On 14/09/18 12:13, Alex Lupsa wrote:
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 ?
Alex
Hi Alex,
If the number of concurrent io threads is higher than your total number
of osds, then there is no point in dividing the load, it actually
can/will reduce performance the higher this ratio is. So in most use
cases where you have many vms doing many io, there would be no point.
In the more specific cases where you have more osds than io operations
and your block sizes are not tiny, for example greater than 32k (you
can know your average io size by dividing cluster bandwidth by iops) you
may try rbd striping feature and adjust your stripe unit/count
accordingly., this could be quite good for example for low concurrency
streaming applications. For small io concurrency with smaller block
sizes, which is not a common use case, then probably go for raid level
striping as you did, but maybe Ceph is not ideal for such cases.
Maged
*Ronny Aasen*ronny+ceph-users at aasen.cx
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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 <[email protected]
<mailto:[email protected]>>:
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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