From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark Nelson Subject: Re: Cache tier READ_FORWARD transition Date: Mon, 07 Jul 2014 14:38:58 -0500 Message-ID: <53BAF752.3080005@inktank.com> References: <53BACB03.5010603@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mail-ie0-f172.google.com ([209.85.223.172]:62519 "EHLO mail-ie0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750979AbaGGTi7 (ORCPT ); Mon, 7 Jul 2014 15:38:59 -0400 Received: by mail-ie0-f172.google.com with SMTP id rd18so4082033iec.3 for ; Mon, 07 Jul 2014 12:38:58 -0700 (PDT) In-Reply-To: Sender: ceph-devel-owner@vger.kernel.org List-ID: To: Sage Weil , Luis Pabon Cc: "ceph-devel@vger.kernel.org" On 07/07/2014 02:29 PM, Sage Weil wrote: > On Mon, 7 Jul 2014, Luis Pabon wrote: >> Hi all, >> I am working on OSDMonitor.cc:5325 and wanted to confirm the following >> read_forward cache tier transition: >> >> readforward -> forward || writeback || (any && num_objects_dirty == 0) >> forward -> writeback || readforward || (any && num_objects_dirty == 0) >> writeback -> readforward || forward >> >> Is this the correct cache tier state transition? > > That looks right to me. > > By the way, I had a thought after we spoke that we probably want something > that is somewhere inbetween the current writeback behavior (promote on > first read) and the read_forward behavior (never promote on read). I > suspect a good all-around policy is something like promote on second read? > This should probably be rolled into the writeback mode as a tunable... That would be a good start I think. What about some kind of scheme that also favours promoting small objects over larger ones? It could be as simple as increasing the number of reads necessary to do a promotion based on the object size. ie something like: <= 64k object = 1 read <= 512k object = 2 read else 3 read That would make the behaviour for default RBD object sizes always 3 read, but could keep big objects out of the cache tier for RGW. Mark > > sage > > > -- > 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 >