Add:
chattr -R +C /var/lib/mysql
to mariadb.install

btrfs and other filesystems with copy-on-write features are becoming more common. Unfortunately they tend to fragment database files because updates will be written somewhere else on disk, instead of replacing the old data with the new one as traditional filesystems do. The above command disables CoW for all (current and future) files in /var/lib/mysql for those filesystems that support it, and should be harmless on others.