[mythtv-users] Fwd: Further Notice of Seagate Hard Drive Class Action and Proposed Settlement

Brian Wood beww at beww.org
Sat Mar 13 00:35:15 UTC 2010


On Friday 12 March 2010 05:18:54 pm Manuel McLure wrote:
> On Fri, Mar 12, 2010 at 2:01 PM, Ben Kamen <bkamen at benjammin.net> wrote:
> > With Intel/IBM gear, I always kind of scratched my head because my job
> > was to repair intel based PC's and they weren't any more reliable because
> > of that 9th bit.
> 
> The point of parity on RAM is not necessarily to correct errors
> (although ECC can do that in _some_ cases) - it's to detect memory
> errors before they can cause extensive data corruption. The idea is
> that once you get a bad bit, there's no telling what consequences it
> could have. For example, a bad bit in a pointer address could cause
> data to be written to the wrong part of memory, clobbering data that
> then could get written to external storage. A bad bit in the counter
> for a loop could cause you to write 66536 bytes instead of 1000 bytes.
> That one bad bit could cause thousands of bytes of data to be
> corrupted. With simple one byte parity you stop the processor as soon
> as the bad bit is detected, and the corruption has little chance to
> spread. This actually may make a machine with bad RAM crash more often
> but greatly reduces the chance of extensive data corruption.
> 
> On more modern systems, multiple bytes (usually 8) and their parity
> bits are combined - this is what's known as ECC. The ECC can detect
> and correct single bit errors, and can detect two bit errors but not
> correct them. In that case it will crash the system exactly in the
> same way as single byte parity does.
> 
Thanks, so it's the detection more than the correction that's important. I 
hadn't thought that through.


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