Re: Slow Ceph: Any plans on torrent-like transfers from OSDs ?

[Ronny Aasen <ronny+ceph-users@xxxxxxxx>]

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


On 09.09.2018 11:20, Alex Lupsa wrote:

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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