The blockchain-less procedure didn't work: NBXplorer was stuck and
would have seemingly never synchronized. This procedure actually
worked in the end, by doing a full rsync.

See #33750

service/BTCpayserver.md

@@ -131,24 +131,49 @@ procedure](#installation) and has an up to date blockchain.
 On server A:
 
     systemctl stop btcpayserver
-    tar -c -z --exclude generated_bitcoin_datadir -f backup.tgz /var/lib/docker/volumes/
-
-Note that this excludes the blockchain, to save some space. It's
-assumed the target already has a copy.
+    tar -c -z -f backup.tgz /var/lib/docker/volumes/
 
 Copy `backup.tgz` to server B.
 
 On server B:
 
     systemctl stop btcpayserver
-    mv /var/lib/docker/volumes/generated_bitcoin_datadir/ /var/lib/docker/generated_bitcoin_datadir.bak/
-    rm -rf /var/lib/docker/volumes/
     tar -C / -x -f -z backup.tgz
-    mv /var/lib/docker/generated_bitcoin_datadir.bak/ /var/lib/docker/volumes/generated_bitcoin_datadir/
-    /root/BTCPayServer/btcpayserver-docker/changedomain.sh btcpay.torproject.org
     systemctl start btcpayserver
 
-Make sure to update the target of the donation form on
+Note that this is likely to run out of disk space because it
+(deliberately) includes the blockchain.
+
+Another option is to stream the content between the two servers, if
+you have a fast link:
+
+    ssh old.example.net 'systemctl stop btcpayserver'
+    ssh new.example.net 'systemctl stop btcpayserver'
+    ssh old.example.net 'tar cf - /var/lib/docker/volumes/' | pv -s 49G | ssh new.example.net tar -C / -x -f -
+    ssh new.example.net 'systemctl start btcpayserver'
+
+Or, alternatively, you can also create an SSH key on the new server,
+copy it on the old one, and just use `rsync`, which is what ended up
+being used in the [actual migration](https://gitlab.torproject.org/tpo/tpa/team/-/issues/33750):
+
+    ssh old.example.net 'systemctl stop btcpayserver'
+    ssh new.example.net 'systemctl stop btcpayserver'
+    ssh new.example.net 'ssh-keygen -t ed25519'
+    ssh new.example.net 'cat .ssh/id_ed25519.pub' | ssh old.example.net 'cat >> .ssh/authorized_keys'
+    ssh new.example.net 'rsync -a --info=progress2 --delete old.example.net:/var/lib/docker/volumes/ /var/lib/docker/volumes/'
+
+It's important that the Docker volumes are synchronized: for example,
+if the NBXplorer volume is ahead or behind the bitcoind volume, it
+will get confused and will not be able to synchronize with the
+blockchain. This is why we copy the full blockchain which, anyways, is
+faster than copying it from the network.
+
+Also, if you are changing to a new hostname, do not forget to change
+it on the new server:
+
+    ssh new.example.net /root/BTCPayServer/btcpayserver-docker/changedomain.sh btcpay.torproject.org
+
+In any case, make sure to update the target of the donation form on
 `donate.torproject.org`. See for example [merge request
 tpo/web/donate-static!76](https://gitlab.torproject.org/tpo/web/donate-static/-/merge_requests/76).