Shlrm.org Blag

This is some interesting technology. Apparently it allows me to emulate a switch on my server such that I can get netflow information, and other such managed switch information, from my virtual machines on my xen box. I will have to build and implement this on my test xen box to see how difficult it’ll be to get it to behave.

I also like that it claims it can distribute across multiple hosts (‘claims’ because I haven’t verified it yet.) I wonder how difficult it will be to set up a switch that extends to my router, so I can just emulate my entire network’s switch in OpenVswitch. It’s certainly got some interesting potential.

There’s potential to get better information on the traffic happening in my network thanks to the netflows provided by this. Maybe it will be enough information to figure out why my traffic shaping doesn’t work well, and how I can improve it.

When I got xm pause working, I was quite excited. However, it doesn’t work in all cases. I’ve done some testing to figure out what combination of things cause it to fail.

All of this is using xen 4.0 hypervisor, the kernel from here, and a pvops domU kernel built by hand.

• Using blktap2 backed disks in the vhd format (vhd-util create -n name -s 1024) xm pause fails. The user-space disk devices are never released, and only a hard reboot will get it to function again.
• Using a different driver for the blktap2, tap:tapdisk:aio, also fails to pause.
• Using phy:/ backed disks, say through LVM works fine. The domU will pause, and resume nicely.

It would seem that there is something not quite ready to be used regarding the blktap2 backend, so I wouldn’t reccomend using it.

I also tried to use the original tap:aio drivers, but I cannot as they don’t seem to exist. I guess I’ll be using the plain old phy:/ drivers for now. That’s not a big deal as I don’t have any second xen machine to do anything fancy that the tap:aio drivers would provide. I guess I should poke on a mailing list or something.

…means I can finally pause/suspend a domU vm again!

Finding this information should’ve been obvious, but for some reason it wasn’t. Maybe I had some silly mental block or something. Anyways, get the kernel source from here. Then build it for your architecture. If you want to build a 32-bit kernel for some 32-bit domUs, and you’re running on 64-bit, it’s as easy as make ARCH=i386 <target>. This does work and I’ve verified it. Had to fool a Fedora domU into beleiving that it really was a 32-bit system.

I’ve found make tarbz2-pkg to be quite handy in packaging up the kernel to be deployed.

I have verified that this works in Xen 3.4.2 on a patched 2.6.29.6 kernel.

On my Xen 4.0 test box, it fails to suspend. I’m using the blktap2 stuff, and perhaps that doesn’t pause well. As far as I understand it, that blktap2 should not affect suspending, but I’m going to test it with some logical volume based vms. I usually use the lvm backed disks anyway, but the blktap2 stuff offers some interesting features, if it were to work. Not being able to pause stuff probably isn’t good, but again, I’m not sure what’s causing it.

Apparently, that is what’s causing it. The domU will pause and unpause just fine when using an lvm backed disk, rather than the blktap2 ones. I should test a couple of other combinations to see if things like tap:aio will behave, as that’s needed for nifty things like remus.