On 27/11/2012 14:56, Roy Sigurd Karlsbakk wrote:
If this system is running RAID-6, recovery should be possible to
check both parity chunks, right?
Yes, of course. (And if anyone ever needs it, it is possible to
extend raid6 to 3 parity chunks. I've done the maths, but it is
not implemented - there doesn't seem to be a big need for it.) But
- again referring back to Neil's blog - if the low-level raid spots
a consistency error, it still cannot correct it reliably even with
2 parity chunks, and should pass on a read error to the higher
level raid. Using raid6 at the low level would let you do a good
consistency check even in the case of a failed drive (or a known
read error on a drive) - or two simultaneous undetected read
errors. And raid6 on the higher level raid would let you correct
such errors, even when there are other errors around. You'd soon
reach the point where it is more likely for your disks to
spontaneously turn into a bowl of petunias than for read errors to
be undetected or unrecoverable.
That would be nice. So what should be done here in the first place,
is to change the code to allow parity data to be read and calculated
also on reads?
Well, what should be done /first/ is to hope that some of the more
experienced md raid experts express an opinion on the idea - is it
possible, is it useful, and is it practical to implement?
The main aim would be to add an option to a md arrays that will turn
each read into an implicit scrub or check of the whole stripe, and that
a consistency error there would return a read error to the next layer of
md raid.
I can see plenty of scope for complications here, such as what to do on
normal (detected) read errors, or how to ensure that the upper layer
re-writes the whole stripe and not just part of it (or perhaps partial
re-writes would be enough). I am fully aware that I'm just giving a
rough idea here - it needs a lot more thought before anyone can start
changing code. But if my theory here is correct, and if it is practical
to implement, then it might be a useful tool for big data producers.
mvh.,
David
Vennlige hilsener / Best regards
roy
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