[bug #45902] BTRFS subvolume - don't resolve symlinks

[Florian]

URL:
  <http://savannah.gnu.org/bugs/?45902>

                 Summary: BTRFS subvolume - don't resolve symlinks
                 Project: GNU GRUB
            Submitted by: ikcalb
            Submitted on: Wed 09 Sep 2015 10:37:19 AM GMT
                Category: Filesystem
                Severity: Major
                Priority: 5 - Normal
              Item Group: Feature Request
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name: Florian Kerle
        Originator Email: address@hidden
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 
                 Release: other
         Reproducibility: Every Time
         Planned Release: None

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Details:

BTRFS subvolumes are a convenient way of making backups - especially for the
currently used root filesystem, as they are COW.

SETUP
=====
Using one physical BTRFS partition:
- mounted on `/mnt`
- using folders to group snapshots for logical partitions
  (i.e. snapshots for `home` stored in address@hidden)
- symlink for the currently used subvolume for each logical partition

```
/mnt
 |-home  --> ./home@/current
 |-home@/
    |-current/
 |-root  --> ./root@/current
 |-root@/
    |-current/
    |-pre-systemd/
    |-2013-05-12/
```

CURRENT BEHAVIOR (steps to reproduce)
=====================================
1) when booted to subvolume `root` in the above configuration, issue
`update-grub`
2) check `/boot/grub/grub.cfg`
   it will resolve the `root` symlink on `/mnt` to `root@/current`

EXPECTED BEHAVIOR
=================
To ease recovery / switching between snapshots, editing one symlink is more
elegant than
- to move the current subvolume (`root`) away and the backup in place
  (not only paths for kernel/initrd when booting had to be updated, but also
fstab - if not replacing the subvolume one way or another)

Unfortunately idk how to determine with which parameters a BTRFS volume was
mounted. (i.e. using the symlink, or the subvolume path directely)
btrfs-tools seem to report the resolved subvolume name only.




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