Normaly total and used should deviate us a few gb.
depend on your write workload you should run
btrfs balance start -dusage=60 /mnt
every week to avoid "ENOSPC"
if you use newer btrfs-progs who supper balance limit filters you should run
btrfs balance start -dusage=99 -dlimit=10 /mnt
every 3 hours.
This will balance 2 Blocks (dlimit=10; corresponds to 10 gb) with are
not filled full into new blocks. You could/should adjust the intervall
and the limit-filter depend on your write workload.
For example if you write (change files + new files) only 10GB a day it
will be enough to run this ever night.
The last option completly avoid the ENOSPC issue but produce aditional
workload for your harddrives.
Note: you should avoid making snapshots during balance. Use a simple
lock-mechanic for that.
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