Hi,
As my previous mail reported some weeks ago ,we are suffering from OSD
crash/ OSD Flipping / System reboot and etc, all these unstable issue really
stop us from digging further into ceph characterization.
Good news is that we seems find out the cause, I explain our
experiments below:
Environment:
We have 2 machines, one for client and one for ceph, connected
via 10GbE.
The client machine is very powerful, with 64 Cores and 256G
RAM.
The ceph machine with 32 Cores and 64G RAM, but we limited the
available RAM to 8GB by the grub configuration.12 OSDs on top of 12* 5400 RPM
1T disk , 4* DCS 3700 SSDs as journals.
Both client and ceph are v0.61.2.
We run 12 rados bench instances in client node as a stress to
ceph node, each instance with 256 concurrent.
Experiment and result:
1.default ceph + default client , OK
2.tuned ceph + default client FAIL,One osd killed by OS due
to OOM, and all swap space is run out. (tuning: Large queue ops/Large queue
bytes/.No flusher/sync_flush =true)
3.tuned ceph WITHOUT large queue bytes + default client OK
4.tuned ceph WITHOUT large queue bytes + aggressive client
FAILED, One osd killed by OOM and one suicide because 150s op thread timeout.
(aggressive client: objecter_inflight_ops, opjecter_inflight_bytes are both set
to 10X of default)
Conclusion.
We would like to say,
a. under heavy load, some tuning will make ceph unstable
,especially queue bytes related ( deduce from 1+2+3)
b. Ceph doesn't do any control on the lenth of OSD Queue,
this is a critical issue, with aggressive client or a lot of concurrent
clients, the osd queue will become too long to fit in memory ,thus result in
osd daemon being killed.(deduce from 3+4)
c. An observation to osd daemon memory usage show that, if I
use "killall rados" to kill all the rados bench instances, the ceph osd daemon
cannot free the allocated memory, instead, it still remain very high memory
usage(a new started ceph used ~0.5 GB , and with load it used ~ 6GB , if killed
rados, it still remain 5~6GB, restart ceph can solve this issue)
We don't capture any log now ,but since it's really easy to reproduce ,
so we can reproduce and provide any log /profiling info per request.
Any inputs/suggestion are highly appreciated. Thanks
Xiaoxi
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