John Robinson wrote:
On 30/01/2010 21:33, Mikael Abrahamsson wrote:
[...]
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.
iirc part of the point of moving to 4K sectors is to improve the error
correction to something like 1 in 10^20 or 22 without losing storage
density, partly by using what was lost before in inter-sector gaps and
partly because you can do better with more bits of ECC over more data.
I remember the other way around: the purpose of 4k was to keep the same
error rate while saving storage space
You have links?
I got my impression from here http://lwn.net/Articles/322777/ but it's
not explicitly written
Frankly I wish they'd sacrificed a little storage density and improved
the error rate a long time ago.
Me too
Anyway I think you'll never know how many ECC bits a brand uses. It's
not declared. They just declare the estimated error rate which I think
is the estimate of how likely is a surface defect and how likely is that
to be fixable by the ECC algorithm. So the discussion has no real meaning...
There is another maybe more important factor: how likely is silent data
corruption. IIRC reed-solomon algorithms can be tuned (at the same
number of bits) for more correction and less detection of errors, or for
more detection and less correction. If the detection is insufficient,
you get garbage data when reading the sector, but the disk does not
return a read error so the OS takes it as good data. Every 1 bit of
error correction costs the same number of ECC bits as 2 bits of error
detection. The disk makers never tell you how their algorithm is balanced.
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