Re: LTTng tracing: ReplicatedPG::log_operation

[Andreas Bluemle <andreas.bluemle@itxperts.de>]

Hi Gregory,

On Tue, 2 Dec 2014 10:32:50 -0800
Gregory Farnum <greg@gregs42.com> wrote:

> On Tue, Dec 2, 2014 at 10:17 AM, Andreas Bluemle
> <andreas.bluemle@itxperts.de> wrote:
> > Hi,
> >
> > during code profiling using LTTng, I encounter that during
> > processing of write requests to the cluster, the ceph-osd
> > spends a lot of time in the ReplicatedPG::log_operation
> > before the the actual writes to journal and object
> > in the FileStore are triggered.
> >
> > This happens in ReplicatedBackend::submit_transaction.
> >
> > What I wonder is
> >   - what is the purpose of the log_operation?
> >     If I am not mistaken, then it is neither the write-to-journal
> >     nor the write-to-object; both of these are triggered from
> >     the queue_operation following that log_operation.
> 
> This is setting up the changes to the pg log, and encoding them into
> the transaction.
> 
> >   - can the sequence between the log_operation and
> >     the actual queue_operation be reversed in
> >     ReplicatedBackend::submit_transaction?
> 
> Nope, it needs to go into the transaction and get journaled.
> I'm kind of surprised this is a big time sink, but there is a lot of
> encoding so if you're running against a fast system I suppose it could
> be relatively large.

From what I see on my test system, adding the pg log entry consumes
about 60 microseconds - which is about 12 % of the overall time spent
for a write request on a replicating OSD, which is sth. like 460
microseconds between receipt of the MSG_OSD_SUBOP at the messenger
until the corresponding MSG_OSD_SUBOPREPLY is sent back to the primary
OSD.

> -Greg
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> 



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Andreas Bluemle                     mailto:Andreas.Bluemle@itxperts.de
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