[4.2.1] KVM/QEMU Snapshots crashing the virtual machines

[Jeff Hair <je...@greenqloud.com>]

Hi,

We are using KVM (kernel version 3.12) and QEMU 2.0.0. We can execute
snapshots of machines that have only a root volume. But when adding
extra volumes, things get weird. When one volume is attached, making a
snapshot of the volume fails every other time. Adding more volumes
causes the machine to start crashing as a result of the snapshot
process.

We have traced the CloudStack code execution down to
KVMStoreProcessor's createSnapshot method:

https://github.com/apache/cloudstack/blob/4.2/plugins/hypervisors/kvm/src/com/cloud/hypervisor/kvm/storage/KVMStorageProcessor.java#L1067

When this runs, we see an "Error while creating snapshot" in the
libvirt logs, and not much else. Increasing the verbosity of the
libvirt logs doesn't really help either. We did notice that it gets an
invalid pointer error at the time it crashes, and we also noticed an
error like this:

> kernel: Clocksource tsc unstable (delta = xxxxx ms)

every time it creates a snapshot. But we're pretty sure that has
nothing to do with it and is just a warning.

Anyone have ideas on this?