How can I get snapper to recognize a snapshot?

Question

Background

I am playing around with BTRFS in a VM.

I followed this guide to make sure my /home/.snapshot folder is its own sub-volume. Everything seems to work, I can take snapshots, list snapshots, undochanges, etc.

Experimenting with Send/Receive

Steps:

• Use send/receive to send snapshot id 1 to a different subvol
• Use snapper to delete the snapshot id 1
• Use send/receive to bring snapshot id 1
• Snapper no longer recognizes snapshot id 1

Details:

I sent an existing snapshot to a different sub-volume (device)

sudo btrfs subvolume list -t /mnt_device3
ID      gen     top level       path
--      ---     ---------       ----
256     9       5               @backup

sudo mkdir /backup/1
sudo btrfs send /home/.snapshots/1/snapshot | sudo btrfs receive /backup/1
sudo btrfs subvolume list -t /backup                                      
ID      gen     top level       path
--      ---     ---------       ----
256     17      5               @backup
258     18      256             1/snapshot

Then I used snapper to delete the snapshot:

sudo snapper -c home delete 1

sudo snapper -c home list
 # | Type   | Pre # | Date                        | User | Cleanup | Description                                      | Userdata
---+--------+-------+-----------------------------+------+---------+--------------------------------------------------+---------
0  | single |       |                             | root |         | current                                          |         
2  | single |       | Tue 04 Jul 2023 03:20:01 PM | root |         | testing send/receive after test.txt |    

Then I used send/receive again to put the snapshot back

sudo mkdir /home/.snapshots/1
sudo btrfs send /backup/1/snapshot | sudo btrfs receive /home/.snapshots/1
ls /home/.snapshots 
total 0
drwxr-xr-x 1 root root  6 Jul  4 16:30 .
drwxr-xr-x 1 root root 32 Jun 29 11:36 ..
drwxr-xr-x 1 root root 16 Jul  4 16:11 1
drwxr-xr-x 1 root root 32 Jul  4 15:20 2

#an excerpt of sudo btrfs subvolume list /home
ID 311 gen 1147 top level 272 path 2/snapshot
ID 313 gen 1214 top level 272 path 1/snapshot

But snapper does not recognize snapshot 1

sudo snapper -c home list
 # | Type   | Pre # | Date                        | User | Cleanup | Description                                      | Userdata
---+--------+-------+-----------------------------+------+---------+--------------------------------------------------+---------
0  | single |       |                             | root |         | current                                          |         
2  | single |       | Tue 04 Jul 2023 03:20:01 PM | root |         | testing send/receive after test.txt |  

So I can't really use snapper to do anything with snapshot 1 anymore.

Is there a way I can get snapper to recognize the imported snapshot?

Answer 1

AFAIK, snapper maintains a database to track the snapshots that it creates. When you did # snapper -c home delete 1, you removed both the snapshot itself and the corresponding entry in the database. Restoring the snapshot directory with BTRFS send/receive doesn't put the information about that snapshot back into the database.

You could try to manually create a new snapshot with the same ID, but this might not be what you're after, since the timestamps and other metadata in the database won't match the restored snapshot:

# manually create snapshot
sudo snapper -c home create --description "Restored snapshot 1"

# replace the snapshot ('3' is an example ID; replace it with the one you actually get)
sudo mv /home/.snapshots/3/snapshot /home/.snapshots/3/snapshot.bak
sudo mv /home/.snapshots/1/snapshot /home/.snapshots/3/snapshot