Tag Archives: libvirt

KVM instances dying at boot

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I recently encountered a crashing KVM instance, where my VM would die at boot once the bootloader tried to unpack initrd.

A check of the log in /var/log/libvirt/qemu/vmname.log showed the following unhelpful line:

Guest moved used index from 6 to 229382013-04-21 \
06:10:36.029+0000: shutting down

The actual cause of this weird error and crash occurs when the host OS lacks disk space on the host server’s filesystems. In my particular case, my filesystem was at 96% full, so whilst the root user could write to disk, the non-root processes including Libvirt/KVM were refused writes.

I’m not totally sure why the error happens, all my VM disks are based on LVM volumes rather than the host root filesystem, I suspect the host OS disk is being used for a temporary file such as unpacking initrd and this need for a small amount of disk leads to this failure.

If you’re having this problem, check your disk space and add some Nagios alerting to avoid a repeat issue!

KVM/libvirt change CDROM

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I was setting up some Windows virtual machines this evening on my Linux KVM/libvirt server, in order to experiment with how Windows handles IPv6 networks.

Installing windows was easy enough – standard virt-install commands, however post-reboot, Windows XP wants to access the CDROM again.

However the reboot causes the CDROM ISO to be unattached from the virtual CDROM drive – so it’s necessary to re-add it to continue installation

However the logical syntax based on virsh help, doesn’t work:

virsh # attach-disk devel-winxp1 /tmp/winxp.iso hdc
error: Failed to attach disk
error: this function is not supported by the connection driver: disk bus 'ide' cannot be hotplugged.

The correct syntax is:

virsh # attach-disk devel-winxp1 /tmp/winxp.iso hdc --type cdrom --mode readonly 
Disk attached successfully

Basically you need to tell libvirt that you’re attaching a *cdrom* and not an actual disk – I’m not sure why it doesn’t just figure that out, based on the fact the user is trying to obviously attach an ISO to a virtual optical drive device – maybe nobody has gotten around to implementing a nice autodetect method yet…