From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755867AbZEERjl (ORCPT ); Tue, 5 May 2009 13:39:41 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752055AbZEERjc (ORCPT ); Tue, 5 May 2009 13:39:32 -0400 Received: from gate.in-addr.de ([212.8.193.158]:54568 "EHLO mx.in-addr.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750908AbZEERjb (ORCPT ); Tue, 5 May 2009 13:39:31 -0400 Date: Tue, 5 May 2009 19:38:54 +0200 From: Lars Marowsky-Bree To: Philipp Reisner , Bart Van Assche Cc: James Bottomley , david@lang.hm, Willy Tarreau , Andrew Morton , linux-kernel@vger.kernel.org, Jens Axboe , Greg KH , Neil Brown , Sam Ravnborg , Dave Jones , Nikanth Karthikesan , Kyle Moffett , Lars Ellenberg Subject: Re: [PATCH 00/16] DRBD: a block device for HA clusters Message-ID: <20090505173854.GC14192@suse.de> References: <1241090812-13516-1-git-send-email-philipp.reisner@linbit.com> <200905051021.33461.philipp.reisner@linbit.com> <200905051757.17100.philipp.reisner@linbit.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <200905051757.17100.philipp.reisner@linbit.com> X-Ctuhulu: HASTUR User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2009-05-05T17:57:15, Philipp Reisner wrote: > Up to now we do not offer barrier support for the layers above us. > That will follow sooner or later. > > Here is an example, why it is not completely trivial: > > Imagine DRBD on top of a dm-linear on both nodes. When you start, > both dm-linear mappings sit on top of something that supports > barriers itself. -- Then the user replaces the backing device > below the dm-linear on the secondary node with something that > does not support barriers. The same problem exists essentially for md raid1 as well, and I'd not consider it objectionable if you took a brutal approach: > When we get a write request with the BIO_RW_BARRIER flag set > in from the FS, we submit this locally, ship it over to the > peer and submit it there. Unfortunately it fails now with > ENOTSUP on the peer. > > We can not ship that error back to the upper layer, because > our mirror is already inconsistent. Disconnect the secondary with a loud error as to why (incompatible change of the device below). (Re-)negotiate barrier capability at connect time; then, resync. Regards, Lars -- SuSE Labs, OPS Engineering, Novell, Inc. SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nürnberg) "Experience is the name everyone gives to their mistakes." -- Oscar Wilde