[Bug 828421] Re: Create new RAID1 array without resync

[Robert Voje]

Thank you, jools, for the info that the raid can be used right away, I did not know that.
It's not mentioned on any of the many guides I've been through..

Anyway, the bug was about mdadm creating "md_d0" and similar devices
when I delete the original raid, even if I clear the disk.

It seems like mdadm has trouble forgetting the raid on a disk, and when trying to clear the superblocks (after deleting the raid), I get no error.
But it still seem to be some kind of info stored either in the OS, or on the disks.

I just bought a couple of large disks that is synchronizing just now,
but when they are done, I'll try again with my 500GB test disks, and try
to describe more step-by-step whats going on.

Basically what I did was this:

Create a raid:

mdadm --create --verbose /dev/md0 --level=1 /dev/sda1 /dev/sdb1

That failed with "no raid devices found" or something like that (don't
remember the exact words), so I ran this instead:

mdadm -Cv /dev/md0 -l1 -n2 /dev/sd[ab]1

And that worked, and started synchronizing.

Then, after a reboot, the device /dev/md_d0 appeared instead of
/dev/md0.

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https://bugs.launchpad.net/bugs/828421

Title:
  Create new RAID1 array without resync

Status in “mdadm” package in Ubuntu:
  New

Bug description:
  I tried several times to create a RAID1 array, but was haunted by the
  resync, that is taking MANY hours to finish.

  Finally, I found a reference to a parameter on the web: --assume-clean
  Of course, this parameter stated that it only should be used "when you know what you're doing".

  Then I answered myself: I know what I'm doing. I'm creating a whole
  new RAID1 array, with blank disks and all.

  The result was an auto creation of a "md_d0" array, which I had to delete, then it created a array named "md_0p1" or something.
  The whole clue is, that mdadm never gave up on the original array, it just renamed it, and I had to delete the member disks from each array that mdadm was auto-creating.

  The main question is still:
  Why should I wait 3-6 hours for the damn daemon to replicate random data from one partition to another.

  The situation is, that I have 2 clean disks. What I weant to do is to
  create a raid1 array on them, but mdadm still demands to sync them.

  Is'nt there a way to "blank" new raid arrays, so we does not have to
  wait several hours on resync?

  Just now, I'm trying this on a couple of 500GB disks, but tomorrow
  I'll receive two 2TB disks wich I also want as a RAID1 array, and I
  fear that the rest of the week will go by synchronizing my empty new
  disks.

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