[pQd]

i've been using BTRFS since 2014 to store backups. there is a noticeable performance penalty when rsync'ing hundreds of thousands of files to a spinning-rust disk connected to USB-SATA dock when BTRFS is used instead of EXT4. i'm accepting it in exchange for ability to run scheduled scrub of the data to detect potential bitrot.

since 2017 i'm also using BTRFS to host mysql replication slaves. every 15 min, 1h, 12h crash-consistent snapshots of the running database files are taken and kept for couple of days. there's consensus that - due to its COW nature - BTRFS is not well suited for hosting vms, databases or any other type of files that change frequently. performance is significantly worse compared to EXT4 - this can lead to slave lag. but slave-lag can be mitigated by: using NVMe drives and relaxing durability of MySQL innodb engine. i've used those snapshots few times each year - it worked fine so far. snapshots should never be the main backup strategy, independently of them there's a full database backup done daily from masters using mysqldump. snapshots are useful whenever you need to very quickly access state of the production data from few minutes or hours ago - for instance after fat fingering some live data.

during those years i've seen kernel crashes most likely due to BTRFS but i did not lose data as long as the underlying drives were healthy.