From mboxrd@z Thu Jan 1 00:00:00 1970 From: Phil Turmel Subject: Re: Suboptimal raid6 linear read speed Date: Sat, 19 Jan 2013 20:37:08 -0500 Message-ID: <50FB4A44.5060004@turmel.org> References: <20130115123301.GA11948@rabbit.us> <50F55046.7050605@turmel.org> <20130115125507.GA12184@rabbit.us> <50F614F7.20104@hardwarefreak.com> <20130116025857.GA31112@rabbit.us> <50F70DB7.6020104@hardwarefreak.com> <50F90857.3010305@hardwarefreak.com> <50F9D32F.7090606@hardwarefreak.com> <50FB22D8.5000803@hardwarefreak.com> <50FB3185.7060009@ultratux.net> <6675858F-43FC-48CE-B146-856F50481289@colorremedies.com> <50FB3F34.3000302@ultratux.net> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <50FB3F34.3000302@ultratux.net> Sender: linux-raid-owner@vger.kernel.org To: Maarten Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 01/19/2013 07:49 PM, Maarten wrote: > On 01/20/13 01:16, Chris Murphy wrote: >>=20 >> On Jan 19, 2013, at 4:51 PM, Maarten wrote: >>=20 >>> On 01/19/13 23:48, Stan Hoeppner wrote: >>>> On 1/19/2013 1:43 AM, Mikael Abrahamsson wrote: >>>>=20 >>>>> With a BER of 10^-14 you have a 16% risk of getting URE when >>>>> reading an entire 2TB drive. >>>> On 1/19/2013 7:21 AM, Roy Sigurd Karlsbakk wrote: >>>>=20 >>>>> ok, perhaps, maybe, but then it's 17% chance of losing data >>>>> after a mirror or raid-5 rebuild with 2TB drives... >>>> Where are you guys coming up with this 16-17% chance of URE on >>>> any single full read of this 2TB, 10E14 drive? The URE rate >>>> here is 1 bit for every 12.5 trillion bytes. Thus, >>>> statistically, one must read this drive more than 6 times to >>>> encounter a URE. Given that, how is any single full read >>>> between the 1st and the 6th going to have a 16-17% chance of >>>> encountering a URE for that one full read? That doesn't make=20 >>>> sense. >>> Sorry but now I have to speak up too. Of course that 16-17% >>> figure is right! Did you miss out on math classes ? It is all >>> statistics. There is a chance of '1.0' to get one URE reading >>> 12.5 TB. That URE may be encountered at the very start of the >>> first TB, or it may not come at all, because that is how >>> statistics work. But *on*average*, you'll get 1.0 URE per 12.5 >>> TB, ergo, 0.16 per 2.0 TB. Basic simple math=E2=80=A6 jeez. >>=20 >> Please explain this basic, simple math, where a URE is equivalent >> to 1 bit of information. And also, explain the simple math where >> bit of error is equal to a URE. And please explain the simple math >> in the context of a conventional HDD 512 byte sector, which is 4096 >> bits. >>=20 >> If you have a URE, you have lost not 1 bit. You have lost 4096 >> bits. A loss of 4096 bits in 12.5TB (not 12.5TiB) is an error rate >> of 1 bit of error in 2.44^10 bits. That is a gross difference from >> published error rates. >>=20 >> And then explain how the manufacturer spec does not actually report >> the URE in anything approaching "on average" terms, but *less than* >> 1 bit in 10^14. If you propose the manufacturers are incorrectly >> reporting the error rate, realize you're basically accusing them of >> a rather massive fraud because less than 1 bit of error in X, is a >> significantly different thing than "on average" 1 bit of error in >> X. This could be up to, but not including, a full order magnitude >> higher error rate than the published spec. It's not an >> insignificant difference. >=20 > All very nice, but that is not the point, is it. The point is, to=20 > calculate (or rather: estimate) the odds of an URE encounter when=20 > reading 2TB, based on the figure one has for reading 12,5 TB. > Whether that 12,5 figure is correct or not, whether endorsed by > manufacturers or not, is totally irrelevant. It simply boils down > to, if there are 10 X's in every 10G Y's, then there are 2 X's in > every 2G Y's. Yes ? On *average* ! The odds of an error within a given period of reading is *not* a linear function of the average. With your simplistic math, the odds of an error while reading 25TB would be 200% ! Ummm, no. Probability goes from 0 to 100%. It would be nice if statistics were simple, as they are very useful in understanding the world around us. Unfortunately, statistics aren't si= mple. Please see my other post on the Poisson distribution. Phil -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html