[SOLVED] btrfs question and problem

[oldgeek]

First, the problem: I want to delete an @home backup on my external hard disk, but I can't. I get an error message: read-only file system. I change the attributes of the file I want to delete, and still get the same thing. I may have done something to the external HD recently when I re-installed 20.04 and used the backup to be my new @home subvolume. That works fine, by the way, but I still cannot delete the old @home file on the HD. I was able to delete the same date's @ subvolume. How do I get ride of the @home subvolume?

The question: I want to make incremental backup t my external storage (hence the desire to erase the oldest one). I have the commands from GreyGeek's manual (very useful) but what I don't know is whether both snapshots have to read-only or read-write.

Thanks in advance.

[jlittle]

Are you using sudo btrfs subvolume delete to delete it? From your post it seems you are trying to delete a file.

btrfs subvolumes can be flagged "readonly", and btrfs send/receive works with read only snapshots. If your external hard disc is mounted on /mnt/external, and the subvolume is @home at the top level,

sudo btrfs subvolume show /mnt/external/@home

shows the flag, with other info. (If you actually want to write to a read-only snapshot, just snapshot it again and write to the new snapshot. If the writeable snapshot should have the name of the read-only one, just delete or rename it, then rename the writeable one.)

Now, if you are using subvolume delete, and still get the "read-only file system" error, there's a few reasons I can think of. If you don't mind, it might be best if you run btrfs su list on the drive in question and post the result here in code /code tags.

I want to make incremental backup t my external storage (hence the desire to erase the oldest one).

For incremental backups, you have to keep a previous snapshot on both source and destination, at least until the incremental backup finishes. You have to start with a full backup (coming from using rsync for incremental backups I had to learn this). After doing the full backup, you don't delete the snapshot that was used for the send. Then, for the incremental backup, that previous snapshot is specified with -p.

[oldgeek]

I don't know why, but the problem resolved itself. I suspect hardware issues, as my computer wouldn't connect to the monitor at all last night, while today everything went smoothly, including the incremental backups, which do save a whole lot of time. My terminology is not always correct; when I say file I often mean subvolume, as I haven't quite gotten used to the vocabulary. The btrfs system takes a little getting used to (and the terminal does not forgive errors) but so far it has worked for me both in emergency situations and in plain old backup and use for new installs. Thanks jlittle for the quick response.