speedup ceph / scaling / find the bottleneck

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Hello list,

i've made some further testing and have the problem that ceph doesn't scale for me. I added a 4th osd server to my existing 3 node osd cluster. I also reformated all to be able to start with a clean system.

While doing random 4k writes from two VMs i see about 8% idle on the osd servers (Single Intel Xeon E5 8 cores 3,6Ghz). I believe that this is the limiting factor and also the reason why i don't see any improvement by adding osd servers.

3 nodes: 2VMS: 7000 IOp/s 4k writes osds: 7-15% idle
4 nodes: 2VMS: 7500 IOp/s 4k writes osds: 7-15% idle

Even the cpu is not the limiting factor i think it would be really important to lower the CPU usage while doing 4k writes. The CPU is only used by the ceph-osd process. I see nearly no usage by other processes (only 5% by kworker and 5% flush).

Could somebody recommand me a way to debug this? So we know where all this CPU usage goes?

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