On 13/04/2012 06:29, Stan Hoeppner wrote:
On 4/12/2012 5:58 AM, Ed W wrote:
The claim by ZFS/BTRFS authors and others is that data silently "bit
rots" on it's own. The claim is therefore that you can have a raid1 pair
where neither drive reports a hardware failure, but each gives you
different data?
You need to read those articles again very carefully. If you don't
understand what they mean by "1 in 10^15 bits non-recoverable read error
rate" and combined probability, let me know.
OK, I'll bite. I only have an honours degree in mathematics from a well
known university, so grateful if you could dumb it down appropriately?
Lets start with what "those articles" are you referring to? I don't see
any articles if I go literally up the chain from this email, but you
might be talking about any one of the lots of other emails in this
thread or even some other email thread?
Wikipedia has it's faults, but it dumbs the "silent corruption" claim
down to:
http://en.wikipedia.org/wiki/ZFS
"an undetected error for every 67TB"
And a CERN study apparently claims "far higher than one in every 10^16 bits"
Now, I'm NOT professing any experience of axe to grind here. I'm simply
asking by what feature do you believe either software or hardware RAID1
is capable of detecting which pair is correct when both pairs of a raid
one disk return different results and there is no hardware failure to
clue us that one pair suffered a read error? Please don't respond with
a maths pissing competition, it's an innocent question about what levels
of data checking are done on each piece of the hardware chain? My
(probably flawed) understanding is that popular RAID 1 implementations
don't add any additional sector checksums over and above what the
drives/filesystem/etc add already offer - is this the case?
And this has zero bearing on RAID1. And RAID1 reads don't work the way
you describe above. I explained this in some detail recently.
Where?
Been working that way for more than 2 decades Ed. :) Note that "RAID1"
has that "1" for a reason. It was the first RAID level.
What should I make of RAID0 then?
Incidentally do you disagree with the history of RAID evolution on
Wikipedia?
http://en.wikipedia.org/wiki/RAID
Regards
Ed W
