Re: overlays on dd images of 4T drives

From: Phillip Susi <phill@thesusis.net>
To: David T-G <davidtg+robot@justpickone.org>
Cc: Linux RAID <linux-raid@vger.kernel.org>
Subject: Re: overlays on dd images of 4T drives
Date: Mon, 15 Nov 2021 14:32:59 -0500	[thread overview]
Message-ID: <87czn1i4xr.fsf@vps.thesusis.net> (raw)
In-Reply-To: <20211115025103.GA254223@opal1.opalstack.com>

David T-G <davidtg+robot@justpickone.org> writes:

>   davidtg@gezebel:/mnt/data/tmp/4Traid> parallel 'size=$(sudo blockdev
> --getsize {}); loop=$(sudo losetup -f --show -- overlay-{/}) ; echo
> $loop' ::: $DEVICES
>   /dev/loop0
>   /dev/loop2
>   /dev/loop1
>
>   davidtg@gezebel:/mnt/data/tmp/4Traid> sudo losetup -a
>   /dev/loop1: [66307]:624372298 (/mnt/data/tmp/4Traid/overlay-sdc1)
>   /dev/loop2: [66307]:624372297 (/mnt/data/tmp/4Traid/overlay-sdb1)
>   /dev/loop0: [66307]:624372296 (/mnt/data/tmp/4Traid/overlay-sda1)
>   ### why are these out of order?!? *sigh*

Because you ran the command to create them with parallel.  Don't do that.

> OK, yeah, we could predict that.  But it gets even more fun:
>
>   diskfarm:/mnt/10Traid50md/tmp # mdadm --assemble --force /dev/md4 /dev/loop{0,1,2}
>   mdadm: no recogniseable superblock on /dev/loop0
>   mdadm: /dev/loop0 has no superblock - assembly aborted
>
> What?!?  Where is my superblock?  This is a bit-for-bit copy of the
> partition itself.
>
> Time to fall back for more help from the RAID gods *sigh*  Any further
> recommendations?

Run mdadm -E /dev/loop{0,1,2}