Several questions regarding btrfs

Hello,

I've recently learned about btrfs and consider to utilize for my needs. I have several questions in this regard:

I manage a dedicated server remotely and have some sort of script that installs an OS from several images. There I can define partitions and their FSs.

1. By default the script provides a small separate partition for /boot with ext3. Does it have any advantages or can I simply have /boot within / all on btrfs? (Note: the OS is Debian9)

2. as for the / I get ca. following written to /etc/fstab:
UUID=blah_blah /dev/sda3 / btrfs ...
So top-level volume is populated after initial installation with the main filesystem dir-structure (/bin /usr /home, etc..). As per btrfs wiki I would like top-level volume to have only subvolumes (at least, the one mounted as /) and snapshots. I can make a snapshot of the top-level volume with / structure, but how can get rid of all the directories within top-lvl volume and keep only the subvolume containing / (and later snapshots), unmount it and then mount the snapshot that I took? rm -rf / - is not a good idea...

3. in my current ext4-based setup I have two servers while one syncs files of certain dir to the other using lsyncd (which launches rsync on inotify events). As far as I have understood it is more efficient to use btrfs send/receive (over ssh) than rsync (over ssh) to sync two boxes. Do you think it would be possible to make lsyncd to use btrfs for syncing instead of rsync? I.e. can btrfs work with inotify events? Did somebody try it already?
Otherwise I can sync using btrfs send/receive from within cron every 10-15 minutes, but it seems less elegant.

4. In a case when compression is used - what quota is based on - (a) amount of GBs the data actually consumes on the hard drive while in compressed state or (b) amount of GBs the data naturally is in uncompressed form. I need to set quotas as in (b). Is it possible? If not - should I file a feature request?

Thank you in advance!

I am on Debian as well and run /boot on multiple systems without any issues. Remember to run grub-install on all your disks and update-grub if you run it in a redundant setup. That way you can loose a disk and still be happy about it. If you run a redundant setup like raid1 / raid10 make sure you have sufficient disks to avoid that the filesystem enters read-only mode. See the status page for details.

There are some tutorials floating around the web for this stuff. Just be careful, after a system update you might run into boot issues. (I suggest you try playing with this in a VM first to see what happens)

Have no idea, but since Debian uses systemd you might be able to cook up something with systemd.path (https://www.freedesktop.org/software/systemd/man/systemd.path.html

No, you should not file a feature request it seems. Look what me and Google found for you :)
https://btrfs.wiki.kernel.org/index.php/Quota_support
(hint: read the "using limits" section)

No worries, good luck!