On Jan 19, 2013, at 11:26 PM, Mikael Abrahamsson <swmike@xxxxxxxxx> wrote: > On Sat, 19 Jan 2013, Chris Murphy wrote: > >> 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. > > I have seen your point of view posted in other discussions, and I don't buy it. I believe the manufacturers are talking about how many bits read before there is one or more bit error (ie can't error correct the bit errors on that sector, so now the whole sector is URE. Since the sector is an atomic unit, the drive can't report a single bit error (even though that's probably what it is), it'll URE the whole 4k bytes. The manufacturer is still talking about what's on the platter, not what the OS sees. You haven't said a single thing that contradicts what I've said. I'm not talking at all about the OS. I am in fact referring to bits read before there is a bit of error. You simply aren't going to get a URE every 12.5TB, with a disk purporting to have "less than 1 bit error in 1E14 bits" because such a rate of error is *NOT* 1 bit in 1E14 bits. It's like arguing 2+2=5 and then blabbing on for 10 minutes asserting your belief it's true. > > Your view on how this works would mean that drives would read more than 10^3 more data before an URE, which from my empirical data isn't right. Also, for a 4k sector drive, with your logic, would have 8 times better BER ratio which I don't believe either. In fact the whole point of the 4K sector size is to improve ECC, and reduce error rate. This is a stated, yet de-emphasized goal of AF disks. Yet it's still consistent with the "less than 1 bit in X bits" language, which is a minimum expected performance criteria. We don't know what the top end is. > > I believe Phil was spot on when it comes to how it works. His post "19 Jan 2013 18:53:41" is exactly how I believe things work. This explanation is identical to religious belief explanations. You believe it because you believe it. It's circular. There is no new information here, at all. Mere disagreement with my position useless. 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
