Re: My superblocks have gone missing, can't reassemble raid5

[Leslie Rhorer <lesrhorer@att.net>]

On 5/19/2021 1:00 PM, Wols Lists wrote:
> On 19/05/21 18:08, Leslie Rhorer wrote:
>>>> There are only two other possible permutations of three disks.  If
>>>> none of those work, you have some more serious problems.
>>>
>>> And here you are oversimplifying the problem immensely. If those three
>>> drives aren't the originals
>>
>>      Hang on.  Which drives do you mean?
> 
> The drives he originally ran --create on to create the array in the
> first place.

	OK, after a double-take, I wasn't sure.

> The ONLY time you can be reasonably confident that running --create WILL
> recover a damaged array is if it is still in its original state - no
> drives swapped, no admin changes to the array, AND you're using the same
> version of mdadm.

	That's a little bit of an overstatement, depending on what you mean by 
"reasonably confident".  Swapped drives should not ordinarily cause an 
issue, especially with RAID 4 or 5.  The parity is, after all, 
numerically unique.  Admin changes to the array should be similarly 
fully established provided the rebuild completed properly.  I don't 
think the parity algorythms have changed over time in mdadm, either. 
Had they done so, mdadm would not be able to assemble arrays from 
previous versions regardless of whether the superblock was intact.

	The main point with respect to my previous post, however, is one 
needn't be confident at all.  Hopeful, perhaps, but one needn't have any 
certainty at all the attempt will work, since one is no worse off if the 
attempt fails than prior to the attempt.  I certainly would not bet the 
farm on it working.  After all, as I mentioned before, there may well be 
something else really, really wrong going on here.