Re: severe librbd performance degradation in Giant

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

Any chance read ahead could be causing issues?

On 09/17/2014 04:29 PM, Somnath Roy wrote:
> I set the following in the client side /etc/ceph/ceph.conf where I am running fio rbd.
>
> rbd_cache_writethrough_until_flush = false
>
> But, no difference. BTW, I am doing Random read, not write. Still this setting applies ?
>
> Next, I tried to tweak the rbd_cache setting to false and I *got back* the old performance. Now, it is similar to firefly throughput !
>
> So, loks like rbd_cache=true was the culprit.
>
> Thanks Josh !
>
> Regards
> Somnath
>
> -----Original Message-----
> From: Josh Durgin [mailto:josh.durgin@inktank.com]
> Sent: Wednesday, September 17, 2014 2:20 PM
> To: Somnath Roy; ceph-devel@vger.kernel.org
> Subject: Re: severe librbd performance degradation in Giant
>
> On 09/17/2014 01:55 PM, Somnath Roy wrote:
>> Hi Sage,
>> We are experiencing severe librbd performance degradation in Giant over firefly release. Here is the experiment we did to isolate it as a librbd problem.
>>
>> 1. Single OSD is running latest Giant and client is running fio rbd on top of firefly based librbd/librados. For one client it is giving ~11-12K  iops (4K RR).
>> 2. Single OSD is running Giant and client is running fio rbd on top of Giant based librbd/librados. For one client it is giving ~1.9K  iops (4K RR).
>> 3. Single OSD is running latest Giant and client is running Giant based ceph_smaiobench on top of giant librados. For one client it is giving ~11-12K  iops (4K RR).
>> 4. Giant RGW on top of Giant OSD is also scaling.
>>
>>
>> So, it is obvious from the above that recent librbd has issues. I will raise a tracker to track this.
>
> For giant the default cache settings changed to:
>
> rbd cache = true
> rbd cache writethrough until flush = true
>
> If fio isn't sending flushes as the test is running, the cache will stay in writethrough mode. Does the difference remain if you set rbd cache writethrough until flush = false ?
>
> Josh
>
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