md devices: Suggestion for in place time and checksum within theRAID

From: Joachim Otahal
Date: Sat Mar 13 2010 - 18:22:08 EST


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).
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.
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.
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.
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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