Received: from malur.postgresql.org ([217.196.149.56]) by arkaria.postgresql.org with esmtp (Exim 4.80) (envelope-from ) id 1WxFxA-0002Cg-3d for pgsql-hackers@arkaria.postgresql.org; Wed, 18 Jun 2014 13:31:04 +0000 Received: from localhost ([127.0.0.1] helo=postgresql.org) by malur.postgresql.org with smtp (Exim 4.80) (envelope-from ) id 1WxFx9-0007mn-HE for pgsql-hackers@arkaria.postgresql.org; Wed, 18 Jun 2014 13:31:03 +0000 Received: from magus.postgresql.org ([2a02:c0:301:0:ffff::29]) by malur.postgresql.org with esmtps (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1WxFx7-0007kY-MR for pgsql-hackers@postgresql.org; Wed, 18 Jun 2014 13:31:01 +0000 Received: from mout.kundenserver.de ([212.227.17.13]) by magus.postgresql.org with esmtps (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1WxFx0-0004iM-GB for pgsql-hackers@postgresql.org; Wed, 18 Jun 2014 13:31:00 +0000 Received: from ayaki.localdomain (106-69-164-206.dyn.iinet.net.au [106.69.164.206]) by mrelayeu.kundenserver.de (node=mreue104) with ESMTP (Nemesis) id 0LwIKE-1Wf3vN1uXe-0186Uc; Wed, 18 Jun 2014 15:30:35 +0200 Message-ID: <53A19472.8020206@2ndquadrant.com> Date: Wed, 18 Jun 2014 21:30:26 +0800 From: Craig Ringer Organization: 2nd Quadrant User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 MIME-Version: 1.0 To: Robert Haas CC: Heikki Linnakangas , Greg Stark , Alvaro Herrera , Andres Freund , Bruce Momjian , Rajeev rastogi , Ants Aasma , PostgreSQL-development , Markus Wanner Subject: Re: Proposal for CSN based snapshots References: <51B1CB35.4050406@bluegap.ch> <51B6C97B.1070000@bluegap.ch> <5370D323.1070606@vmware.com> <20140512144112.GB9619@awork2.anarazel.de> <5370E267.2020904@vmware.com> <20140515183436.GD25053@momjian.us> <20140515200632.GB16014@awork2.anarazel.de> <20140515213714.GB7857@eldon.alvh.no-ip.org> <53889CCB.7030005@vmware.com> <5388A063.5000209@vmware.com> <539E7985.5090801@2ndquadrant.com> <53A0E4B5.3090207@2ndquadrant.com> In-Reply-To: X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Provags-ID: V02:K0:XUHQcTP3/+FvTBs3twJ08VjlZTGWV7BzSXCvTcdTFx4 NccuL9ESz+VsoNpKKHwe7lpwcS0rIXqn1PMmo0SpxoDl3Z6nIT mKvsUwkbYukl4z44zSvPTzHJUmxAqL9gqoBs+KTXXrPMwtrkHG 1x4lNYRZ5ZVwTXK4kNcKDxikiAXgsIYPHQYPK8s0t2A5rmgO9E Yz/J7kITybv2/U1C1MexULJL3x9rekFpTyc2kXFI3Eu4UrhGC2 WWRlOOkswxwk3uuEZxMOBYjSyjTB1g5KyAPpDPa+298VyGhGJS uk+eIsKbG1+i09tzYXlDu4cGsIF9ljq5mIQxSOabDjdAOvCrSG QxlV9LFouaIDvu6KQ7hw7bBzAaAKwp35qgVSf4Xj8 X-Pg-Spam-Score: -1.9 (-) List-Archive: List-Help: List-ID: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: X-Mailing-List: pgsql-hackers Precedence: bulk Sender: pgsql-hackers-owner@postgresql.org On 06/18/2014 09:12 PM, Robert Haas wrote: > No. Otherwise, one of those bits could get changed after a backend > takes a snapshot and before it finishes using it - so that the > transaction snapshot is in effect changing underneath it. You could > avoid that by memorizing the contents of CLOG when taking a snapshot, > but that would defeat the whole purpose of CSN-based snapshots, which > is to make the small and fixed-size. Ah. Thankyou. I appreciate the explanation. -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers