From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josh Durgin Subject: Re: rados read ordering Date: Mon, 08 Dec 2014 15:38:48 -0800 Message-ID: <54863688.20202@inktank.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mail-ig0-f179.google.com ([209.85.213.179]:49553 "EHLO mail-ig0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752339AbaLHXhf (ORCPT ); Mon, 8 Dec 2014 18:37:35 -0500 Received: by mail-ig0-f179.google.com with SMTP id r2so13713igi.0 for ; Mon, 08 Dec 2014 15:37:34 -0800 (PST) In-Reply-To: Sender: ceph-devel-owner@vger.kernel.org List-ID: To: Sage Weil , sjust@redhat.com, jdurgin@redhat.com, yehuda@redhat.com, dillaman@redhat.com Cc: zhiqiang.wang@intel.com, ceph-devel@vger.kernel.org On 12/08/2014 09:03 AM, Sage Weil wrote: > The current RADOS behavior is that reads (on any given object) are always > processed in the order they are submitted by the client. This causes a > few headaches for the cache tiering that it would be nice to avoid. It > also occurs to me that there are likely cases where we could go a lot > faster by not strictly ordering things. For example, a stat can respond > more quickly than a large read, and some reads may hit cache while others > go to disk. This doesn't happen currently because of the (lame) way we do > reads synchronously, but hope that can change too. > > I propose we drop this semantic. If a client wants reads to have a strict > ordering, they can set the existing RWORDERED flag (which also orders them > with respect to writes). That's not the most general thing ever, but I'm > not sure we care about callers who want reads ordered with respect to each > other but not writes. > > The real question is whether there are any users that want/need this > currently. I can't think of any offhand. In several places we submit > multiple *writes* and expect them to be strictly ordered (e.g., we > set a completion on teh last write only). I don't think we do this > anywhere for reads though... > > Josh, Yehuda, Jason--can you think of any in RBD or RGW that would depend > on this? Nope, I've thought we should fix this since I found out about it.