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Fedora Core 5 Xen Virtualization Guide
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Troubleshooting

If you're running into issues with your Xen installation there are two log files on the host system to help you out. The file /var/log/xend.log holds the same information as you receive when running 'xm log', and contains basic information. Unfortunately these log messages are often very short and contain little useful information. The following is the output of trying to create a domain running the kernel for NetBSD/xen.

  • [2005-06-27 02:23:02 xend] ERROR (SrvBase:163) op=create: Error creating domain:(0, 'Error')
    Traceback (most recent call last):
      File "/usr/lib/python2.4/site-packages/xen/xend/server/SrvBase.py", line 107, in _perform
        val = op_method(op, req)
      File "/usr/lib/python2.4/site-packages/xen/xend/server/SrvDomainDir.py", line 71, in op_create
        raise XendError("Error creating domain: " + str(ex))
    XendError: Error creating domain: (0, 'Error')
    

The second file, /var/log/xend-debug.log usually contains much more detailed information. Trying to start the NetBSD/xen kernel will result in the following log output:

  • ERROR: Will only load images built for Xen v3.0
    ERROR: Actually saw: 'GUEST_OS=netbsd,GUEST_VER=2.0,XEN_VER=2.0,LOADER=generic,BSD_SYMTAB'
    ERROR: Error constructing guest OS
    

If you are reporting errors or need help with your Xen setup you should always include both the xend.log and xend-debug.log messages that you receive.

Serial Console

For more difficult problems, serial console can be very helpful. If the Xen kernel itself has died and the hypervisor has generated an error, there is no way to record the error persistently on the local host. Serial console lets you capture it on a remote host.

You need to set up the Xen host for serial console output, and set up a remote host to capture it. For the console output, you need to set appropriate options in /etc/grub.conf, for example:

  • title Fedora Core (2.6.16-1.2080_FC5xen0)
            root (hd0,2)
            kernel /xen.gz-2.6.16-1.2080_FC5 com1=38400,8n1 sync_console
            module /vmlinuz-2.6.16-1.2080_FC5xen0 ro root=LABEL=/ rhgb quiet console=ttyS0 console=tty pnpacpi=off
            module /initrd-2.6.16-1.2080_FC5xen0.img
    

for a 38400-bps serial console on com1 (ie. /dev/ttyS0 on Linux.) The "sync_console" works around a problem that can cause hangs with asynchronous hypervisor console output, and the "pnpacpi=off" works around a problem that breaks input on serial console. "console=ttyS0 console=tty" means that kernel errors get logged both on the normal VGA console and on serial console. Once that is done, you can install and set up ttywatch (from fedora-extras) to capture the information on a remote host connected by a standard null-modem cable.

Hints

Q: I am trying to start then xend service and nothing happens, then when I do a `xm list1 I get the following:

  • Error: Error connecting to xend: Connection refused.  Is xend running?
    

Alternatively, I run xend start manually and get the following error:

  • ERROR: Could not obtain handle on privileged command interface (2 = No such file or directory)
    Traceback (most recent call last):
      File "/usr/sbin/xend", line 33, in ?
        from xen.xend.server import SrvDaemon
      File "/usr/lib/python2.4/site-packages/xen/xend/server/SrvDaemon.py", line 21, in ?
        import relocate
      File "/usr/lib/python2.4/site-packages/xen/xend/server/relocate.py", line 26, in ?
        from xen.xend import XendDomain
      File "/usr/lib/python2.4/site-packages/xen/xend/XendDomain.py", line 33, in ?
        import XendDomainInfo
      File "/usr/lib/python2.4/site-packages/xen/xend/XendDomainInfo.py", line 37, in ?
        import image
      File "/usr/lib/python2.4/site-packages/xen/xend/image.py", line 30, in ?
        xc = xen.lowlevel.xc.xc()
    RuntimeError: (2, 'No such file or directory')
    

A: You have rebooted your host into a kernel that is not a xen-hypervisor kernel. Yes I did this myself in testing :)

You either need to select the xen-hypervisor kernel at boot time or set the xen-hypervisor kernel as default in your grub.conf file.

Getting Help

If the Troubleshooting section above does not help you to solve your problem, check the [WWW] Red Hat Bugzilla for existing bug reports on Xen in FC5. The product is "Fedora Core", and the component is "kernel" for bugs related to the xen0 kernel and "xen" for bugs related to the tools. These reports contain useful advice from fellow xen testers and often describe work-arounds.

For general Xen issues and useful information check the Xen project [WWW] documentation, and [WWW] mailing list archives.

Finally, discussion on Fedora Xen support issues occur on the [WWW] Fedora Xen mailing list

Footnotes

  • 1This is required because you actually boot the Xen hypervisor and it then starts the Linux kernel. It does this using the MultiBoot standard.
  • 2

    Note that you can change so that future kernel-xen0 packages are the default kernel by editing /etc/sysconfig/kernel

  • 3

    To check that you are running a xen0 kernel use 'uname -r' - the name of the kernel should contain xen0

  • 4

    This information can all be passed as command line options; run with an argument of --help for more details

Fedora Core 5 Xen Virtualization Guide
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  Published under the terms of the GNU General Public License Design by Interspire