From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Why does one get mismatches? Date: Tue, 02 Feb 2010 10:11:03 -0500 Message-ID: <4B684087.50001@tmr.com> References: <869541.92104.qm@web51304.mail.re2.yahoo.com> <4B67451F.8040206@tmr.com> <20100202093738.44b4fece@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: <20100202093738.44b4fece@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: Jon@eHardcastle.com, linux-raid@vger.kernel.org List-Id: linux-raid.ids Neil Brown wrote: > On Mon, 01 Feb 2010 16:18:23 -0500 > Bill Davidsen wrote: > > >> Comment: when there is a three way RAID-1, why doesn't repair *vote* on >> the correct value instead of just making a guess? >> >> > > Because truth is not democratic. > > (and I defy you to define "correct" in any general way in this context). > If you are willing to accept that the reconstructed data from RAID-[56] is "correct" then the data from RAID-1 majority opinion is "correct." If you say that such recovered data is the "most likely to match what was written," then data consistent on (N+1)/2 drives of a RAID-1 should be viewed in the same light. Call it "most likely to be correct" if you prefer, but picking a value from a drive at random is less likely. This whole discussion simply shows that for RAID-1 software RAID is less reliable than hardware RAID (no, I don't mean fake-RAID), because it doesn't pin the data buffer until all copies are written. -- Bill Davidsen "We can't solve today's problems by using the same thinking we used in creating them." - Einstein