Hi,
Thank you for the answer Ronny. I did indeed try 2x RBD drives (rdb-cache was already active), striping them, and got double write/read speed instantly. So I am chalking this one on KVM who is single-threaded and not fully ceph-aware it seems. Although I can see some threads talking about multi-threads coming to KVM.
I am however still of the opinion that all ceph osd replicas should be read from in the future, because the code is there already in the form of recovery so the amount of time vs the tremendous speeds should be worth it!
About dm-cache or bcache on osd's, which one would you recommend ?
About dm-cache or bcache on osd's, which one would you recommend ?
Alex
Sun Sep 9 03:13:47 PDT 2018
Ronny Aasen ronny+ceph-users at aasen.cx
Sun Sep 9 03:13:47 PDT 2018
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ceph is a distributed system, it scales by concurrent access to nodes. generally a single client will access a single OSD at the time, iow max possible single thread read is the read speak of the drive. and max possible write is single drive write / (replication size-1) but when you have many vm's accessing the same cluster the load is spread all over (just like when you see the recovery running) A single spinning disk should be able to do 100-150MB/s depending on make and model. even with the overhead of ceph and networking so i still think 20MB/s is a bit on the low side, depending on how you benchmark. I would start by going thru this benchmarking guide, and see if you find some issues: https://tracker.ceph.com/projects/ceph/wiki/Benchmark_Ceph_Cluster_Performance in order to get more singlethread performance out of ceph you must get faster individual parts ( nvram disks/fast ram and processors/fast network/etc/etc) or you can cheat by either spreading the load over more disks. eg you can do rbd fancy striping, or attach multiple disk's with individual controllers in the vm. or use caching and /or readahead. when it comes to cache tiering i would remove that, it does not get the love it needs. and redhat have even stopped supporting it in deployments. but you can use dm-cache or bcache on osd's or/and rbd-cache on kvm clients. good luck Ronny Aasen
Den sön 9 sep. 2018 kl 11:20 skrev Alex Lupsa <alex@xxxxxxxx>:
Hi,
Any ideas about the below ?
Thanks,
Alex---------- Hi, I have a really small homelab 3-node ceph cluster on consumer hw - thanks to Proxmox for making it easy to deploy it. The problem I am having is very very bad transfer rates, ie 20mb/sec for both read and write on 17 OSDs with cache layer. However during recovery the speed hover between 250 to 700mb/sec which proves that the cluster IS capable of reaching way above those 20mb/sec in KVM. Reading the documentation, I see that during recovery "nearly all OSDs participate in resilvering a new drive" - kind of a torrent of data incoming from multiple sources at once, causing a huge deluge. However I believe this does not happen during the normal transfers, so my question is simply - is there any hidden tunables I can enable for this with the implied cost of network and heavy usage of disks ? Will there be in the future if not ? I have tried disabling authx, upgrading the network to 10gbit, have bigger journals, more bluestore cache and disabled the debugging logs as it has been advised on the list. The only thing that did help a bit was cache tiering, but this only helps somewhat as the ops do not get promoted unless I am very adamant about keeping programs in KVM open for very long times so that the writes/reads are promoted. To add some to the injury, once the cache gets full - the whole 3-node cluster grinds to a full halt until I start forcefully evict data from the cache... manually! So I am therefore guessing a really bad misconfiguration from my side. Next step would be removing the cache layer and using those SSDs as bcache instead as it seems to yeld 5x the results, even though it does add yet another layer of complexity and RAM requirements. Full config details: https://pastebin.com/xUM7VF9k rados bench -p ceph_pool 30 write Total time run: 30.983343 Total writes made: 762 Write size: 4194304 Object size: 4194304 Bandwidth (MB/sec): 98.3754 Stddev Bandwidth: 20.9586 Max bandwidth (MB/sec): 132 Min bandwidth (MB/sec): 16 Average IOPS: 24 Stddev IOPS: 5 Max IOPS: 33 Min IOPS: 4 Average Latency(s): 0.645017 Stddev Latency(s): 0.326411 Max latency(s): 2.08067 Min latency(s): 0.0355789 Cleaning up (deleting benchmark objects) Removed 762 objects Clean up completed and total clean up time :3.925631 Thanks, Alex
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