Re: assemble vs create an array.......

[David Greaves <david@dgreaves.com>]

Neil Brown wrote:
> On Thursday November 29, dragos@mpigani.org wrote:
>> 2. Do you know of any way to recover from this mistake? Or at least what 
>> filesystem it was formated with.
It may not have been lost - yet.


> If you created the same array with the same devices and layout etc,
> the data will still be there, untouched.
> Try to assemble the array and use "fsck" on it.
To be safe I'd use fsck -n (check the man page as this is odd for reiserfs)


> When you create a RAID5 array, all that is changed is the metadata (at
> the end of the device) and one drive is changed to be the xor of all
> the others.
In other words, one of your 3 drives has just been erased.
Unless you know the *exact* command you used and have the dmesg output to hand
then we won't know which one.

Now what you need to do is to try all the permutations of creating a degraded
array using 2 of the drives and specify the 3rd as 'missing':

So something like:
mdadm --create /dev/md0 --raid-devices=3 --level=5 missing /dev/sdb1 /dev/sdc1
mdadm --create /dev/md0 --raid-devices=3 --level=5 /dev/sdb1 missing /dev/sdc1
mdadm --create /dev/md0 --raid-devices=3 --level=5 /dev/sdb1 /dev/sdc1 missing
mdadm --create /dev/md0 --raid-devices=3 --level=5 missing /dev/sdb1 /dev/sdd1
mdadm --create /dev/md0 --raid-devices=3 --level=5 /dev/sdb1 missing /dev/sdd1
mdadm --create /dev/md0 --raid-devices=3 --level=5 /dev/sdb1 /dev/sdd1 missing
etc etc

It is important to create the array using a 'missing' device so the xor data
isn't written.

There is a program here: http://linux-raid.osdl.org/index.php/Permute_array.pl
that may help...

David