Hi all, hope you are doing fine. The following discussion emerged from PR #5297[¹].
It is a known fact that, regarding KVM functionalities, CentOS7 default's QEMU binary is quite limited. This is due to the removal of some features of KVM default binary in CentOS (like VM's volume live migration, memory/CPU hotplug/hot-unplug, live disk snapshot, and so on); more information can be found in CentOS forum's thread[²]. In my point of view, such limitations in the default QEMU binary in CentOS make it unfeasible to build a cloud with CentOS and the default QEMU binary, as operators lose a lot of useful/important operations or have to go through workarounds, which cause VM's disruption (e.g. having to stop a VM to migrate the volume between different storage pools, which triggers a secondary storage usage). There is an alternative binary, "qemu-kvm-ev", which supports more features than the default one. Probably, most people using KVM with CentOS are using the "ev" binary (I might be wrong though, however, that seems to be the case when looking at the users' list). PR #5297[¹] ran into one of the CentOS7 default's QEMU binary limitations (live disk snapshot). The easiest solution (and, IHMO, is the best option) is to guide users to upgrade CentOS7 QEMU binary to "qemu-kvm-ev". Further, it is important to mention that in our experience, it is not possible to run a highly available cloud environment with CentOS7 and the default binaries. In a cloud environment with thousands of VMs, sooner or later the need to hotplug (increase) CPU/RAM, migrate volumes across different storage pool tyles (such as iSCSI <> NFS), or some other type of operation appear, and operators/final customers do not want service disruption. Our customers, for instance, never shut down VMs for these kinds of operations, and that is only possible because they are all using KVM with Ubuntu now. Moreover, CentOS7 is getting close to its EOL. Therefore, We do not think that CloudStack should limit its features due to a dying operating system that presents very limited features by default. With that said, it would be interesting if dev/users that use CentOS7 could share their experiences with the "qemu-kvm-ev" in this thread, so we can decide which way to go. Or, users that only use the default binary, if they are satisfied with it. If almost no one is relying on default CentOS7 binaries, we could define as a step in the documentation, that when using CentOS7 people must use the "ev" binary. This would free us to evolve ACS more freely and avoid headaches with workarounds to a limited operating system when there are alternatives out there. [¹] https://github.com/apache/cloudstack/pull/5297 [²] https://forums.centos.org/viewtopic.php?t=65618
