Bruce Allen wrote:
Dear LKML,
Apologies in advance for potential mis-use of LKML, but I don't know
where else to ask.
An ongoing study on datasets of several Petabytes have shown that there
can be 'silent data corruption' at rates much larger than one might
naively expect from the expected error rates in RAID arrays and the
expected probability of single bit uncorrected errors in hard disks.
The origin of this data corruption is still unknown. See for example
http://cern.ch/Peter.Kelemen/talk/2007/kelemen-2007-C5-Silent_Corruptions.pdf
In thinking about this, I began to wonder about the following. Suppose
that a (possibly RAID) disk controller correctly reads data from disk
and has correct data in the controller memory and buffers. However when
that data is DMA'd into system memory some errors occur (cosmic rays,
electrical noise, etc). Am I correct that these errors would NOT be
detected, even on a 'reliable' server with ECC memory? In other words
the ECC bits would be calculated in server memory based on incorrect
data from the disk.
It depends where the data got corrupted. Normally transfers over the PCI
or PCI Express bus are protected by parity (or CRC or something, I
assume on PCI-E) so errors there would get detected. This is quite rare
unless the motherboard or expansion card is faulty or badly designed
with timing problems.
However, it's conceivable that data could get corrupted inside the
controller, or inside the chipset. This seems quite rare however, except
in the presence of design flaws (like some VIA southbridges that had
nasty problems with losing data if PCI bus masters kept the CPU off the
PCI bus too long, which we have to work around).
The alternative is that disk controllers (or at least ones that are
meant to be reliable) DMA both the data AND the ECC byte into system
memory. So that if an error occurs in this transfer, then it would most
likely be picked up and corrected by the ECC mechanism. But I don't
think that 'this is how it works'. Could someone knowledgable please
confirm or contradict?
I don't know any controller that works in this way. This would greatly
increase CPU overhead since the CPU would need to perform this CRC
calculation.
--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from [email protected]
Home Page: http://www.roberthancock.com/
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