From mboxrd@z Thu Jan 1 00:00:00 1970 From: keld@keldix.com Subject: Re: bug: 4-disk md raid10 far2 can be assembled clean with only two disks, causing silent data corruption Date: Wed, 26 Sep 2012 11:08:03 +0200 Message-ID: <20120926090803.GA21499@www5.open-std.org> References: <50606207.7040804@gooseman.cz> <20120925141959.0c22de7d@notabene.brown> <152edbf7bdad33717477f174f94116b7@192.168.93.35> <20120925223227.130ef8e3@notabene.brown> <50628B39.90205@gooseman.cz> <20120926154107.1568a115@notabene.brown> <20120926082848.GA18616@www5.open-std.org> <5062C3FE.7090906@yuiop.co.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <5062C3FE.7090906@yuiop.co.uk> Sender: linux-raid-owner@vger.kernel.org To: John Robinson Cc: NeilBrown , Jakub Hus?k , Mikael Abrahamsson , linux-raid@vger.kernel.org List-Id: linux-raid.ids On Wed, Sep 26, 2012 at 09:59:42AM +0100, John Robinson wrote: > On 26/09/2012 09:28, keld@keldix.com wrote: > [...] > >I also understand it now, I think. raid10,f2 with 4 disks cannot in the > >current implementation > >survive 2 failing disks. > > It can, but only two non-adjacent discs. With 4 drives sda-sdd, than > means you can only lose both sda and sdc, or sdb and sdd. Agree > >We have discussed earlier how to implement raid10,far that would mean > >better survival chances with more disks failing. This is not implemented > >yet. > > No, but even if/when it is, there will still be some combinations of two > discs that you cannot afford to lose. The layout change to try to > improve redundancy will not be generic, as it doesn't work for an odd > number of discs, so the existing layout would have to be retained as an > option. Well, at least for backward compatibility we need an option for the current layout. For odd number of disks, I do think we can improve the chances for more failing disks, as discussed earlier. best regards keld