From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lars Marowsky-Bree Subject: Re: [PATCH 1/2] md bitmap bug fixes Date: Sat, 19 Mar 2005 13:58:21 +0100 Message-ID: <20050319125821.GO18819@marowsky-bree.de> References: <20050318103326.GA18819@marowsky-bree.de> <6ivqg2-qsn.ln1@news.it.uc3m.es> <20050318134255.GS18819@marowsky-bree.de> <7e6rg2-pj1.ln1@news.it.uc3m.es> <423B09EF.8070708@steeleye.com> <23krg2-4rr.ln1@news.it.uc3m.es> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Content-Disposition: inline In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: "Peter T. Breuer" , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 2005-03-19T12:43:41, "Peter T. Breuer" wrote: > Well, there is the "right data" from our point of view, and it is wha= t > should by on (one/both?) device by now. One doesn't get to recover t= hat > "right data" by copying one disk over another, however efficiently on= e > does it. It's about conflict resolution and recovery after a split-brain and concurrent service activation has occured. Read up on that here: http://www.linux-mag.com/2003-11/availability_01.html (see the blob about split-brain with drbd). It all depends on the kind of guarantees you need. > But neither mirror is necessarily right. We are already in a bad > situation. There is no good way out. You can merely choose which of > the two data possibilities you want for each block. They're not > necesarily either of them "right", but one of them may be, but which = one > we don't know. It's quite clear that you won't get a consistent state of the system by mixing blocks from either side; you need to declare one the 'winner', throwing out the modifications on the other side (probably after having them saved manually, and then re-entering them later). For some scenarios, this is acceptable. > Why should one think that copying all of one disk to the other (moral= ly) > gets one data that is more right than copying some of it? Nothing one > can do at this point will help. It's not a moral problem. It is about regaining consistency. Which one of the datasets you choose you could either arbitate via some automatic mechanisms (drbd-0.8 has a couple) or let a human decide. The default with drbd-0.7 is that they will detect this situation has occured and refuse to start replication unless the admin intervenes and decides which side wins. Sincerely, Lars Marowsky-Br=E9e --=20 High Availability & Clustering SUSE Labs, Research and Development SUSE LINUX Products GmbH - A Novell Business - To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html