On Jan 19, 2013, at 4:51 PM, Maarten <maarten@xxxxxxxxxxxx> wrote: > On 01/19/13 23:48, Stan Hoeppner wrote: >> On 1/19/2013 1:43 AM, Mikael Abrahamsson wrote: >> >>> 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: >> >>> 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 >> 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… jeez. 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. 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. 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. Chris Murphy-- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
