Re: RBD fio Performance concerns

[Mark Nelson <mark.nelson@inktank.com>]

Responding to my own message. :)

Talked to Sage a bit offline about this.  I think there are two opposing 
forces:

On one hand, random IO may be spreading reads/writes out across more 
OSDs than sequential IO that presumably would be hitting a single OSD 
more regularly.

On the other hand, you'd expect that sequential writes would be getting 
coalesced either at the RBD layer or on the OSD, and that the 
drive/controller/filesystem underneath the OSD would be doing some kind 
of readahead or prefetching.

On the third hand, maybe coalescing/prefetching is in fact happening but 
we are IOP limited by some per-osd limitation.

It could be interesting to do the test with a single OSD and see what 
happens.

Mark

On 11/21/2012 09:52 AM, Mark Nelson wrote:
> Hi Guys,
>
> I'm late to this thread but thought I'd chime in.  Crazy that you are
> getting higher performance with random reads/writes vs sequential!  It
> would be interesting to see what kind of throughput smalliobench reports
> (should be packaged in bobtail) and also see if this behavior happens
> with cephfs.  It's still too early in the morning for me right now to
> come up with a reasonable explanation for what's going on.  It might be
> worth running blktrace and seekwatcher to see what the io patterns on
> the underlying disk look like in each case.  Maybe something unexpected
> is going on.
>
> Mark
>
> On 11/19/2012 02:57 PM, Sébastien Han wrote:
>> Which iodepth did you use for those benchs?
>>
>>
>>> I really don't understand why I can't get more rand read iops with 4K
>>> block ...
>>
>> Me neither, hope to get some clarification from the Inktank guys. It
>> doesn't make any sense to me...
>> --
>> Bien cordialement.
>> Sébastien HAN.
>>
>>
>> On Mon, Nov 19, 2012 at 8:11 PM, Alexandre DERUMIER
>> <aderumier@odiso.com> wrote:
>>>>> @Alexandre: is it the same for you? or do you always get more IOPS
>>>>> with seq?
>>>
>>> rand read 4K : 6000 iops
>>> seq read 4K : 3500 iops
>>> seq read 4M : 31iops (1gigabit client bandwith limit)
>>>
>>> rand write 4k: 6000iops  (tmpfs journal)
>>> seq write 4k: 1600iops
>>> seq write 4M : 31iops (1gigabit client bandwith limit)
>>>
>>>
>>> I really don't understand why I can't get more rand read iops with 4K
>>> block ...
>>>
>>> I try with high end cpu for client, it doesn't change nothing.
>>> But test cluster use  old 8 cores E5420  @ 2.50GHZ (But cpu is around
>>> 15% on cluster during read bench)
>>>
>>>
>>> ----- Mail original -----
>>>
>>> De: "Sébastien Han" <han.sebastien@gmail.com>
>>> À: "Mark Kampe" <mark.kampe@inktank.com>
>>> Cc: "Alexandre DERUMIER" <aderumier@odiso.com>, "ceph-devel"
>>> <ceph-devel@vger.kernel.org>
>>> Envoyé: Lundi 19 Novembre 2012 19:03:40
>>> Objet: Re: RBD fio Performance concerns
>>>
>>> @Sage, thanks for the info :)
>>> @Mark:
>>>
>>>> If you want to do sequential I/O, you should do it buffered
>>>> (so that the writes can be aggregated) or with a 4M block size
>>>> (very efficient and avoiding object serialization).
>>>
>>> The original benchmark has been performed with 4M block size. And as
>>> you can see I still get more IOPS with rand than seq... I just tried
>>> with 4M without direct I/O, still the same. I can print fio results if
