Re: RAID 6, 6 device array - all devices lost superblock

[Peter Sanders <plsander@gmail.com>]

Couple more questions.

Mdadm -create ... Do I use the /dev/sdx or /dev/mapper/sdx name for
the overlayed device?

And reset the mapping between each create attempt by doing:
remove the loop-device/overlay association
   dmsetup remove on all devices
remove the overlay files
  rm
remove the loop back devices
  losetup -d ...
rebuild the loop back devices
  mknod -m 660 ...
build the overlay files
  truncate -s 300G overlay-...
reassociate the loop-devices and the overlays
  losetup... dmsetup..

and try again.

(Yeah, I recognize that there is code to do this (I think) in the
article, but my script-fu is not up to fully understanding those
examples.)

On Mon, Aug 29, 2022 at 6:58 PM Eyal Lebedinsky <fedora@eyal.emu.id.au> wrote:
>
>
> On 30/08/2022 07.45, John Stoffel wrote:
> >>>>>> "Peter" == Peter Sanders <plsander@gmail.com> writes:
> >
> > Peter> Phil,
> > Peter> fstab from the working config -
> >
> > Peter> # <file system> <mount point>   <type>  <options>       <dump>  <pass>
> > Peter> # / was on /dev/sda1 during installation
> > Peter> UUID=50976432-b750-4809-80ac-3bbdd2773163 /               ext4
> > Peter> errors=remount-ro 0       1
> > Peter> # /home was on /dev/sda6 during installation
> > Peter> UUID=eb93a2c4-0190-41fa-a41d-7a5966c6bc47 /home           ext4
> > Peter> defaults        0       2
> > Peter> # /var was on /dev/sda5 during installation
> > Peter> UUID=d1aa6d1f-3ee9-48a8-9350-b15149f738c4 /var            ext4
> > Peter> defaults        0       2
> > Peter> /dev/sr0        /media/cdrom0   udf,iso9660 user,noauto     0       0
> > Peter> /dev/sr1        /media/cdrom1   udf,iso9660 user,noauto     0       0
> > Peter> # raid array
> > Peter> /dev/md0    /mnt/raid6    ext4    defaults    0    2
> >
> > Peter> No LVM, one large EXT4 partition
> >
> > Peter> I have several large files ( NEF and various mpg files) I can identify
> > Peter> and have backup copies available.
> >
> > Peter> I have the overlays created. 300G for each of the six drives.
> >
> > So that's good.  Now you have to try and figure out which order they
> > were created in.  As the docs show, you setup the overlayfs on top of
> > each of the six drives.
> >
> > Keep track by noting the drive serial numbers, since Linux can move
> > them around and change drive letters on reboots.
> >
> >
> > Then using the overlays, do an:
> >
> >       mdadm --create /dev/md0 --level=raid6 -n 6 /dev/sd[bcdefg]
> >       fsck -n /dev/md0
> >
> > and see what you get.  If it doesn't look like a real filesystem, then
> > you can break it down, and then modify the order you give the drive
> > letters, like:
> >
> >        /dev/sd[cdefge]
> >
> > and rinse and repeat as it goes.  Not fun... but should hopefully fix
> > things for you.
> >
> > John
>
> An aside, I would think the way to specify a list in a nominated order is something like
>
> $ echo /dev/sd{c,d,a,b}
> /dev/sdc /dev/sdd /dev/sda /dev/sdb
>
> rather than
>
> $ echo /dev/sd[cdab]
> /dev/sda /dev/sdb /dev/sdc /dev/sdd
>
> which will be in sorting order, regardless of the order of the letter.
>
> --
> Eyal Lebedinsky (fedora@eyal.emu.id.au)