Help on first dangerous scrub / suggestions

[Asdo <asdo@shiftmail.org>]

Hi all
we have a server with a 12 disks raid-6.
It has been up for 1 year now but I have never scrubbed it because at 
the time I did not know about this good practice (a note on man mdadm 
would help).
The array is currently not degraded and has spares.

Now I am scared about initiating the first scrub because if it turns out 
that 3 areas in different disks have bad sectors I think am gonna lose 
the whole array.

Doing backups now it's also scary because if I hit a bad (uncorrectable) 
area in anyone of the disks while reading, a rebuild will start on the 
spare and that's like initiating the scrub with all associated risks.

About this point, I would like to suggest a new "mode" of the array, 
let's call it "nodegrade" in which no degradation can occur, and I/O in 
unreadable areas simply fails with I/O error. By temporarily putting the 
array in that mode, at least one could backup without anxiety. I 
understand it would not be possible to add a spare / rebuild in this 
mode but that's ok.

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?

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?)

But then if it turns out that 3 of them indeed have unreadable areas I 
am screwed anyway. Even with dd_rescue there's no strategy that can save 
my data, even if the unreadable areas have different placement in the 3 
disks (and that's a case where it should instead be possible to get data 
back).

This brings to my second suggestion:
I would like to see 12 (in my case) devices like:
  /dev/md0_fromparity/{sda1,sdb1,...}   (all readonly)
that behave like this: when reading from /dev/md0_fromparity/sda1 , what 
comes out is the bytes that should be in sda1, but computed from the 
other disks. Reading from these devices should never degrade an array, 
at most give read error.

Why is this useful?
Because one could recover sda1 from a disastered array with multiple 
unreadable areas (unless too many are overlapping) in this way:
With the array in "nodegrade" mode and blockdevice marked as readonly:
  1- dd_rescue if=/dev/sda1 of=/dev/sdz1   [sdz is a good drive to 
eventually take sda place]
     take note of failed sectors
  2- dd_rescue from /dev/md0_fromparity/sda1 to /dev/sdz1 only for the 
sectors that were unreadable from above
  3- stop array, take out sda1, and reassemble the array with sdz1 in 
place of sda1
... repeat for all the other drives to get a good array back.

What do you think?

I have another question on scrubbing: I am not sure about the exact 
behaviour of "check" and "repair":
- will "check" degrade an array if it finds an uncorrectable read-error? 
The manual only mentions what happens if the checksums of the parity 
disks don't match with data, but that's not what I'm interested in right 
now.
- will "repair" .... (same question as above)

Thanks for your comments