Another BIG hint.
While doing random 4k I/O from one VM i archieve 14k I/Os. This is
around 54MB/s. But EACH ceph-osd machine is writing between 500MB/s and
750MB/s. What do they write?!?!
Just an idea?:
Do they completely rewrite EACH 4MB block for each 4k write?
Stefan
Am 29.06.2012 15:02, schrieb Stefan Priebe - Profihost AG:
Am 29.06.2012 13:49, schrieb Mark Nelson:
I'll try to replicate your findings in house. I've got some other
things I have to do today, but hopefully I can take a look next week. If
I recall correctly, in the other thread you said that sequential writes
are using much less CPU time on your systems?
Random 4k writes: 10% idle
Seq 4k writes: !! 99,7% !! idle
Seq 4M writes: 90% idle
> Do you see better scaling in that case?
3 osd nodes:
1 VM:
Rand 4k writes: 7000 iops
Seq 4k writes: 19900 iops
2 VMs:
Rand 4k writes: 6000 iops each
Seq 4k writes: 4000 iops each VM 1
Seq 4k writes: 18500 iops each VM 2
4 osd nodes:
1 VM:
Rand 4k writes: 14400 iops
Seq 4k writes: 19000 iops
2 VMs:
Rand 4k writes: 7000 iops each
Seq 4k writes: 18000 iops each
To figure out where CPU is being used, you could try various options:
oprofile, perf, valgrind, strace. Each has it's own advantages.
Here's how you can create a simple callgraph with perf:
http://lwn.net/Articles/340010/
10s perf data output while doing random 4k writes:
https://raw.github.com/gist/2c16136faebec381ae35/09e6de68a5461a198430a9ec19dfd5392f276706/gistfile1.txt
Stefan
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