(Sorry for sending this twice... Forgot to reply to the list)
Is rbd caching safe to enable when you may need to do a live migration of the guest later on? It was my understanding that it wasn't, and that libvirt prevented you from doing the migration of it knew about the caching setting.
If it isn't, is there anything else that could help performance? Like, some tuning of block size parameters for the rbd image or the qemu
On May 10, 2013 8:57 PM, "Mark Nelson" <mark.nelson@xxxxxxxxxxx> wrote:
On 05/10/2013 07:21 PM, Yun Mao wrote:
Hi Mark,
Given the same hardware, optimal configuration (I have no idea what that
means exactly but feel free to specify), which is supposed to perform
better, kernel rbd or qemu/kvm? Thanks,
Yun
Hi Yun,
I'm in the process of actually running some tests right now.
In previous testing, it looked like kernel rbd and qemu/kvm performed about the same with cache off. With cache on (in cuttlefish), small sequential write performance improved pretty dramatically vs without cache. Large write performance seemed to take more concurrency to reach peak performance, but ultimately aggregate throughput was about the same.
Hopefully I should have some new results published in the near future.
Mark
On Fri, May 10, 2013 at 6:56 PM, Mark Nelson <mark.nelson@xxxxxxxxxxx
<mailto:mark.nelson@inktank.com>> wrote:
On 05/10/2013 12:16 PM, Greg wrote:
Hello folks,
I'm in the process of testing CEPH and RBD, I have set up a small
cluster of hosts running each a MON and an OSD with both
journal and
data on the same SSD (ok this is stupid but this is simple to
verify the
disks are not the bottleneck for 1 client). All nodes are
connected on a
1Gb network (no dedicated network for OSDs, shame on me :).
Summary : the RBD performance is poor compared to benchmark
A 5 seconds seq read benchmark shows something like this :
sec Cur ops started finished avg MB/s cur MB/s
last lat avg
lat
0 0 0 0 0 0 -
0
1 16 39 23 91.9586 92
0.966117 0.431249
2 16 64 48 95.9602 100
0.513435 0.53849
3 16 90 74 98.6317 104
0.25631 0.55494
4 11 95 84 83.9735 40
1.80038 0.58712
Total time run: 4.165747
Total reads made: 95
Read size: 4194304
Bandwidth (MB/sec): 91.220
Average Latency: 0.678901
Max latency: 1.80038
Min latency: 0.104719
91MB read performance, quite good !
Now the RBD performance :
root@client:~# dd if=/dev/rbd1 of=/dev/null bs=4M count=100
100+0 records in
100+0 records out
419430400 bytes (419 MB) copied, 13.0568 s, 32.1 MB/s
There is a 3x performance factor (same for write: ~60M
benchmark, ~20M
dd on block device)
The network is ok, the CPU is also ok on all OSDs.
CEPH is Bobtail 0.56.4, linux is 3.8.1 arm (vanilla release + some
patches for the SoC being used)
Can you show me the starting point for digging into this ?
Hi Greg, First things first, are you doing kernel rbd or qemu/kvm?
If you are doing qemu/kvm, make sure you are using virtio disks.
This can have a pretty big performance impact. Next, are you
using RBD cache? With 0.56.4 there are some performance issues with
large sequential writes if cache is on, but it does provide benefit
for small sequential writes. In general RBD cache behaviour has
improved with Cuttlefish.
Beyond that, are the pools being targeted by RBD and rados bench
setup the same way? Same number of Pgs? Same replication?
Thanks!
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