I ran into this same problem. The workarounds suggested here seemed
really hacky so I tried to dig a bit deeper for a proper solution...
I found the "btrfs device ready $DEV" command which is equivalent to
"btrfs device scan $DEV" except it only has exit code 0 if all devices
that are part of the same filesystem are known to the kernel, implying
it is ready to be mounted. Note that contrary to what the btrfs-device
manpage and "btrfs device --help" say it does not actually wait for
this condition, it merely checks and exits.
I also found that udev has a "btrfs ready" builtin which is used by
/lib/udev/rules.d/64-btrfs.rules to set the ID_BTRFS_READY property
based on this same criterion, and it also sets SYSTEMD_READY=0 until
the filesystem is ready, which causes systemd to pretend the device
doesn't exist yet.
I tried testing the ID_BTRFS_READY property in an initramfs script but
for some reason it doesn't seem to get set, even though udev should be
running if I understand correctly. I didn't get around to try using
"btrfs device ready" since I found a much easier workaround:
apt-get install dracut
apt-get purge initramfs-tools-core
Matthijs van Duin
