Control: reasign -1 fsarchiver Michael Biebl <[email protected]> writes:
> On Thu, 23 Jul 2026 19:02:23 +0200 Michael Biebl <[email protected]> wrote: > >> < interestingly, running it once again then works again > >> >> # mkfs.btrfs -f -q -L Test -U 'f42896b1-3815-4e1e-a01a-9261c5bab6f8' $DEVICE >> ERROR: superblock magic doesn't match I get this: # mkfs.btrfs -f -q -L Test -U 'f42896b1-3815-4e1e-a01a-9261c5bab6f8' $DEVICE ERROR: cannot open device '/dev/loop0': Device or resource busy ERROR: open ctree failed # mkfs.btrfs -f -q -L Test -U 'f42896b1-3815-4e1e-a01a-9261c5bab6f8' $DEVICE ERROR: superblock magic doesn't match # mkfs.btrfs -f -q -L Test -U 'f42896b1-3815-4e1e-a01a-9261c5bab6f8' $DEVICE ERROR: cannot open device '/dev/loop0': Device or resource busy ERROR: open ctree failed # sync # and wait a few seconds # mkfs.btrfs -f -q -L Test -U 'f42896b1-3815-4e1e-a01a-9261c5bab6f8' $DEVICE ERROR: superblock magic doesn't match And the pattern repeats infinitely. > > It does output an error here, but the resulting fs has the correct UUID, > btrfsck does not complain and the fs can be mounted successfully. > The return code is also "0". > > So it looks like "ERROR: superblock magic doesn't match" can be ignored? You're inducing a UUID collision, and this was a fs corruption footgun until the kernel introduced a check for it. UUIDs are supposed to be unique, by definition, and you need to wipefs -a $DEVICE to eliminate this collision before the second mkfs. I'm reassigning this bug to fsarchiver, because it should be running wipefs before mkfs. As an asside, this is also best practices when changing filesystems. Best, Nicholas
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