On Tue, Jun 10, 2014 at 11:48:46PM -0700, Bart Kus wrote:

Hello,

As far as I understand, md-raid relies on the underlying devices to inform
it of IO errors before it'll seek redundant/parity data to fulfill the read
request.  I have, however, seen certain hard drives report successful reads
while returning garbage data.

Is it possible to set md-raid into a paranoid mode, in which it reads all
available data and confirms integrity?  Here's how it would work:

RAID6: read data + parity 1 + parity 2.  If 1 of the 3 mismatches, correct
it, and write corrected data to the corrupt source.  Log the event.  If all
3 disagree, alert user somehow.
RAID5: read data + parity.  If they mismatch, alert user somehow.
RAID1: read data 1 + data 2.  If they mismatch, alert user somehow.

You can see this is mostly useful for RAID6 mode, where there is a chance at
automated recovery.  However, it can also be used to prevent silent data
corruption in the other modes, by making it not silent.

Hi Bart,

this was discussed some times ago, mainly for RAID6.
One compromise was "raid6check", which run in user space.

The main objection is the performance drop, that such a
reading method will have, which, of course, will require
an enable/disable switch.

I do not want to write for Neil, but I guess reasonable
patches, doing what you propose, will be accepted.

bye,

pg

--Bart

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piergiorgio