I’m seeing a strange queue depth behaviour with a kernel mapped RBD, librbd does not show this problem. Cluster is comprised of 4 nodes, 10GB networking, not including OSDs as test sample is small so fits in page cache. Running fio against a kernel mapped RBD fio --randrepeat=1 --ioengine=libaio --direct=1 --gtod_reduce=1 --name=test --filename=/dev/rbd/cache1/test2 --bs=4k --readwrite=randread --iodepth=1 --runtime=10 --size=1g
See how initially I get a very high number of IOs at queue depth 1, but this drops dramatically as soon as I start increasing the queue depth. It’s not until a depth or 32 IOs that I start to get similar performance. Incidentally when changing the read type to sequential instead of random the oddity goes away. Running fio with librbd engine and the same test options I get the following
As you can see the performance scales up nicely, although the top end IO’s seem limited to around 18k. I don’t know if this is due to kernel/userspace performance differences or if there is a lower max queue depth limit in librbd. Both tests were run on a small sample size to force the OSD data into page cache to rule out any device latency. Does anyone know why kernel mapped RBD’s show this weird behaviour? I don’t think it can be OSD/ceph config related as it only happens with krbd’s. Nick |
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