Bill Davidsen schrieb:
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.
In over 20 years (including our customer drives) about ten harddrives of
that type. Does indeed not happen often. Were not cable issues, we
replaced the drive with the same type and vendor and RMA'd the original.
It is not vendor specific, it's like every vendor does have such
problematic drives during their existence. The last case was just a few
month ago.
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.
From what I know from the docs: The "stripe" is normally 64k, so the
"chunk" on each drive when using raid5 with three drives is 32k, smaller
with more drives. At least that is what I am referring to : ). The
filesystem level never sees what is done on the raid level not even in
the ZFS implementation on linux which was originally designed for such a
case.
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.
That is mainly meant to know whether the stripe is consistent (after
power fail etc), and if not, correct it. Currently that cannot be
detected, especially since the the partiy is not read in the current
implementation (at least the docs say so!). If it can be reconstructed
using the ECC and/or parity write the corrected data back silently (if
mounted rw) to get the data consistent again. For successful silent
correction only one syslog line would be enough, if correction is not
possible it can still go back to the current default behaviour, read
whatever is there, but at least we could _detect_ such inconsistency.
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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