Switching from PV to HVM for a quick cd boot

Today I needed to boot a rescue system. Obviously all my virtual machines are running para-virtualized to allow live migration. So how can we switch a PV machine to HVM to allow CD booting?

First you will need to add a storage repository which hosts your ISO files. I just added a CIFS share and put my ISO onto it. Then use xe vm-list to determine the VM's UUID. Determine the name of the CD via xe cd-list. Attach the CD to the VM:

xe vm-cd-add vm=<vm_uuid> cd ...
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Interval coalescence in SQL

Recently I needed to coalescence intervals in SQL. There are already existing solutions to this problem, but for my taste neither of them was really nice or easy to understand (At least none of them had good explanations). So here is my solution for this problem -- and hopefully explained well enough.

Assume you have a database table like this (x is the start time and y the end time):

t x y
a 10:00 11:00
a 11:00 12:00
a 14:00 16:00
b 12:00 ...
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Moving existing Xen machines into XenServer

My current Xen machines use one logical volume per disk, for XenServer I need those in VDI files/volumes. This is how I moved them onto XenServer:

  • Prepare the new machine via XenCenter or use xe vm-install.

  • Determine the uuid of the new VDI via xe vdi-list and write it down.

  • Create a new VBD for the control domain:

    xe vbd-create device=0 vm-uuid=uuid_of_xen_control_domain vdi-uuid=uuid_of_vdi bootable=false mode=RW type=Disk
  • Attach it to the Dom0 via xe vbd-plug.

  • Use fdisk to create a partition table and filesystem ...

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DRBD and crappy RAIDS

Since my last blog post I had quite some fun with crappy DRBD performance. Long story short, it looks like as if I can't get a decent random write speed out of DRBD without disabling disk flushes and barriers. Oddly enough the latency tests on the backend device were great, so either something is entirely wrong with my DRBD config or the 3ware raid cards are as broken as many people say. Usually one shouldn't risk running a DRBD raid using drain without BBUs, but since my machines ...

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Installing XenServer via PXE and building DRBD

As promised here is the next part of my Xen series.

I had to setup the servers via PXE since I didn't order CD drives (on purpose since I didn't play much with PXE yet). I installed the TFTP server following the docs provided by Citrix and added the following line to our dnsmasq config:


This instructs PXE clients to load the pxelinux.0 from via TFTP.

The installation of XenServer itself was straight forward then ...

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Server infrastructure for small companies

I work for a small company with six employees during the summer holidays. This means that money for many servers is a no-go, but we still need servers for some internal stuff (internet gateway, some Django apps and test systems). As every other company we would like a reliable service, outsourcing is no option with our requirements.

Currently our services run on an old dual core Intel Xeon with one GB ram. We use Xen for virtualization, but the machine is near at it's limit. Also it's just ...

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