Btrfs – Meaning of @ in Pathnames

The redistribution of files when removing a device is transparent. I looked at the source ( a 1 Gb git clone) but I have never been able to find anything in the user-land utilities (now bundled in btrfs IIRC) to do this without actually removing (btrfs device delete) and re-adding (btrfs device add) the device. I specifically looked at mapping subvolumes to specific devices without results.

The removing of a device takes time equivalent to the amount of data to be moved, during which the device is not used for new data. Also, there is no control to leave frequently accessed items on the device. So that was not a useful option.

I have my setup now so that the slower 'backend' is a separate filesystem to which specific data is move based on file type and access information. That is, of course, not as convenient as having one big filesystem with respect to running out of space (at least 'backend' storage is relatively cheap).

If you have not already done so you might also want to look at this entry on the btrfs Wiki.

The replace command doesn't make a backup of sda1, it replaces sda1 with sdb1 in the filesystem, but since it's a one device filesystem and btrfs doesn't bother wiping the data from sda1 when it replaces it they end up being indentical copies of the filesystem. However you do NOT want to do this as both will have the same UUID, and currently it's not safe to mount two btrfs filesystems with the same UUID as it can cause MASSIVE DATA CORRUPTION (see btrfswiki's Gotchas page). If you want to use btrfs's incremental backup feature you should format you're backup drive /dev/sdb1 to a new btrfs filesystem. Then you should make a read-only snapshot of watever subvolume(s) you want to backup on your filesystem by using

btrfs su sn -r @subvolume-name @subvolume-name-RO

on each subvolume. Then you should mount the blank btrfs filesystem and run

btrfs send /path/to/@subvolume-name-RO | btrfs rec /path/to/backup-directory/

This will be the first send and btrfs will have to transfer all of the data this time. Next time you want to send a backup to this drive you can use incremental sends to only send what data has changed since the previous backup you sent. It will also use Copy On Write so you'll save a lot of space as well. Just make sure you keep the latest snapshot on both filesystems. When it's done you can rename the sent snapshot to whatever you want.

Now if you want to send another snapshot just rename the orignal one and take a new snapshot with something like

mv @subvolume-name-RO @subvolume-name-RO-old
btrfs su sn -r @subvolume-name @subvolume-name-RO

Then you can use send to send the latest snapshot using

btrfs send -p @subvolume-name-RO-old @subvolume-name-RO | btrfs rec /path/to/backup-directory/

and if the previous snapshot still exists on your backup drive it will send your new snapshot by only having to copy whatever changes you've made since the previous one.