>>> it's needed.
>>>
>>>> We do direct writes for benchmarking, not because it is a reasonable
>>>> way to do I/O, but because it bypasses the buffer cache and enables
>>>> us to directly measure cluster I/O throughput (which is what we are
>>>> trying to optimize). Applications should usually do buffered I/O,
>>>> to get the (very significant) benefits of caching and write
>>>> aggregation.
>>>
>>> I know why I use direct I/O. It's synthetic benchmarks, it's far away
>>> from a real life scenario and how common applications works. I just
>>> try to see the maximum I/O throughput that I can get from my RBD. All
>>> my applications use buffered I/O.
>>>
>>> @Alexandre: is it the same for you? or do you always get more IOPS
>>> with seq?
>>>
>>> Thanks to all of you..
>>>
>>>
>>> On Mon, Nov 19, 2012 at 5:54 PM, Mark Kampe <mark.kampe@inktank.com>
>>> wrote:
>>>> Recall:
>>>> 1. RBD volumes are striped (4M wide) across RADOS objects
>>>> 2. distinct writes to a single RADOS object are serialized
>>>>
>>>> Your sequential 4K writes are direct, depth=256, so there are
>>>> (at all times) 256 writes queued to the same object. All of
>>>> your writes are waiting through a very long line, which is adding
>>>> horrendous latency.
>>>>
>>>> If you want to do sequential I/O, you should do it buffered
>>>> (so that the writes can be aggregated) or with a 4M block size
>>>> (very efficient and avoiding object serialization).
>>>>
>>>> We do direct writes for benchmarking, not because it is a reasonable
>>>> way to do I/O, but because it bypasses the buffer cache and enables
>>>> us to directly measure cluster I/O throughput (which is what we are
>>>> trying to optimize). Applications should usually do buffered I/O,
>>>> to get the (very significant) benefits of caching and write
>>>> aggregation.
>>>>
>>>>
>>>>> That's correct for some of the benchmarks. However even with 4K for
>>>>> seq, I still get less IOPS. See below my last fio:
>>>>>
>>>>> # fio rbd-bench.fio
>>>>> seq-read: (g=0): rw=read, bs=4K-4K/4K-4K, ioengine=libaio, iodepth=256
>>>>> rand-read: (g=1): rw=randread, bs=4K-4K/4K-4K, ioengine=libaio,
>>>>> iodepth=256
>>>>> seq-write: (g=2): rw=write, bs=4K-4K/4K-4K, ioengine=libaio,
>>>>> iodepth=256
>>>>> rand-write: (g=3): rw=randwrite, bs=4K-4K/4K-4K, ioengine=libaio,
>>>>> iodepth=256
>>>>> fio 1.59
>>>>> Starting 4 processes
>>>>> Jobs: 1 (f=1): [___w] [57.6% done] [0K/405K /s] [0 /99 iops] [eta
>>>>> 02m:59s]
>>>>> seq-read: (groupid=0, jobs=1): err= 0: pid=15096
>>>>> read : io=801892KB, bw=13353KB/s, iops=3338 , runt= 60053msec
>>>>> slat (usec): min=8 , max=45921 , avg=296.69, stdev=1584.90
>>>>> clat (msec): min=18 , max=133 , avg=76.37, stdev=16.63
>>>>> lat (msec): min=18 , max=133 , avg=76.67, stdev=16.62
>>>>> bw (KB/s) : min= 0, max=14406, per=31.89%, avg=4258.24,
>>>>> stdev=6239.06
>>>>> cpu : usr=0.87%, sys=5.57%, ctx=165281, majf=0, minf=279
>>>>> IO depths : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%,
>>>>>> =64=100.0%
>>>>> submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.0%
>>>>> complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.1%
>>>>> issued r/w/d: total=200473/0/0, short=0/0/0
>>>>>
>>>>> lat (msec): 20=0.01%, 50=9.46%, 100=90.45%, 250=0.10%
>>>>> rand-read: (groupid=1, jobs=1): err= 0: pid=16846
