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The first way is mkfs.btrfs turns off metadata duplication on a single device when /sys/block/device/queue/rotational is zero for the single specified device.
This is equivalent to specifying -m singleon the command line. It can be overridden and duplicate metadata forced by providing the -m dup option. Duplication is not required due to SSD firmware potentially losing both copies. This wastes space and is a performance cost.
Illetve:
Explanation: on SSDs, mkfs.btrfs creates metadata in single mode (because of widely spread SSD deduplication algorithms negating duplicate entries). However, second copy of metadata increases recovery chances, especially so if your SSD does not deduplicate writes. Hence the desire to add metadata/systemdata duplication after the filesystem is created.