Re: changing uuid of mdadm array
On Fri, 21 Oct 2022, Dan Ritter wrote:
Tim Woodall wrote:
On Wed, 19 Oct 2022, Lucas Castro wrote:
The array must be identified somehow, in mdadm, array is by uuid!
And I have two disks, which have the same uuid but are not part of the
same array.
How do I tell mdadm that they aren't part of the same array?
I can't believe I'm the only person who has cloned a machine by breaking
a raid1 array.
Or someone who has used dd to clone a disk that they then want to mount
in parallel with the original
mdadm --zero-superblock --force
can be used, carefully.
You will need to re-add the disk to the new array and let it
sync.
-dsr-
But why would I want to do that? If I wanted it to be part of the array
I'd have added it to the array, not cloned it with dd.
I have an array. I use dd to copy it onto a new disk. I now want to
start the original array (which is unchanged) and I also want to start
the cloned disk as a new array.
Or actually, what caused me grief:
I had a machine with two disks as part of a raid 1 pair.
I wanted to clone the machine, so sdb was taken out and put in a new
machine and two new disks were added as sdb in each machine. Arrays were
rebuilt, tweaks were made to fix up things like ip addresses and then
the two machines slowly diverged.
On one of the machines, one of the disks started reporting issues. The
other machine was getting tight for space, so I bought two new larger
disks, swapped one on the ok machine, rebuilt the array, swapped the
other, rebuilt, resized.
And then... Swapped the going faulty disk with one of the disks taken
out of the other machine - except that mdadm thought they were part of
the same array and things ended up in a terrible mess (partly because I
didn't realize what was going on so made some mistakes with hindsight)
Yes, I should have failed the disk I was going to remove and then zeroed
the superblock before removing it - but I'd completely forgotten that
the two machines had the same uuid for their raids.
So recently, when I ended up with a disk that had been a raid member, I
thought I'd take the opportunity to see how to change the uuid, which is
what I should have done on splitting the array originally.
It surprises me that there isn't an easy safe way to change the uuid,
but, as per my OP, there are 'risky' ways to achieve this if necessary.
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