Let's add a special tweak to the btrfs builtin: if /dev/btrfs-control is not there, let's consider all btrfs file systems as SYSTEMD_READY=0. This is useful in initrds, where btrfs.ko might be missing. After the initrd → host transition we can then retigger the device and undo the SYSTEMD_READY=0 marking.
System and Service Manager
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