Ok, here is the reality.
Yes, things like that CAN occur, but do they ever?
I have read what you guys type above, and also that that stuff is never needed because you would not end up in a situation like that UNLESS you have multiple disks.
I am one of those persons that if I see conflicting information, just test myself so I created a situation for testing this about 6 months ago.
I use this drive as “everything drive”, ie if I download something, this is where it goes. I create big img files to work with my rpi:s, this is where it goes.
I even intentionally did something that is not recommended on btrfs, download torrents to it, because it can create a situation where balance could be needed, because of all the tiny chunks that are being created when downloading.
The data exchange on this drive is massive.
Not an ssd, but that doesn’t matter.
NEVER balanced it.
Lets take a look of how much space I would “save” if I ran a balance.
$ sudo btrfs fi df /media/fil-fitt
Data, single: total=1.23TiB, used=1.22TiB
System, DUP: total=8.00MiB, used=160.00KiB
Metadata, DUP: total=2.25GiB, used=1.53GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
sudo btrfs fi usage /media/fil-fitt
Overall:
Device size: 2.47TiB
Device allocated: 1.24TiB
Device unallocated: 1.23TiB
Device missing: 0.00B
Device slack: 0.00B
Used: 1.23TiB
Free (estimated): 1.24TiB (min: 639.74GiB)
Free (statfs, df): 1.24TiB
Data ratio: 1.00
Metadata ratio: 2.00
Global reserve: 512.00MiB (used: 0.00B)
Multiple profiles: no
Data,single: Size:1.23TiB, Used:1.22TiB (99.26%)
/dev/sdb3 1.23TiB
Metadata,DUP: Size:2.25GiB, Used:1.53GiB (67.81%)
/dev/sdb3 4.50GiB
System,DUP: Size:8.00MiB, Used:160.00KiB (1.95%)
/dev/sdb3 16.00MiB
Unallocated:
/dev/sdb3 1.23TiB
99,26%…
Again, you guys do you, but imho, a balance is never needed on a single device filesystem.
But ask me again in a year, but if this is 6 months of heavy usage, I do not think it will be much different in any amount of time.