64-btrfs.rules and degraded boot

[Chris Murphy <lists@colorremedies.com>]

For some reason I thought it was possible to do degraded Btrfs boots
by removing root=UUID= in favor of a remaining good block device, e.g.
root=/dev/vda2, and then adding degraded to rootflags. But this
doesn't work either on CentOS 7.2 or Fedora Rawhide. What happens is
systemd waits for vda2 (or by UUID) indefinitely, it doesn't even try
to mount the volume.

I think it's due to the udev rule that's basically saying the device
isn't ready because not all of its devices are there.

[root@f24m ~]# cat /usr/lib/udev/rules.d/64-btrfs.rules
# do not edit this file, it will be overwritten on update

SUBSYSTEM!="block", GOTO="btrfs_end"
ACTION=="remove", GOTO="btrfs_end"
ENV{ID_FS_TYPE}!="btrfs", GOTO="btrfs_end"

# let the kernel know about this btrfs filesystem, and check if it is complete
IMPORT{builtin}="btrfs ready $devnode"

# mark the device as not ready to be used by the system
ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0"

LABEL="btrfs_end"


I am kinda confused about this "btrfs ready $devnode" portion. Isn't
it "btrfs device ready $devnode" if this is based on user space tools?




-- 
Chris Murphy