[REQ] Help to gain access to btrfsraid6 pool that won't mount

[TheJay <obiwantje@gmail.com>]

$ uname -a
Linux thejay-srv03 4.0.0-040000-generic #201504121935 SMP Sun Apr 12 
23:36:33 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

$ btrfs --version
btrfs-progs v3.19.1

$ sudo btrfs fi show
Label: 'roms03'  uuid: b0025417-4045-4c63-a432-0625b7cc41ea
	Total devices 4 FS bytes used 2.67TiB
	devid    1 size 1.36TiB used 1.36TiB path /dev/sdz
	devid    3 size 1.36TiB used 1.36TiB path /dev/sdaa
	devid    4 size 1.36TiB used 1.36TiB path /dev/sdab
	*** Some devices missing

This pool is a btrfsraid6 pool

when trying to mount I get:

$ sudo mount -t btrfs -o recovery,ro /dev/sdaa /mnt/roms03
mount: wrong fs type, bad option, bad superblock on /dev/sdaa,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

$ dmesg -T|tail
[Mon May  4 08:35:08 2015] BTRFS info (device sdz): enabling auto recovery
[Mon May  4 08:35:08 2015] BTRFS info (device sdz): disk space caching is 
enabled
[Mon May  4 08:35:08 2015] BTRFS: has skinny extents
[Mon May  4 08:35:08 2015] BTRFS: failed to read chunk tree on sdz
[Mon May  4 08:35:08 2015] BTRFS: open_ctree failed

What could I try to see if I can mount this pool and gain access to the 
data?

While I have a backup of the data, next time I may not, and I am trying 
to learn how to go about.