From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andreas Bluemle Subject: Re: LTTng tracing: ReplicatedPG::log_operation Date: Wed, 3 Dec 2014 09:58:03 +0100 Message-ID: <20141203095803.1276de6e@doppio> References: <20141202191758.5eded52c@doppio> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Return-path: Received: from mail.itxperts.de ([212.202.108.166]:57247 "EHLO mail.itxperts.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751576AbaLCOz4 convert rfc822-to-8bit (ORCPT ); Wed, 3 Dec 2014 09:55:56 -0500 In-Reply-To: Sender: ceph-devel-owner@vger.kernel.org List-ID: To: Gregory Farnum Cc: Ceph Development Hi Gregory, On Tue, 2 Dec 2014 10:32:50 -0800 Gregory Farnum wrote: > On Tue, Dec 2, 2014 at 10:17 AM, Andreas Bluemle > 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 > -- > 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 > > -- Andreas Bluemle mailto:Andreas.Bluemle@itxperts.de ITXperts GmbH http://www.itxperts.de Balanstrasse 73, Geb. 08 Phone: (+49) 89 89044917 D-81541 Muenchen (Germany) Fax: (+49) 89 89044910 Company details: http://www.itxperts.de/imprint.htm