it was using non-standard ports. They could look at the traffic itself and determine what “it looks like” and base their throttling on that.

And that would work today.

My prediction is, though, that if Deep Packet Inspection were to become common, inevitably people would circumvent that e.g. with encryption. Deep Packet Inspection won’t work on encrypted traffic (because by definition, encrypted traffic can’t be inspected).

So I come to a conclusion:

If Deep Packet Inspection becomes widespread, the consequence will be that encryption becomes equally widespread, making the internet a safer place for all of us!

Deep Packet Inspection leads to Encryption which would also make it impossible for the government to snoop anybody’s emails and have many other benefits. Wonderful. So bring it on!


Now, do I want to run Xen? Hmm… I’m using it to host itself, and I think its great.

Compared to VMware workstation, which is my other favorite, I love that it is open source, and I love that it is detachable. That is, I can start it and don’t have any GUI artifacts hanging around. What I do with VMware workstation is start it under a VNC server, so I don’t have to worry about the GUI, but this isn’t about VMware but about Xen. [I’ve since learned that this isn’t true about VMware workstation – it has become detachable]

I also love Ubuntu, and it seems Xen dom0 is not supported on Ubuntu. As that thread shows, I finally did git Xen running on Ubuntu by following this howto.

But to really get it working, I ran into these problems too:

Other than that, I think my Xen on Ubuntu is running fine.

Now, the way I work is that I’d like to have some virtual machines running all the time, and some for debugging and short term trials. For the former, I’d like to use Xen, but for the latter, I’d like to use VMware workstation (especially because it has multilevel snapshots, and LVM which Xen uses for snapshots, doesn’t support a snapshot of a snapshot.)

But alas, You cannot run VMware products on a xen kernel

So, now, I guess, I have to ask: How much do I really want to run VMware Workstation? Am I prepared to give that up to run Xen instead?

I wish I could have run VMware under a Xen dom0 and use Nvidia graphics drivers! 🙁 VMware workstation for now…