On Thu, 26 Nov 2009, Asdo wrote:

Justin Piszcz wrote:

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On Thu, 26 Nov 2009, Asdo wrote:

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BTW I would like to ask an info on "readonly" mode mentioned here:
http://www.mjmwired.net/kernel/Documentation/md.txt
upon read error, will it initiate a rebuild / degrade the array or not?

This is a good question but it is difficult to test as each use case is
different. That would be a question for Neil.

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Anyway the "nodegrade" mode I suggest above would be still more useful 
because you do not need to put the array in readonly mode, which is 
important for doing backups during normal operation.

Coming back to my problem, I have thought that the best approach would 
probably be to first collect information on how good are my 12 drives, 
and I probably can do that by reading each device like
dd if=/dev/sda of=/dev/null
and see how many of them read with errors. I just hope my 3ware disk 
controllers won't disconnect the whole drive upon read error.
(anyone has a better strategy?)

I see where you're going here.  Read below but if you go this route I 
assume
you would first stop the array (?) mdadm -S /dev/mdX and then test each
individual disk one at a time?

I don't plan to stop the array prior to reading the drives. Reads should not 
be harmful...
You think otherwise?

Depends on the drives, ever try to copy a file from a failing drive? Some
drives will start to click/reset/ATA errors, etc.  Depends on how it is
failing and what is wrong with it.

I think I did read the drives in the past on a mounted array and it was no 
problem.

Good to hear, let us know which option you choose & what the outcome is!

Justin.