Disk reliability (Was: Node cloning)

[Josip Loncaric]

Mark Hahn wrote:
> 
> > What size of CRCs are being used?
> 
> hmm, I'll admit I never actually looked at the details.
> the CRC is 16b (not really surprising, since ATA is that wide):
> G(X) = X15 + X12 + X5 + 1.

The ATA channel is usually quite reliable.  BadCRC errors are rarely
seen, and the probability of missing one despite the 16-bit CRC is 2^16
times lower.  This means that there is no need to worry unless the
system is regularly reporting lots of BadCRC events.  If your target
MTBF is one year, "lots" means about a few hundred BadCRC events per
day.

Sincerely,
Josip

P.S.  You need at least UltraDMA mode 2 to get CRC protection (=> you do
not want to turn off DMA without a really good reason).  Lower modes
(PIO or multiword DMA) do not have CRC protection.

-- 
Dr. Josip Loncaric, Research Fellow               mailto:josip at icase.edu
ICASE, Mail Stop 132C           PGP key at http://www.icase.edu./~josip/
NASA Langley Research Center             mailto:j.loncaric at larc.nasa.gov
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