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langfingaz

Member

From: Germany

Registered: 2020-01-11

Posts: 8

BTRFS multiple mounts on same mountpoint

If I'm not wrong, one should usually get the message "already mounted" if one tries to mount a device for the second time at the same mountpoint.

But after my last system updates, I can issue this command multiple times:

sudo mount /mnt/data/

And I don't get an error message, but instead just exit code 0.

After each additional mount, when running

findmnt -no uuid -T '/mnt/data'

, I get one more UUID back:

First only

c385a436-0288-486f-a2b9-c64c2db667e7

, then

c385a436-0288-486f-a2b9-c64c2db667e7
c385a436-0288-486f-a2b9-c64c2db667e7

, etc.

Do you think that this is a bug? Or is this intended?

Additional information:
- util-linux 2.39-8
- btrfs-progs 6.3.1-1
- /mnt/data is a BTRFS filesystem. From /etc/fstab:

/dev/mapper/12tb1 			  /mnt/data   btrfs defaults,noauto,nofail,noatime,compress=zstd:1 0 0

. And from /etc/crypttab:

12tb1 UUID=68c349e8-5118-4773-9fd5-5dbad9acee4e none noauto,nofail

Last edited by langfingaz (2023-06-12 11:15:18)

just4arch

Member

Registered: 2023-01-07

Posts: 126

Re: BTRFS multiple mounts on same mountpoint

AFAIK this has to do with subvolumes and is needed to mount different subvolumes on one FS - internally this should just be bind mounts.
https://btrfs.readthedocs.io/en/latest/Subvolumes.html:

This is similar to a bind mount, and in fact the subvolume mount does exactly that.