From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from fw6a-delilah.wadns.net ([196.220.39.210]:25582 "EHLO fw6a.wadns.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751288AbaEBJ22 (ORCPT ); Fri, 2 May 2014 05:28:28 -0400 Message-ID: <5363652D.2080602@swiftspirit.co.za> Date: Fri, 02 May 2014 11:28:13 +0200 From: Brendan Hide MIME-Version: 1.0 To: Duncan <1i5t5.duncan@cox.net>, linux-btrfs@vger.kernel.org CC: Russell Coker Subject: Re: Help with space References: <2809235.lZD2oazSeA@xev> <201405021148.07577.russell@coker.com.au> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 02/05/14 10:23, Duncan wrote: > Russell Coker posted on Fri, 02 May 2014 11:48:07 +1000 as excerpted: > >> On Thu, 1 May 2014, Duncan <1i5t5.duncan@cox.net> wrote: >> [snip] >> http://www.eecs.berkeley.edu/Pubs/TechRpts/1987/CSD-87-391.pdf‎ >> >> Whether a true RAID-1 means just 2 copies or N copies is a matter of >> opinion. Papers such as the above seem to clearly imply that RAID-1 is >> strictly 2 copies of data. > Thanks for that link. =:^) > > My position would be that reflects the original, but not the modern, > definition. The paper seems to describe as raid1 what would later come > to be called raid1+0, which quickly morphed into raid10, leaving the > raid1 description only covering pure mirror-raid. Personally I'm flexible on using the terminology in day-to-day operations and discussion due to the fact that the end-result is "close enough". But ... The definition of "RAID 1" is still only a mirror of two devices. As far as I'm aware, Linux's mdraid is the only raid system in the world that allows N-way mirroring while still referring to it as "RAID1". Due to the way it handles data in chunks, and also due to its "rampant layering violations", *technically* btrfs's "RAID-like" features are not "RAID". To differentiate from "RAID", we're already using lowercase "raid" and, in the long term, some of us are also looking to do away with "raid{x}" terms altogether with what Hugo and I last termed as "csp notation". Changing the terminology is important - but it is particularly non-urgent. -- __________ Brendan Hide http://swiftspirit.co.za/ http://www.webafrica.co.za/?AFF1E97