Re: [PATCH] mkfs.btrfs: allow UUID specification at mkfs time

[James Shubin <purpleidea@redhat.com>]

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On Wed, 2014-05-14 at 16:39 +0200, Goffredo Baroncelli wrote:
> Hi Eric,
> 
> On 05/14/2014 03:18 AM, Eric Sandeen wrote:
> > Allow the specification of the filesystem UUID at mkfs time.
> 
> I suggest to add some warning when this options is used, because the behavior could be very different than the one expected.
> 
> I suspect that BTRFS tracks the filesystem by UUID and not by devices. When two filesystems have the same UUID at the same time, it may mount the wrong one.
> 
> $ #
> $ # Make two *different* filesystems with the *same* UUID
> $ #
> $ UUID=e285c9bd-ea97-40b3-ad7d-8713dcfd5eea
> $ sudo ./mkfs.btrfs -f -U $UUID /dev/vdg
> $ sudo ./mkfs.btrfs -f -U $UUID /dev/vdh
> 
> $ #
> $ # from the beginning "btrfs fi show" reports wrong information
> $ #
> $ sudo btrfs fi show
> Label: none  uuid: e285c9bd-ea97-40b3-ad7d-8713dcfd5eea
> 	Total devices 1 FS bytes used 96.00KB
> 	devid    1 size 50.00GB used 4.00MB path /dev/vdh
> 	devid    1 size 50.00GB used 2.04GB path /dev/vdg
> 
> $ #
> $ # mount the first one, create a new file then un-mount it
> $ #
> $ sudo mount /dev/vdg /mnt/btrfs1
> $ sudo touch /mnt/btrfs1/dev-vdg
> $ sudo umount /dev/vdg
> 
> $ #
> $ # mount the second one, it should be empty
> $ # instead btrfs mount the first one
> $ #
> $ sudo mount /dev/vdh /mnt/btrfs2
> $ ls -l /mnt/btrfs2
> total 0
> -rw-r--r-- 1 root root 0 May 14 16:12 dev-vdg
> 
> 
> I am not against this option; I am suggesting to add a explicit warning to the user about the risk of doing that, both on the man pages and into the program. 
> The warning should say that this option is only for testing. Better ask for a confirmation (even with an undocumented switch like '--I-know-that-I-am-doing-something-really-dangerous').

I don't think is necessary since xfs and ext4 both have these options
too... There probably *should* be a conflict, because typically this
will cause an entry to show up in
/dev/disks/by-uuid/$UUID

I'm sure this same bug exists if two "randomly generated UUID's" get
created too. I think a warning in the man page would be enough.

> 
> For the record, BTRFS seems unable to mount at the same time two different filesystems with the same UUID:
> 
> $ #
> $ # try to mount two fs with the same UUID, but BTRFS doesn't allow it
> $ #
> $ sudo mount /dev/vdh /mnt/btrfs2
> $ sudo mount /dev/vdg /mnt/btrfs1
> ERROR: mount failed : 16 - Device or resource busy
> 
> 
> BR
> G.Baroncelli
> 



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