Re: [PATCH] btrfs-progs: drop unique uuid test for btrfstune -M

[Anand Jain <anand.jain@oracle.com>]

Nikolay,

>>> This is intended. Otherwise it's an open avenue for the user to shoot
>>> themselves in the foot.
>>
>>   I don't understand how?

  Again. Any idea how? Is there any test case?

  <snip>

> UUID, by definition, are Unique. What you want to is
> to toss the Unique part, meaning we'll really be left with some sort of
> ID. What's more - the UUID is used by libblkid to populate the available
> devices so not making it unique can cause subtle bugs.

- Irrespective of the fstype, when fs image file is copied/snapshot-ed
   the fsid is indeed going to be duplicate.

- xfs and btrfs kernel doesn't allow duplicate fsid to be mounted which
   is fair and accepted. (I remember fixing the duplicate fsid bugs in
   the kernel, just fyi if you are concerned about them).

- ext4 kernel does allow duplicate fsid to be mounted.

- btrfstume -M <uuid> isn't the place to check if the fsid is a
   duplicate. Because, libblkid will be unaware of the complete list of
   fs images with its fsid.

- As I said before, its a genuine use-case here where the user wants to
   revert the fsid change, so that btrfs fs root image can be booted.
   Its up-to the user if fsid is duplicate in the user space, as btrfs
   kernel rightly fails the mount if its duplicate fsid anyways.

Thanks, Anand