BTRFS/Rollback & Yum Snapshot Plugin

[Jorge Fábregas]

On 01/30/2014 09:02 PM, Jorge Fábregas wrote:
> Now, here's the thing.  I wanted to permanently run off these snapshots
> so I wanted to delete the parent "subvolumes" for these snapshots (e.g.
> "commit" the snapshot) by doing:
> 
> # mount -o subvolid=5 /dev/vda3 /mnt
> # btrfs subvolume delete /mnt/root
> Delete subvolume '/mnt/root'
> ERROR: cannot delete '/mnt/root' - Device or resource busy

Well, I found out you can rename the subvolumes and not only that;  you
can also rename them ("move" them) while they're being used.  This is
what I'm doing when I want to rollback my system to the previous state:

1) Modify GRUB2:  Change "rootflags=subvol=root/yum_*" accordingly & reboot

2) Make sure you're running off the snapshot subvolume:
# cat /proc/selfmountinfo | grep btrfs

3) Move/Rename snapshot subvol & delete old root fs
# mount -o subvolid=5 /dev/vda3 /mnt
# mv /mnt/root/yum_20140203080009 /mnt
# btrfs subvolume delete /mnt/root
# mv /mnt/yum_20140203080009 /mnt/root

4) Modify GRUB2:  revert rootflags=subvol=root/yum_* to subvol=root

Since I used the default setup for btrfs, Anaconda created a /boot of
its own (ext4). (I presume a btrfs /boot is not ready yet).  This
creates some time paradox issues when I want to run "yum upgrade" again:

- yum wants to install a kernel that I already have (the one I'm
currently running with)
- yum wants  to remove the oldest kernel (which it already removed on
previous existence)

All of this due to the fact that I restored the previous RPM database
when I rollbacked my system.

There is more to this yum-plugin/btrfs/rollback than meets the eye :(

-- 
Jorge