Slow IOPS on RBD compared to journal and backing devices

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Am 14.05.2014 11:29, schrieb Josef Johansson:
> Hi Christian,
> 
> I missed this thread, haven't been reading the list that well the last
> weeks.
> 
> You already know my setup, since we discussed it in an earlier thread. I
> don't have a fast backing store, but I see the slow IOPS when doing
> randwrite inside the VM, with rbd cache. Still running dumpling here though.
> 
> A thought struck me that I could test with a pool that consists of OSDs
> that have tempfs-based disks, think I have a bit more latency than your
> IPoIB but I've pushed 100k IOPS with the same network devices before.
> This would verify if the problem is with the journal disks. I'll also
> try to run the journal devices in tempfs as well, as it would test
> purely Ceph itself.

i did the same with bobtail a year ago and was still limited to nearly
the same values. No idea what firefly will say. I'm pretty sure the
limit is in the ceph code itself.

There were a short discussion here:
http://www.spinics.net/lists/ceph-devel/msg18731.html

Stefan

> I'll get back to you with the results, hopefully I'll manage to get them
> done during this night.
> 
> Cheers,
> Josef
> 
> On 13/05/14 11:03, Christian Balzer wrote:
>> I'm clearly talking to myself, but whatever.
>>
>> For Greg, I've played with all the pertinent journal and filestore options
>> and TCP nodelay, no changes at all.
>>
>> Is there anybody on this ML who's running a Ceph cluster with a fast
>> network and FAST filestore, so like me with a big HW cache in front of a
>> RAID/JBODs or using SSDs for final storage?
>>
>> If so, what results do you get out of the fio statement below per OSD?
>> In my case with 4 OSDs and 3200 IOPS that's about 800 IOPS per OSD, which
>> is of course vastly faster than the normal indvidual HDDs could do.
>>
>> So I'm wondering if I'm hitting some inherent limitation of how fast a
>> single OSD (as in the software) can handle IOPS, given that everything else
>> has been ruled out from where I stand.
>>
>> This would also explain why none of the option changes or the use of
>> RBD caching has any measurable effect in the test case below. 
>> As in, a slow OSD aka single HDD with journal on the same disk would
>> clearly benefit from even the small 32MB standard RBD cache, while in my
>> test case the only time the caching becomes noticeable is if I increase
>> the cache size to something larger than the test data size. ^o^
>>
>> On the other hand if people here regularly get thousands or tens of
>> thousands IOPS per OSD with the appropriate HW I'm stumped. 
>>
>> Christian
>>
>> On Fri, 9 May 2014 11:01:26 +0900 Christian Balzer wrote:
>>
>>> On Wed, 7 May 2014 22:13:53 -0700 Gregory Farnum wrote:
>>>
>>>> Oh, I didn't notice that. I bet you aren't getting the expected
>>>> throughput on the RAID array with OSD access patterns, and that's
>>>> applying back pressure on the journal.
>>>>
>>> In the a "picture" being worth a thousand words tradition, I give you
>>> this iostat -x output taken during a fio run:
>>>
>>> avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>>>           50.82    0.00   19.43    0.17    0.00   29.58
>>>
>>> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s
>>> avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
>>> sda               0.00    51.50    0.00 1633.50     0.00  7460.00
>>> 9.13     0.18    0.11    0.00    0.11   0.01   1.40 sdb
>>> 0.00     0.00    0.00 1240.50     0.00  5244.00     8.45     0.30
>>> 0.25    0.00    0.25   0.02   2.00 sdc               0.00     5.00
>>> 0.00 2468.50     0.00 13419.00    10.87     0.24    0.10    0.00
>>> 0.10   0.09  22.00 sdd               0.00     6.50    0.00 1913.00
>>> 0.00 10313.00    10.78     0.20    0.10    0.00    0.10   0.09  16.60
>>>
>>> The %user CPU utilization is pretty much entirely the 2 OSD processes,
>>> note the nearly complete absence of iowait.
>>>
>>> sda and sdb are the OSDs RAIDs, sdc and sdd are the journal SSDs.
>>> Look at these numbers, the lack of queues, the low wait and service
>>> times (this is in ms) plus overall utilization.
>>>
>>> The only conclusion I can draw from these numbers and the network results
>>> below is that the latency happens within the OSD processes.
>>>
>>> Regards,
>>>
>>> Christian
>>>> When I suggested other tests, I meant with and without Ceph. One
>>>> particular one is OSD bench. That should be interesting to try at a
>>>> variety of block sizes. You could also try runnin RADOS bench and
>>>> smalliobench at a few different sizes.
>>>> -Greg
>>>>
>>>> On Wednesday, May 7, 2014, Alexandre DERUMIER <aderumier at odiso.com>
>>>> wrote:
>>>>
>>>>> Hi Christian,
>>>>>
>>>>> Do you have tried without raid6, to have more osd ?
>>>>> (how many disks do you have begin the raid6 ?)
>>>>>
>>>>>
>>>>> Aslo, I known that direct ios can be quite slow with ceph,
>>>>>
