On Sat, 30 Jan 2010, John Robinson wrote:

I think the problem is that modern discs haven't got more reliable, but they 
have got much, much bigger; "modern" 2G discs had a read error rate of 1 bit 
per 10^14, and current 2T discs have the same, so while on a 2G disc you 
could read the whole surface of the disc tens of thousands of times before 
being likely to get a read error, now it's only tens of times. Someone did 
recently post links to a formal article analysing this subject to this list, 
but I can't find it :-(

I think the 4k sector size on WD20EARS (for instance) is supposed to add 
more ECC information but I'm not sure how this will affect the 10^14 error 
rate. I think the manufacturers need to work a bit more on it, I don't see 
how these drives can be used in single drive configuration with the 
current 10^14 value.

I don't have experience with any other drives than the WD20EADS but they 
develop read errors at a rate higher than any other drive I've 
experienced before (and I started doing computer stuff in the 80ties). 
Yes, the large size is absolutely a factor, so the per-block read error 
rate might not be too high, but the read error per drive is definitely so 
(and I use the default ubuntu behaviour and read scrub every week).

I'm going RAID6 as soon as I can because of this, I'm tired of this 
bit-rot when I ddrescue things because of this (hopefully it's just 4k of 
data in the middle of a file which I care little about).

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se