Ceph RBD Speedup
It turns out that virtual machines in KVM tend to behave not very well under high I/O load. In my case there was a busy nagios / smokeping machine that had troubles saving all it's data in Ceph.
Some tweaks to the caching-settings of libvirt helped to improve the responsiveness of the virtual machine.
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<disk type='network' device='disk'> <driver name='qemu' type='raw'/> <source protocol='rbd' name='rrs/checker2.kt-net.net:rbd_cache=1:rbd_cache_max_dirty=134217728:rbd_cache_size=268435456:rbd_cache_max_dirty_age=20'> <host name='rd-c2.ceph' port='6789'/> <host name='rd-c1.ceph' port='6789'/> <host name='rd-c3.ceph' port='6789'/> </source> <target dev='vdb' bus='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/> </disk>