Linux – mdadm & raid6: does “re-create(with different chunk size) + resync” destroy orig. data

linuxmdadmraidraid6

I would like to rescue data on my software raid-6 array. I did some stupid actions (described bellow) with this original array.

Main Question:

I need to know if original data stored on raid-6 array are definitely lost (or not) after the following actions have been prepared upon this array (executed in the order listed bellow):

  1. zeroing super-blocks of all the
    active disks/partitions registered
    in the array

  2. executing the "mdadm –create …"
    command using different options
    (see bellow for list) than have been
    used when array have been created
    originally:
    -> different chunk size
    -> different layout
    -> different disks order

  3. resync-ing the array

Note: Specific values of mdadm parameters should not be relevant here, since this is about the principle how mdadm works …

I think points 1) & 2) should not even touch the original data since they are supposed to manipulate just superblocks

I see the 3) point as most critical from data lost point of view: I'm not sure what exactly is happening with array during resync, but based on heavy activity of all involved hard drives (for ~7 hours) I assume the data storage area is completely re-processed …

Sub-questions:

  1. Does ordering of hard drives/partitions (as they are ordered on mdadm command line) play role for raid6 creation & initial resync?

  2. What everything is necessary to backup after array creation to be able to safely re-create the array in similar situation as my is (e.g. backup superblocks informations & partition table info for each disk involved in the array …)?

Comment:

The mdadm wiki article (http://en.wikipedia.org/wiki/Mdadm) should be revised and author should be kicked in to ass a little bit, or more then just a little bit …
The article mentions the zeroing superblocks & subsequent array re-creation as solution for getting rid of the "mdadm: Cannot open …: Device or resource busy" issue.
Author somehow forgot to mention important step – to backup the parameters the original array (superblocks) as the first step … and also my investigation seems to point out that the ordering of the involved disks/partitions plays role as well …

Thanks for answers,
Peter

Best Answer

I looked at the wikipedia article, and the bit about deleting the superblock etc. is right, but it's supposed to be used for solving a different problem than what you were apparently trying to solve. The intention in the article is to "clean out" a drive that has old RAID settings on it so that you can use it in a new array.

You should, of course, never clear the superblock on a drive that currently contains data that you want to preserve. The superblock contains critical information regarding how the data on that drive is organized, so erasing or changing the superblock it is unhelpful unless you intend to throw away the data with it.

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