Hi, In a couple of situations I have encountered that Virtual Machines running on RBD had a high I/O-wait, nearly 100%, on their vdX (VirtIO) or sdX (Virtio-SCSI) devices while they were performing CPU intensive tasks.
These servers would be running a very CPU intensive application while *not* doing that many disk I/O. I however noticed that the I/O-wait of the disk(s) in the VM went up to 100%. This VM is CPU limited by Libvirt by putting that KVM process in it's own cgroup with a CPU limitation. Now, my theory is: KVM (qemu-kvm) is completely userspace and librbd runs inside qemu-kvm as a library. All threads for disk I/O are part of the same PID and thus part of that cgroup. If a process inside the Virtual Machine now starts to consume all CPU time there is nothing left for librbd which slows it down. This then causes a increased I/O-wait inside the Virtual Machine. Even though the VM is not performing a lot of disk I/O. The wait of the I/O goes up due to this. Is my theory sane? Can somebody confirm this? Thanks, Wido _______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
