Re: Wow! Is memory ever cheap!

[H. Peter Anvin]

Followup to:  <[e-mail]>
By author:    Edgar Toernig <[e-mail]>
In newsgroup: linux.dev.kernel
> 
> I think you have a wrong idea why the ECC is there.  ECC deals with
> the inherit shortcommings of DRAM.
> 
> DRAMs are not perfect.  They have a probability to lose a bit.
> Normally this probability is low enough to live with it.  Lets say
> you have a system with 1MByte and let's say the probability for a
> single bit error is around 1 error in 100 years.  Good enough.
> Now put 1GByte in the system. You'll get a probability of 10 errors
> per year.  Maybe good enough for a Windows box but not acceptable
> for your server.  So you put in ECC to bring this probability back
> into reasonable numbers.  ECC can correct the single bit errors.
> You only have to deal with double bit errors.  Chance for them is
> much much lower.
> 

Yes, ECC, unlike parity, is a technique for reducing the error rate,
with the side benefit of intercepting an error when it happens.

I am not disagreeing with Larry that integrity checks are a Good
Thing[TM], and in general are a hallmark of good engineering.
However, they are not a replacement for ECC for the purpose of driving
the failure rate down into an acceptable probability range.

It is of course a very nice thing that DRAM prices have come down into
the range where buying them in gigabyte quantities are reasonable :)

	-hpa
-- 
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