journal writes when running rados bench seq on ec pool

journal writes when running rados bench seq on ec pool

[Deneau, Tom]

I am running a rados bench seq read benchmark on an erasure-coded pool (k=2, m=1),
and recording the disk activity.   I notice that I always see a small number of writes
to the journal partitions during the seq read run.  (I always drop the caches on the osd nodes
before starting the seq read run).

I never see this journal write behavior with a replicated pool and I didn't think there
would be any journal writes with either kind of pool on a read benchmark.

The write activity is low enough that it's probably not that important performance-wise
but I as just curious as to what would cause this.

-- Tom Deneau

RE: journal writes when running rados bench seq on ec pool

[Somnath Roy]

Tom,
Good that you brought this up !
I was also seeing the small writes during reads but forgot to mention in my last report on EC.
Basically, Ceph code base seems to be issuing small writes during reads and it is basically going to both data/jounal (considering filestore backend) ..But, since data partition is syncing on every 5 sec , it is not obvious there.

Loic,
Could you please explain a bit more on this ? What exactly we are writing during reads ?

Thanks & Regards
Somnath

-----Original Message-----
From: ceph-devel-owner@vger.kernel.org [mailto:ceph-devel-owner@vger.kernel.org] On Behalf Of Deneau, Tom
Sent: Wednesday, May 13, 2015 4:40 PM
To: ceph-devel
Subject: journal writes when running rados bench seq on ec pool

I am running a rados bench seq read benchmark on an erasure-coded pool (k=2, m=1),
and recording the disk activity.   I notice that I always see a small number of writes
to the journal partitions during the seq read run.  (I always drop the caches on the osd nodes before starting the seq read run).

I never see this journal write behavior with a replicated pool and I didn't think there would be any journal writes with either kind of pool on a read benchmark.

The write activity is low enough that it's probably not that important performance-wise but I as just curious as to what would cause this.

-- Tom Deneau



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RE: journal writes when running rados bench seq on ec pool

[Deneau, Tom]

Hi --

I never saw an explanation for the writes that are occurring during ecpool reads.

-- Tom


> -----Original Message-----
> From: Somnath Roy [mailto:Somnath.Roy@sandisk.com]
> Sent: Wednesday, May 13, 2015 6:48 PM
> To: Deneau, Tom; ceph-devel
> Subject: RE: journal writes when running rados bench seq on ec pool
> 
> Tom,
> Good that you brought this up !
> I was also seeing the small writes during reads but forgot to mention in my
> last report on EC.
> Basically, Ceph code base seems to be issuing small writes during reads and
> it is basically going to both data/jounal (considering filestore backend)
> ..But, since data partition is syncing on every 5 sec , it is not obvious
> there.
> 
> Loic,
> Could you please explain a bit more on this ? What exactly we are writing
> during reads ?
> 
> Thanks & Regards
> Somnath
> 
> -----Original Message-----
> From: ceph-devel-owner@vger.kernel.org [mailto:ceph-devel-
> owner@vger.kernel.org] On Behalf Of Deneau, Tom
> Sent: Wednesday, May 13, 2015 4:40 PM
> To: ceph-devel
> Subject: journal writes when running rados bench seq on ec pool
> 
> I am running a rados bench seq read benchmark on an erasure-coded pool (k=2,
> m=1),
> and recording the disk activity.   I notice that I always see a small number
> of writes
> to the journal partitions during the seq read run.  (I always drop the caches
> on the osd nodes before starting the seq read run).
> 
> I never see this journal write behavior with a replicated pool and I didn't
> think there would be any journal writes with either kind of pool on a read
> benchmark.
> 
> The write activity is low enough that it's probably not that important
> performance-wise but I as just curious as to what would cause this.
> 
> -- Tom Deneau
> 
> 
> 
> --
> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the
> body of a message to majordomo@vger.kernel.org More majordomo info at
> http://vger.kernel.org/majordomo-info.html
> 
> ________________________________
> 
> PLEASE NOTE: The information contained in this electronic mail message is
> intended only for the use of the designated recipient(s) named above. If the
> reader of this message is not the intended recipient, you are hereby notified
> that you have received this message in error and that any review,
> dissemination, distribution, or copying of this message is strictly
> prohibited. If you have received this communication in error, please notify
> the sender by telephone or e-mail (as shown above) immediately and destroy
> any and all copies of this message in your possession (whether hard copies or
> electronically stored copies).

Re: journal writes when running rados bench seq on ec pool

[Tom Deneau]

Somnath Roy <Somnath.Roy <at> sandisk.com> writes:

> 
> Tom,
> Good that you brought this up !
> I was also seeing the small writes during reads but forgot to mention in
my last report on EC.
> Basically, Ceph code base seems to be issuing small writes during reads
and it is basically going to both
> data/jounal (considering filestore backend) ..But, since data partition is
syncing on every 5 sec , it is
> not obvious there.
> 
> Loic,
> Could you please explain a bit more on this ? What exactly we are writing
during reads ?
> 
> Thanks & Regards
> Somnath
> 
> -----Original Message-----
> From: ceph-devel-owner <at> vger.kernel.org [mailto:ceph-devel-owner <at>
vger.kernel.org] On Behalf Of
> Deneau, Tom
> Sent: Wednesday, May 13, 2015 4:40 PM
> To: ceph-devel
> Subject: journal writes when running rados bench seq on ec pool
> 
> I am running a rados bench seq read benchmark on an erasure-coded pool
(k=2, m=1),
> and recording the disk activity.   I notice that I always see a small
number of writes
> to the journal partitions during the seq read run.  (I always drop the
caches on the osd nodes before starting
> the seq read run).
> 
> I never see this journal write behavior with a replicated pool and I
didn't think there would be any journal
> writes with either kind of pool on a read benchmark.
> 
> The write activity is low enough that it's probably not that important
performance-wise but I as just
> curious as to what would cause this.
> 

Was anyone able to explain these journal/data writes that occur during EC
pool reads?