>>>>> read : io=6376.4MB, bw=108814KB/s, iops=27203 , runt= 60005msec
>>>>> slat (usec): min=8 , max=12723 , avg=33.54, stdev=59.87
>>>>> clat (usec): min=4642 , max=55760 , avg=9374.10, stdev=970.40
>>>>> lat (usec): min=4671 , max=55788 , avg=9408.00, stdev=971.21
>>>>> bw (KB/s) : min=105496, max=109136, per=100.00%, avg=108815.48,
>>>>> stdev=648.62
>>>>> cpu : usr=8.26%, sys=49.11%, ctx=1486259, majf=0, minf=278
>>>>> IO depths : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%,
>>>>>> =64=100.0%
>>>>> submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.0%
>>>>> complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.1%
>>>>> issued r/w/d: total=1632349/0/0, short=0/0/0
>>>>>
>>>>> lat (msec): 10=83.39%, 20=16.56%, 50=0.04%, 100=0.01%
>>>>> seq-write: (groupid=2, jobs=1): err= 0: pid=18653
>>>>> write: io=44684KB, bw=753502 B/s, iops=183 , runt= 60725msec
>>>>> slat (usec): min=8 , max=1246.8K, avg=5402.76, stdev=40024.97
>>>>> clat (msec): min=25 , max=4868 , avg=1384.22, stdev=470.19
>>>>> lat (msec): min=25 , max=4868 , avg=1389.62, stdev=470.17
>>>>> bw (KB/s) : min= 7, max= 2165, per=104.03%, avg=764.65,
>>>>> stdev=353.97
>>>>> cpu : usr=0.05%, sys=0.35%, ctx=5478, majf=0, minf=21
>>>>> IO depths : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.3%,
>>>>>> =64=99.4%
>>>>> submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.0%
>>>>> complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.1%
>>>>> issued r/w/d: total=0/11171/0, short=0/0/0
>>>>>
>>>>> lat (msec): 50=0.21%, 100=0.44%, 250=0.97%, 500=1.49%, 750=4.60%
>>>>> lat (msec): 1000=12.73%, 2000=66.36%, >=2000=13.20%
>>>>> rand-write: (groupid=3, jobs=1): err= 0: pid=20446
>>>>> write: io=208588KB, bw=3429.5KB/s, iops=857 , runt= 60822msec
>>>>> slat (usec): min=10 , max=1693.9K, avg=1148.15, stdev=15210.37
>>>>> clat (msec): min=22 , max=5639 , avg=297.37, stdev=430.27
>>>>> lat (msec): min=22 , max=5639 , avg=298.52, stdev=430.84
>>>>> bw (KB/s) : min= 0, max= 7728, per=31.44%, avg=1078.21,
>>>>> stdev=2000.45
>>>>> cpu : usr=0.34%, sys=1.61%, ctx=37183, majf=0, minf=19
>>>>> IO depths : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%,
>>>>>> =64=99.9%
>>>>> submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.0%
>>>>> complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.1%
>>>>> issued r/w/d: total=0/52147/0, short=0/0/0
>>>>>
>>>>> lat (msec): 50=2.82%, 100=25.63%, 250=46.12%, 500=10.36%, 750=5.10%
>>>>> lat (msec): 1000=2.91%, 2000=5.75%, >=2000=1.33%
>>>>>
>>>>> Run status group 0 (all jobs):
>>>>> READ: io=801892KB, aggrb=13353KB/s, minb=13673KB/s, maxb=13673KB/s,
>>>>> mint=60053msec, maxt=60053msec
>>>>>
>>>>> Run status group 1 (all jobs):
>>>>> READ: io=6376.4MB, aggrb=108814KB/s, minb=111425KB/s,
>>>>> maxb=111425KB/s, mint=60005msec, maxt=60005msec
>>>>>
>>>>> Run status group 2 (all jobs):
>>>>> WRITE: io=44684KB, aggrb=735KB/s, minb=753KB/s, maxb=753KB/s,
>>>>> mint=60725msec, maxt=60725msec
>>>>>
>>>>> Run status group 3 (all jobs):
>>>>> WRITE: io=208588KB, aggrb=3429KB/s, minb=3511KB/s, maxb=3511KB/s,
>>>>> mint=60822msec, maxt=60822msec
>>>>>
>>>>> Disk stats (read/write):
>>>>> rbd1: ios=1832984/63270, merge=0/0, ticks=16374236/17012132,
>>>>> in_queue=33434120, util=99.79%
>> --
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