From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754690AbZEDI2t (ORCPT ); Mon, 4 May 2009 04:28:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754351AbZEDI2g (ORCPT ); Mon, 4 May 2009 04:28:36 -0400 Received: from mail09.linbit.com ([212.69.161.110]:50136 "EHLO mail09.linbit.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754287AbZEDI2f (ORCPT ); Mon, 4 May 2009 04:28:35 -0400 From: Philipp Reisner Organization: LINBIT To: James Bottomley Subject: Re: [PATCH 00/16] DRBD: a block device for HA clusters Date: Mon, 4 May 2009 10:28:12 +0200 User-Agent: KMail/1.11.2 (Linux/2.6.28-11-generic; KDE/4.2.2; i686; ; ) Cc: david@lang.hm, Willy Tarreau , Bart Van Assche , Andrew Morton , linux-kernel@vger.kernel.org, Jens Axboe , Greg KH , Neil Brown , Sam Ravnborg , Dave Jones , Nikanth Karthikesan , "Lars Marowsky-Bree" , Kyle Moffett , Lars Ellenberg References: <1241090812-13516-1-git-send-email-philipp.reisner@linbit.com> <1241361925.5596.27.camel@mulgrave.int.hansenpartnership.com> In-Reply-To: <1241361925.5596.27.camel@mulgrave.int.hansenpartnership.com> X-OTRS-FollowUp-SenderType: agent MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200905041028.13865.philipp.reisner@linbit.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sunday 03 May 2009 16:45:25 James Bottomley wrote: > On Sun, 2009-05-03 at 07:36 -0700, david@lang.hm wrote: > > On Sun, 3 May 2009, James Bottomley wrote: > > > Subject: Re: [PATCH 00/16] DRBD: a block device for HA clusters > > > > > > On Sat, 2009-05-02 at 22:40 -0700, david@lang.hm wrote: > > >> On Sun, 3 May 2009, Willy Tarreau wrote: > > >>> On Sat, May 02, 2009 at 09:33:35AM +0200, Bart Van Assche wrote: > > >>>> On Fri, May 1, 2009 at 10:59 AM, Andrew Morton > > >>>> > > >>>> wrote: > > >>>>> On Thu, 30 Apr 2009 13:26:36 +0200 Philipp Reisner wrote: > > >>>>>> This is a repost of DRBD > > >>>>> > > >>>>> Is it being used anywhere for anything? If so, where and what? > > >>>> > > >>>> One popular application is to run iSCSI and HA software on top of > > >>>> DRBD in order to build a highly available iSCSI storage target. > > >>> > > >>> Confirmed, I have several customers who're doing exactly that. > > >> > > >> I will also say that there are a lot of us out here who would have a > > >> use for DRDB in our HA setups, but have held off implementing it > > >> specificly because it's not yet in the upstream kernel. > > > > > > Actually, that's not a particularly strong reason because we already > > > have an in-kernel replicator that has much of the functionality of drbd > > > that you could use. The main reason for wanting drbd in kernel is that > > > it has a *current* user base. > > > > > > Both the in kernel md/nbd and drbd do sync and async replication with > > > primary side bitmaps. The main differences are: > > > > > > * md/nbd can do 1 to N replication, > > > * drbd can do active/active replication (useful for cluster > > > filesystems) > > > * The chunk size of the md/nbd is tunable > > > * With the updated nbd-tools, current md/nbd can do point in time > > > rollback on transaction logged secondaries (a BCS requirement) > > > * drbd manages the mirror state explicitly, md/nbd needs a user > > > space helper > > > > > > And probably a few others I forget. > > > > one very big one: > > > > DRDB has better support for dealing with split brain situations and > > recovering from them. > > I don't really think so. The decision about which (or if a) node should > be killed lies with the HA harness outside of the province of the > replication. > > One could argue that the symmetric active mode of drbd allows both nodes > to continue rather than having the harness make a kill decision about > one. However, if they both alter the same data, you get an > irreconcilable data corruption fault which, one can argue, is directly > counter to HA principles and so allowing drbd continuation is arguably > the wrong thing to do. > When you do asynchronous replication, how do you ensure that implicit write-after-write dependencies in the stream of writes you get from the file system above, are not violated on the secondary ? There might be a disk scheduler on the secondary. -Phil -- : Dipl-Ing Philipp Reisner : LINBIT | Your Way to High Availability : Tel: +43-1-8178292-50, Fax: +43-1-8178292-82 : http://www.linbit.com DRBD(R) and LINBIT(R) are registered trademarks of LINBIT, Austria.