Re: Wow! Is memory ever cheap!

H. Peter Anvin
2001-05-10T22:45:24Z

Followup to: <3AF9B0D2.F2E330F9@gmx.de>
By author: Edgar Toernig <froese@gmx.de>
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
--
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."