From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gionatan Danti Subject: RAID 10 far and offset on-disk layouts Date: Fri, 27 Dec 2013 15:29:49 +0100 Message-ID: <52BD8EDD.10809@assyoma.it> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids Hi all, I (think of) quite well understand how far and offset work, but I can not find any data on the precise on-disk layout. 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? OFFSET LAYOUT md(4) states: "When 'offset' replicas are chosen, the multiple copies of a given chunk are laid out on consecutive drives and at consecutive offsets. Effectively each stripe is duplicated and the copies are offset by one device." This means a schema like this: 3) A1 A2 A3 A4 A4 A1 A2 A3 .. .. .. .. However, this is susceptible to any consecutive two-disk failures. A schema like 4) A1 A2 A3 A4 A2 A1 A4 A3 would not suffer from this problem (eg: disk 2 & 3 can fail and the array is still working). Question 2: apart from simplicity, why the offset layout use the schema as n.3? I miss something? 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