From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Boldi Subject: Re: Two-disk RAID5? Date: Thu, 27 Apr 2006 07:15:43 +0300 Message-ID: <200604270715.43531.a1426z@gawab.com> References: <216D6FA68B0F304DA93E95C5B786F2D12FCBBC@bart.corp.egenera.com> <17487.63969.767902.768109@cse.unsw.edu.au> Mime-Version: 1.0 Content-Type: text/plain; charset="windows-1256" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <17487.63969.767902.768109@cse.unsw.edu.au> Content-Disposition: inline Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Neil Brown wrote: > (*) The term 'mirror' for raid1 has always bothered me because a > mirror presents a reflected image, while raid1 copies the data without > any transformation. > > With a 2drive raid5, one drive gets the original data, and the other > drive gets the data after it has been 'reflected' through an XOR > operation, so maybe a 2drive raid5 is really a 'mirrored' pair.... > Except that the data is still the same as XOR with 0 produces no > change. > So, if we made a tiny change to raid5 and got the xor operation to > start with 0xff in every byte, then the XOR would "reflect" each byte > in a reasonable meaningful way, and we might actually get a "mirrored > pair"!!! > > But I don't think that would provide any real value :-) Why not? Consider disks w/ 100mb/s thruput (theoretical): 2disk raid0 stripes data to yield 200mb/s read/write thruput. 2disk raid5 stripes data to yield 100mb/s write, 200mb/s read thruput always. 2disk raid1 mirrors data to yield 100mb/s write, 100/200mb/s single/multiple read thruput only. Would you think that this is enough of a real value? Thanks! -- Al