From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gionatan Danti Subject: Re: On URE and RAID rebuild - again! Date: Tue, 05 Aug 2014 08:52:01 +0200 Message-ID: <53E07F11.5050103@assyoma.it> References: <53D8ACF0.1070202@assyoma.it> <53D8ED99.90606@assyoma.it> <20140731073121.38cd1773@notabene.brown> <53D9ED48.9000307@assyoma.it> <1370eb7a35b628323646a86094a26912@assyoma.it> <20140803134834.7773b0ab@notabene.brown> <53DF8A31.8060609@assyoma.it> <35916d10dab6084e6f28da2e0975fce7@assyoma.it> <20140805092951.7d8f8e6d@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: <20140805092951.7d8f8e6d@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: Mikael Abrahamsson , linux-raid@vger.kernel.org, g.danti@assyoma.it List-Id: linux-raid.ids On 05/08/2014 01:29, NeilBrown wrote: > On Tue, 05 Aug 2014 00:44:04 +0200 Gionatan Danti wrote: > > > Probabilities are often calculated by examining a statistical record - the > two concepts are not separate. > There is probably some theoretical analysis, some statistical analysis, some > marketing and maybe even some actuarial analysis that goes in to the quoted > figure. I remember when CPU speed was measured in "MIPS". > This stood for > Meaningless Indicators of Performance for Salesmen > > URE rates numbers are probably equally trustworthy. > > The probability number doesn't tell you much at all about your drive. > Your drive probably works much better than the quoted rate, but could be much > worse. > The quoted number might say something useful about a collection of 10,000 > drives, but if you can afford those, you can probably afford to competent > statistician to explain the details too. > > > > I'm not an electro-magnetic engineer, but I would guess that UREs are caused > by some combination of: > - irregularities in the physical media > - imperfections in positioning of the write head > - fluctuations in temperature and pressure which could > affect precise performance of resistors and capacitors etc. > > and probably various quantum effects that I know nothing about. > > Maybe most UREs come from a spec of dust that was in the wrong place at the > wrong time. > > If think a better summary would be: > in normal conditions and typical loads, a collection of 10^14 drives will > exhibit errors somewhere in the collection on a regular basis. > > > NeilBrown > VERY informative post. Thank you Neil. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8