Hi folks, i just received a mail of a customer. He reclaimed that the "ping latency" of his VM rises, if he does a lot of IO inside of the VM. I have done the same with a test VM. I could reproduce this behavior. If i disable the rbd cache the VM IO is slower but the latency is ok. Even SSH und other programs are affected .... so its not a problem of slow ICMP. Device Setting: virtio0: rbd:9997/vm-1171-disk-1.rbd:rbd_cache=true:rbd_cache_size=16777216:rbd_cache_max_dirty=8388608:rbd_cache_target_dirty=4194304,cache=none normal ping: 64 bytes from 109.75.x.x: icmp_seq=38 ttl=56 time=29.2 ms 64 bytes from 109.75.x.x: icmp_seq=39 ttl=56 time=20.8 ms 64 bytes from 109.75.x.x: icmp_seq=40 ttl=56 time=22.4 ms with lots of IO: 64 bytes from 109.75.x.x: icmp_seq=87 ttl=56 time=28.1 ms 64 bytes from 109.75.x.x: icmp_seq=88 ttl=56 time=665 ms 64 bytes from 109.75.x.x: icmp_seq=89 ttl=56 time=226 ms 64 bytes from 109.75.x.x: icmp_seq=90 ttl=56 time=179 ms 64 bytes from 109.75.x.x: icmp_seq=91 ttl=56 time=140 ms 64 bytes from 109.75.x.x: icmp_seq=92 ttl=56 time=25.6 ms 64 bytes from 109.75.x.x: icmp_seq=93 ttl=56 time=568 ms 64 bytes from 109.75.x.x: icmp_seq=94 ttl=56 time=405 ms 64 bytes from 109.75.x.x: icmp_seq=95 ttl=56 time=223 ms 64 bytes from 109.75.x.x: icmp_seq=96 ttl=56 time=24.5 ms 64 bytes from 109.75.x.x: icmp_seq=97 ttl=56 time=321 ms 64 bytes from 109.75.x.x: icmp_seq=98 ttl=56 time=391 ms 64 bytes from 109.75.x.x: icmp_seq=99 ttl=56 time=4200 ms 64 bytes from 109.75.x.x: icmp_seq=101 ttl=56 time=2194 ms But if i disable caching: virtio0: rbd:9997/vm-1171-disk-1.rbd:rbd_cache=false,cache=writeback with lots of IO: 64 bytes from 109.75.x.x: icmp_seq=62 ttl=56 time=22.1 ms 64 bytes from 109.75.x.x: icmp_seq=63 ttl=56 time=26.5 ms 64 bytes from 109.75.x.x: icmp_seq=64 ttl=56 time=30.7 ms 64 bytes from 109.75.x.x: icmp_seq=65 ttl=56 time=24.8 ms 64 bytes from 109.75.x.x: icmp_seq=66 ttl=56 time=21.9 ms Can someone please explain me this behavior ? Why is the latency of the VM spiky if i enable rbd caching ? I've played around with the caching parameters but with caching enabled its always the same. KVM Version: 1.2.1 Ceph Version: ceph version 0.48.2argonaut (commit:3e02b2fad88c2a95d9c0c86878f10d1beb780bfe) Thanks a lot !! -- mit freundlichen Grüssen Jens Rehpöhler ---------------------------------------------------------------------- filoo GmbH Moltkestr. 25a 33330 Gütersloh HRB4355 AG Gütersloh Geschäftsführer: S.Grewing | J.Rehpöhler | Dr. C.Kunz Telefon: +49 5241 8673012 | Mobil: +49 151 54645798 Hotline: +49 5241 8673026| Fax: +49 5241 8673020 Folgen Sie uns auf: Twitter: http://twitter.com/filoogmbh Facebook: http://facebook.com/filoogmbh
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