Joachim Otahal wrote:

Current Situation in RAID:
If a drive fails silently and is giving out wrong data instead of read 
errors there is no way to detect that corruption (no fun, I had that a 
few times already).

That is almost certainly a hardware issue, the chances of silent bad 
data are tiny, the chances of bad hardware messing the data is more 
likely. Often cable issues.

Even in RAID1 with three drives there is no "two over three" voting 
mechanism.

A workaround for that problem would be:
Adding one sector to each chunk to store the time (in nanoseconds 
resolution) + CRC or ECC value of the whole stripe, making it possible 
to see and handle such errors below the filesystem level.
Time in nanoseconds only to differ between those many writes that 
actually happen, it does not really matter how precise the time 
actually is, just every stripe update should have a different time 
value from the previous update.

Unlikely to have meaning, there is so much caching and delay that it 
would be inaccurate. A simple monotonic counter of writes would do as 
well. And I think you need to do it at a lower level than chuck, like 
sector. Have to look at that code again.

It would be an easy way to know which chunks are actually the latest 
(or which contain correct data in case one out of three+ chunks has a 
wrong time upon reading). A random uniqe ID or counter could also do 
the job of the time value if anyone prefers, but I doubt since the 
collision possibility would be higher.

You can only know the time when the buffer is filled, after that you 
have write cache, drive cache, and rotational delay. A count does as 
well and doesn't depend on time between PCUs being the same at ns level.

The use of CRC or ECC or whatever hash should be obvious, their 
existence would make it easy to detect drive degration, even in a 
RAID0 or LINEAR.

There is a ton of that in the drive already.

Bad side: Adding this might break the on the fly raid expansion 
capabilities. A workaround might be using 8K(+ one sector) chunks by 
default upon creation or the need to specify the chunk size on 
creation (like 8k+1 sector) if future expansion capabilities are 
actually wanted with RAID0/4/5/6, but that is a different issue anyway.

Question:
Will RAID4/5/6 in the future use the parity upon read too? Currently 
it would not detect wrong data reads from the parity chunk, resulting 
in a disaster when it is actually needed.

Do those plans already exist and my post was completely useless?

Sorry that I cannot give patches, my last kernel patch + compile was 
2.2.26, since then I never compiled a kernel.

Joachim Otahal

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Bill Davidsen [off-list ref]
  "We can't solve today's problems by using the same thinking we
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