Re: How to rollback a snapshot of a subvolume with nested subvolumes?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-05-06 07:48, Niccolò Belli wrote:
> The following are my subvolumes:
>
> $ sudo btrfs subvol list /
> [sudo] password di niko: ID 257 gen 1040 top level 5 path @
> ID 258 gen 1040 top level 5 path @home
> ID 270 gen 889 top level 257 path var/cache/pacman/pkg
> ID 271 gen 15 top level 257 path var/abs
> ID 272 gen 972 top level 257 path var/tmp
> ID 273 gen 37 top level 257 path tmp
> ID 274 gen 20 top level 257 path srv
> ID 276 gen 25 top level 258 path @home/niko/.cache/pacaur
> ID 280 gen 993 top level 257 path .snapshots
> ID 281 gen 993 top level 258 path @home/.snapshots
> ID 282 gen 169 top level 280 path .snapshots/1/snapshot
> ID 283 gen 171 top level 280 path .snapshots/2/snapshot
> ID 284 gen 173 top level 280 path .snapshots/3/snapshot
> ID 285 gen 124 top level 281 path @home/.snapshots/1/snapshot
> ID 286 gen 175 top level 280 path .snapshots/4/snapshot
> ID 288 gen 177 top level 280 path .snapshots/5/snapshot
> ID 290 gen 237 top level 280 path .snapshots/6/snapshot
> ID 291 gen 238 top level 281 path @home/.snapshots/2/snapshot
> ID 292 gen 308 top level 280 path .snapshots/7/snapshot
> ID 293 gen 309 top level 281 path @home/.snapshots/3/snapshot
> ID 294 gen 376 top level 280 path .snapshots/8/snapshot
> ID 295 gen 377 top level 281 path
> @home/.snapshots/4/snapshot
>                                                 ID 296 gen 442 top level
> 280 path .snapshots/9/snapshot
>                                                 ID 297 gen 443 top level
> 281 path @home/.snapshots/5/snapshot
>                                                 ID 298 gen 511 top level
> 280 path .snapshots/10/snapshot
>                                                 ID 299 gen 512 top level
> 281 path @home/.snapshots/6/snapshot
>                                                 ID 300 gen 578 top level
> 280 path .snapshots/11/snapshot
>                                                 ID 301 gen 579 top level
> 281 path @home/.snapshots/7/snapshot
>                                                 ID 302 gen 648 top level
> 280 path .snapshots/12/snapshot
>                                                 ID 303 gen 649 top level
> 281 path @home/.snapshots/8/snapshot
>                                                 ID 304 gen 716 top level
> 280 path .snapshots/13/snapshot
>                                                 ID 305 gen 717 top level
> 281 path @home/.snapshots/9/snapshot
>                                                 ID 306 gen 967 top level
> 280 path .snapshots/14/snapshot
>                                                 ID 307 gen 789 top level
> 281 path @home/.snapshots/10/snapshot
>                                                 ID 309 gen 834 top level
> 280 path .snapshots/15/snapshot
>                                                 ID 310 gen 874 top level
> 280 path .snapshots/16/snapshot
>                                                 ID 311 gen 875 top level
> 281 path @home/.snapshots/11/snapshot
>                                                 ID 312 gen 887 top level
> 280 path .snapshots/17/snapshot
>
>
>                         ID 313 gen 888 top level 280 path
> .snapshots/18/snapshot
>
>
>                         ID 314 gen 904 top level 280 path
> .snapshots/19/snapshot
>
>
>                         ID 316 gen 938 top level 280 path
> .snapshots/20/snapshot
>
>
>                         ID 317 gen 939 top level 281 path
> @home/.snapshots/12/snapshot
>
>
>                         ID 318 gen 991 top level 280 path
> .snapshots/21/snapshot
>
>
>                         ID 319 gen 992 top level 281 path
> @home/.snapshots/13/snapshot
>
> I would like to rollback to .snapshots/14/snapshot, restoring my @
> subvolume to such a previous state.
> So I booted into a livecd, mounted my disk into /mnt and typed:
> btrfs subvol snapshot /mnt/@/.snapshots/14/snapshot /mnt/@
>
> Unfortunately it simply creates a new /mnt/@/snapshot subvolume instead
> of replacing @.
> If @ didn't have any nested subvolume I would simply delete it and run
> the previous command again, but
> 1) I dont't want to lose the @ nested subvolumes like var/cache/pacman/pkg
> 2) btrfs subvolume delete /mnt/@ doesn't want to delete it (cannot
> delete directory not empty), probably because of the nested subvolumes
>
> So, how am I supposed to rollback to a previous snapshot?
General procedure:
1. Rename the subvolume you want to replace to something else.
2. run the command you had used above.
3. Remove the empty directories in the new subvolume that result from 
the nested snapshots/subvolumes.
4. Snapshot all the nested subvolumes from the old subvolume into the 
required locations in the new one.
5. If you're running off of the subvolume you're replacing, reboot 
(completely, not just drop to single user or emergency mode or 
whatever), if not (for example, if you're using a live-CD, just continue 
to the next step.
6. Delete the old subvolume (you'll have to delete any nested subvolumes 
first).  This can be deferred to whatever point in time you determine 
that you don't need any of the data from it.

Now, as to a slightly higher level discussion, this is part of why usage 
of nested subvolumes is generally considered a bad practice.  In 
general, the only valid use for nested subvolumes is to exclude things 
from snapshots.

Part of what you have listed (/var/cache/pacman/pkg, /var/tmp, and /tmp) 
is stuff that makes some sense to use like this, as it's all data that 
you don't want to snapshot, and it's trivially reproducible.  I have a 
couple of cases where I'm using nested subvolumes like this to exclude 
data that is trivial to reproduce from snapshots (and by extension, from 
backups), but I also just drop them and create new ones when I have to 
roll-back.