From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gionatan Danti Subject: Re: RAID 10 far and offset on-disk layouts Date: Mon, 13 Jan 2014 11:15:13 +0100 Message-ID: <52D3BCB1.1010200@assyoma.it> References: <52BD8EDD.10809@assyoma.it> <20131227151927.GA4003@www5.open-std.org> <52BD9B4F.3000509@assyoma.it> <20131227154952.GA6539@www5.open-std.org> <52CE57D9.1030501@assyoma.it> <20140113102021.1ef3e203@notabene.brown> <52D3A962.4000308@assyoma.it> <20140113204534.737a98f6@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20140113204534.737a98f6@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: linux-raid@vger.kernel.org, keld@keldix.com, Gionatan Danti List-Id: linux-raid.ids On 01/13/2014 10:45 AM, NeilBrown wrote: > On Mon, 13 Jan 2014 09:52:50 +0100 Gionatan Danti wrote: > >> Hi Neil, >> let me recap from a previous message: >> >> >FAR LAYOUT >> >md(4) states: >> >"The first copy of all data blocks will be striped across the early >part >> >of all drives in RAID0 fashion, and then the next copy of all blocks >> >will be striped across a later section of all drives, always ensuring >> >that all copies of any given block are on different drives" >> > >> >The "on different drives" part let me wonder _how_ are chunks >> >distributed. On a 4-disk array, I can imagine some different schemas: >> > >> >1) A1 A2 A3 A4 >> > .. .. .. .. >> > A4 A1 A2 A3 >> > >> >2) A1 A2 A3 A4 >> > .. .. .. .. >> > A2 A1 A4 A3 >> > >> >The first schema is the one depicted by SuSe documentation [1], while >> >the second is the one described by Wikipedia [2]. >> > >> >Question 1: as the two schema have different reliability >> >characteristics, which is really used? >> >> SuSe entry: >> https://www.suse.com/documentation/sles11/stor_admin/data/raidmdadmr10cpx.html#b7cynnk >> >> Wikipedia entry: >> http://en.wikipedia.org/wiki/Linux_MD_RAID_10#LINUX-MD-RAID-10 (see how >> far layout is depicted) >> >> Keld kindly told me that the SuSe is simply not updated, as it depict a >> situation changed with newer kernels. So my two questions: > > I cannot see an important difference between the two pages you reference. > Both appear to be correct. Mmm... they seem different to me. SeSe FAR Layout: sda1 sdb1 sdc1 sde1 0 1 2 3 4 5 6 7 . . . 3 0 1 2 7 4 5 6 Notice how (for example) sdb1 is coupled both to sda1 (0,4) and sdc1(1,5). If sdb1 fails, any sda1 or sdc1 failure lead to data loss. Now, Wikipedia FAR Layout: 4 drives (sda1, sdb1, sdc1, sdd1) -------------------- A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 .. .. .. .. A2 A1 A4 A3 A6 A5 A8 A7 A10 A9 A12 A11 .. .. .. .. Notice now how a single disk (eg: sdb1) is coupled to only another _single_ disk (eg: sda1). In this case, if sdb1 fails, you had to lose sda1 to have a data loss. Losing sdc1 or sdd1 will _not_ lead to data loss. I am wrong? Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8