>>>>> maybe can you try without --direct=1
>>>>>
>>>>> and also enable rbd_cache
>>>>>
>>>>> ceph.conf
>>>>> [client]
>>>>> rbd cache = true
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> ----- Mail original -----
>>>>>
>>>>> De: "Christian Balzer" <chibi at gol.com <javascript:;>>
>>>>> ?: "Gregory Farnum" <greg at inktank.com <javascript:;>>,
>>>>> ceph-users at lists.ceph.com <javascript:;>
>>>>> Envoy?: Jeudi 8 Mai 2014 04:49:16
>>>>> Objet: Re: Slow IOPS on RBD compared to journal and
>>>>> backing devices
>>>>>
>>>>> On Wed, 7 May 2014 18:37:48 -0700 Gregory Farnum wrote:
>>>>>
>>>>>> On Wed, May 7, 2014 at 5:57 PM, Christian Balzer
>>>>>> <chibi at gol.com<javascript:;>>
>>>>> wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> ceph 0.72 on Debian Jessie, 2 storage nodes with 2 OSDs each. The
>>>>>>> journals are on (separate) DC 3700s, the actual OSDs are RAID6
>>>>>>> behind an Areca 1882 with 4GB of cache.
>>>>>>>
>>>>>>> Running this fio:
>>>>>>>
>>>>>>> fio --size=400m --ioengine=libaio --invalidate=1 --direct=1
>>>>>>> --numjobs=1 --rw=randwrite --name=fiojob --blocksize=4k
>>>>>>> --iodepth=128
>>>>>>>
>>>>>>> results in:
>>>>>>>
>>>>>>> 30k IOPS on the journal SSD (as expected)
>>>>>>> 110k IOPS on the OSD (it fits neatly into the cache, no surprise
>>>>>>> there) 3200 IOPS from a VM using userspace RBD
>>>>>>> 2900 IOPS from a host kernelspace mounted RBD
>>>>>>>
>>>>>>> When running the fio from the VM RBD the utilization of the
>>>>>>> journals is about 20% (2400 IOPS) and the OSDs are bored at 2%
>>>>>>> (1500 IOPS after some obvious merging).
>>>>>>> The OSD processes are quite busy, reading well over 200% on atop,
>>>>>>> but the system is not CPU or otherwise resource starved at that
>>>>>>> moment.
>>>>>>>
>>>>>>> Running multiple instances of this test from several VMs on
>>>>>>> different hosts changes nothing, as in the aggregated IOPS for
>>>>>>> the whole cluster will still be around 3200 IOPS.
>>>>>>>
>>>>>>> Now clearly RBD has to deal with latency here, but the network is
>>>>>>> IPoIB with the associated low latency and the journal SSDs are
>>>>>>> the (consistently) fasted ones around.
>>>>>>>
>>>>>>> I guess what I am wondering about is if this is normal and to be
>>>>>>> expected or if not where all that potential performance got lost.
>>>>>> Hmm, with 128 IOs at a time (I believe I'm reading that correctly?)
>>>>> Yes, but going down to 32 doesn't change things one iota.
>>>>> Also note the multiple instances I mention up there, so that would be
>>>>> 256 IOs at a time, coming from different hosts over different links
>>>>> and nothing changes.
>>>>>
>>>>>> that's about 40ms of latency per op (for userspace RBD), which
>>>>>> seems awfully long. You should check what your client-side objecter
>>>>>> settings are; it might be limiting you to fewer outstanding ops
>>>>>> than that.
>>>>> Googling for client-side objecter gives a few hits on ceph devel and
>>>>> bugs and nothing at all as far as configuration options are
>>>>> concerned. Care to enlighten me where one can find those?
>>>>>
>>>>> Also note the kernelspace (3.13 if it matters) speed, which is very
>>>>> much in the same (junior league) ballpark.
>>>>>
>>>>>> If
>>>>>> it's available to you, testing with Firefly or even master would be
>>>>>> interesting ? there's some performance work that should reduce
>>>>>> latencies.
>>>>>>
>>>>> Not an option, this is going into production next week.
>>>>>
>>>>>> But a well-tuned (or even default-tuned, I thought) Ceph cluster
>>>>>> certainly doesn't require 40ms/op, so you should probably run a
>>>>>> wider array of experiments to try and figure out where it's coming
>>>>>> from.
>>>>> I think we can rule out the network, NPtcp gives me:
>>>>> ---
>>>>> 56: 4096 bytes 1546 times --> 979.22 Mbps in 31.91 usec
>>>>> ---
>>>>>
>>>>> For comparison at about 512KB it reaches maximum throughput and still
>>>>> isn't that laggy:
>>>>> ---
>>>>> 98: 524288 bytes 121 times --> 9700.57 Mbps in 412.35 usec
>>>>> ---
>>>>>
>>>>> So with the network performing as well as my lengthy experience with
>>>>> IPoIB led me to believe, what else is there to look at?
>>>>> The storage nodes perform just as expected, indicated by the local
>>>>> fio tests.
>>>>>
>>>>> That pretty much leaves only Ceph/RBD to look at and I'm not really
>>>>> sure what experiments I should run on that. ^o^
>>>>>
>>>>> Regards,
>>>>>
>>>>> Christian
>>>>>
>>>>>> -Greg
>>>>>> Software Engineer #42 @ http://inktank.com | http://ceph.com
>>>>>>
>>>>>
>>>>> --
>>>>> Christian Balzer Network/Systems Engineer
>>>>> chibi at gol.com <javascript:;> Global OnLine Japan/Fusion
>>>>> Communications http://www.gol.com/
>>>>> _______________________________________________
>>>>> ceph-users mailing list
>>>>> ceph-users at lists.ceph.com <javascript:;>
>>>>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
>>>>>
>>>>
>>>
>>
> 
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