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I upgraded a XenServer version 6.2 host to 7.1 One of the VMs (CentOS 6.9, PV type - paravirtualized) on the host runs Asterisk phone software which uses a module called "DAHDI" which is a set of hardware drivers for telephony purposes.

Before upgrade the 6.2 host, I migrated the VMs off to another 6.2 host. After upgrade to 7.1 migrated back and it seemed fine except that the DAHDI functionality had disappeared.

As it turns out, DAHDI has kernel modules so upon installation it modifies or adds kernel modules.

What seems to happen is that the XenServer host upgrade changes the Linux kernel, which then has the side-effect that the previously installed DAHDI kernel modules disappear.

Original kernel version of this VM:

2.6.32-431.29.2.el6.i686 #1 SMP Tue Sep 9 20:14:52 UTC 2014

Kernel version after host upgrade:

 2.6.32-696.6.3.el6.i686 #1 SMP Wed Jul 12 13:38:41 UTC 2017 

My questions:

  1. Is my conclusion correct that the modified kernel causes this problem?

  2. Is this known behavior and so intended or at least "normal" and to be expected? Any documents I find on Xen or Citrix XenServer leave me with the impression that to upgrade I merely need to migrate the VMs off the server or export/import, with no mention that on PV VMs you might have lost functionality after.

  3. If it is not normal, what do I need to do to prevent it? What does it depend on? I do not necessarily know which VMs may have some sort of special module which will crash upon upgrade.

  4. I am aware that most or all newer Linux versions seem to be implemented as HVM only on XenServer. Is that then the only way to prevent this problem?

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What I found out is the following:

  1. XenServer upgrades do not change the kernel version of guest operating systems. Incompatibilities may exist, in which case a kernel upgrade is required before upgrading the host (and often one finds notes to that effect in the Citrix docs). In this specific case, log files revealed that the kernel version had been changed by another administrator just before the Xen host upgrade. So this was not the reason for the problem.

There were a number of updates and installs on the VM just before the Xen host upgrade (unbeknownst to me). It is apparently possible that these updates would have made DAHDI disappear upon a reboot. The first reboot after the VM installations was at the time of the XenServer upgrade - so it may have just been a coincidence.

In summary, despite not entirely clear why the crash occurred, one should be able to say that PV VM upgrades do not randomly and uncontrollably change Linux kernels and should not be the cause for the problem. So one can feel safe upgrading PV Linux VMs on Xen!

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