Safe to hard reboot while balancing is running?

JasperA, 

I just saw your post now after the weekend.

The problem you ran into is clearly not a problem with BTRFS. The problem you encountered could just as easily be run into when using EXT4. A 4GB EXT4 partition has a limited number of inodes and systemd is available for use with EXT4 as well as BTRFS. If you have a huge number of directories on a 4GB EXT4 partition you will use up all the inodes.

Did you happen to make a configuration backup before running into this problem? If so, that would be useful for restoring your configuration.

If you fill up the root volume (again this is the case with both EXT4 and BTRFS) then some config may get corrupted when the system attempts to write to it. If there's no space to write to it then an attempt to overwrite the config may result in an empty file.

BTRFS stores both data and metadata (data about data). As it needs to it preallocates space. If the space on the volume is fully allocated but the allocation for data or metadata is fully used then you will get the errors about no space being left on the device like you would if you were to fill all the inodes on an EXT4 volume.

You can check the usage of the root volume by using commands such as

# btrfs fi show

and
# btrfs fi df /

For the data volume it's best to keep volume usage under 80% and schedule a regular balance. For the root volume you shouldn't need to run a balance, but if you do choose to do so as the volume is quite small you should get the root volume usage well under 50% (probably under about 30%) before doing so. My root volume usage is under 15%. You can use the -dusage filter when running a balance to start by only balancing chunks that are nearly empty.

When you are running a balance and shutdown the system without cancelling it, it will automatically start again on the next boot. There is a command you can run to cancel a balance.

BTRFS is a great filesystem and it's inevitable that more and more NAS vendors will use it as time progresses.

Which apps did you install on your system? Have you installed any packages via SSH? If so, which ones? Did you check to see if updates were available for the apps you had installed before updating the firmware on your system?

Also there is now a LogitechMediaServer app available from NASTools: https://github.com/NAStools/logitechmediaserver/releases

You may wish to uninstall the LogitechMediaServer app you have installed and use this one instead.

From how I see it you have installed some apps on your system and you have done some things using SSH (the use of which is unsupported) which may have made the problem worse.

We'd certainly like to get some more information to see if we can reproduce what happened on your system. Some logs would be very helpful (see the Sending Logs link in my sig).

I just had a look at my RN516 and I can see no systemd-private directories under /var/tmp