Comment 34 for bug 1519470

Ok, I did as you suggested and it still fails:
root@supermicro:~# mdadm --stop /dev/md0; mdadm --zero-superblock /dev/sda1; mdadm --zero-superblock /dev/sdb1
mdadm: error opening /dev/md0: No such file or directory
mdadm: Unrecognised md component device - /dev/sda1
root@supermicro:~# mdadm -v -v --create /dev/md0 --run --level=0 --raid-devices=2 /dev/sda1 /dev/sdb1
mdadm: chunk size defaults to 512K
mdadm: Defaulting to version 1.2 metadata
mdadm: RUN_ARRAY failed: Invalid argument

FWIW, this is the version of mdadm used by the ephemeral
root@supermicro:~# mdadm --version
mdadm - v3.2.5 - 18th May 2012

and finally, I do not believe the RAID module is actually loaded.

root@supermicro:~# lsmod
Module Size Used by
dm_crypt 24576 0
overlay 45056 1
iscsi_tcp 20480 2
libiscsi_tcp 28672 1 iscsi_tcp
libiscsi 57344 2 libiscsi_tcp,iscsi_tcp
scsi_transport_iscsi 102400 3 iscsi_tcp,libiscsi
hid_logitech_dj 20480 0
hid_generic 16384 0
i2c_algo_bit 16384 0
ttm 94208 0
drm_kms_helper 126976 0
psmouse 114688 0
drm 344064 2 ttm,drm_kms_helper
ahci 36864 0
libahci 32768 1 ahci
pata_acpi 16384 0
usbhid 53248 0
e1000e 237568 0
hid 110592 3 hid_generic,usbhid,hid_logitech_dj
ptp 20480 1 e1000e
pps_core 20480 1 ptp

And finally, THAT seems to be the root cause. There are no software RAID modules that I can find on the ephemeral:

root@supermicro:/lib/modules/3.19.0-43-generic/kernel/drivers/md# ls
bcache dm-crypt.ko

Compared to my desktop running Trusty:
bladernr@sulaco:/lib/modules/3.13.0-74-generic/kernel/drivers/md$ ls
bcache dm-crypt.ko dm-multipath.ko dm-snapshot.ko linear.ko raid456.ko
dm-bio-prison.ko dm-delay.ko dm-queue-length.ko dm-switch.ko multipath.ko
dm-bufio.ko dm-flakey.ko dm-raid.ko dm-thin-pool.ko persistent-data
dm-cache-cleaner.ko dm-log.ko dm-region-hash.ko dm-verity.ko raid0.ko
dm-cache.ko dm-log-userspace.ko dm-round-robin.ko dm-zero.ko raid10.ko
dm-cache-mq.ko dm-mirror.ko dm-service-time.ko faulty.ko raid1.ko

but they DO exist on the image mounted on the tmpfs:

from the output of mount:
/dev/sdc on /media/root-ro type ext4 (ro)

root@supermicro:/media/root-ro/lib/modules/3.13.0-74-generic/kernel/drivers/md# ls
bcache dm-crypt.ko dm-multipath.ko dm-snapshot.ko linear.ko raid456.ko
dm-bio-prison.ko dm-delay.ko dm-queue-length.ko dm-switch.ko multipath.ko
dm-bufio.ko dm-flakey.ko dm-raid.ko dm-thin-pool.ko persistent-data
dm-cache-cleaner.ko dm-log.ko dm-region-hash.ko dm-verity.ko raid0.ko
dm-cache.ko dm-log-userspace.ko dm-round-robin.ko dm-zero.ko raid10.ko
dm-cache-mq.ko dm-mirror.ko dm-service-time.ko faulty.ko raid1.ko

But those are for an entirely different kernel.

Other than those, there are no available RAID drivers.

root@supermicro:~# find / -name raid*.ko
/media/root-ro/lib/modules/3.13.0-74-generic/kernel/crypto/async_tx/raid6test.ko
/media/root-ro/lib/modules/3.13.0-74-generic/kernel/drivers/scsi/raid_class.ko
/media/root-ro/lib/modules/3.13.0-74-generic/kernel/drivers/md/raid1.ko
/media/root-ro/lib/modules/3.13.0-74-generic/kernel/drivers/md/raid456.ko
/media/root-ro/lib/modules/3.13.0-74-generic/kernel/drivers/md/raid10.ko
/media/root-ro/lib/modules/3.13.0-74-generic/kernel/drivers/md/raid0.ko
/media/root-ro/lib/modules/3.13.0-74-generic/kernel/lib/raid6/raid6_pq.ko
/lib/modules/3.19.0-43-generic/kernel/crypto/async_tx/raid6test.ko
/lib/modules/3.19.0-43-generic/kernel/drivers/scsi/raid_class.ko
/lib/modules/3.19.0-43-generic/kernel/lib/raid6/raid6_pq.ko