The install completes but the system is unable to boot and comes up with a open_ctree failed with an initrd shell. This usually happens when you install with a btrfs handled raid (not a mdadm handled one). You may get the occasional boot success, but the root issue is that one or both disks are not initialised fully by btrfs before the kernel tries to mount the fs.
There are two ways to fix this. The second is the fastest, safest, easiest way 
The fix is to append boot commands such as
rootflags=device=/dev/sda2,device=/dev/sdb2,rootfstype=btrfs
or
rootflags=device=/dev/disk/by-partuuid/6dc5624c-2d54-4726-b2fa-a7a988d337a4,device=/dev/disk/by-partuuid/b57f2240-fa2e-4516-9049-603d2c5029b5,rootfstype=btrfs
to the grub entry or GRUB_CMD in /etc/default/grub. This really needs to be done on install.
(I also had to remove the existing rootflags=).
—edit—
It seems that all that is required is for btrfs device scan to be run in the initrd on boot.
Steps to fix new install
NOTE: I have not tried this with /boot on the root partition, only on a separate partition. I imagine booting from a btrfs raid setup may not actually work. In fact the Oracle website recommends the partitioning below too.
Partitions required
Single partitions
Preferably on a single disk but you can put these on the two disk system; you will need to mirror the partition setup however to help keep things consistent.
-
/boot/efi, EFI type, can be small (50mb) -
/boot, 200mb at least, this is the minimum size for two kernels
Raid partitions
- a swap partition, preferably set up as raid 1 through the GUI
- the remainder of space can be simply selected as BTRFS type, and given the
/mount point
An alternative is to have / sized and separate from the remaining disk space - you seem to only need 3GiB.
Once the install is done, don’t reboot yet. Ctrl+Alt+F2 to a terminal then:
-
chroot /mnt/sysimage, -
lsblkto see which parts are mounted - the part at/will have a matching part on the other disk. btrfs doesn’t mount both, but uses them both internally. -
blkidwill show you the matching partition of the btrfs raid, the two partitions will have the same UUID blkid |grep /dev/sd[part of / ] >> /etc/default/grub-
blkid |grep /dev/sd[matching of / ] >> /etc/default/grub(these two steps are to make editing the grub easier) vi /etc/default/grub
You will have lines similar to
/dev/sda2: LABEL="rockstor" UUID="54dcbbf3-1fe0-4b56-befc-8b275120c872" UUID_SUB="8a191f4c-7a12-4d33-ab9c-156ad72598ec" TYPE="btrfs" PARTUUID="0c6cc0a6-d19a-4884-938a-41516ebb4f74"
/dev/sdb2: LABEL="rockstor" UUID="54dcbbf3-1fe0-4b56-befc-8b275120c872" UUID_SUB="ab092784-25cc-474a-8967-52316c790b06" TYPE="btrfs" PARTUUID="a5516130-605f-4d70-83aa-5c159380df77"
at the end of the file for referencing. Change the existing GRUB_CMDLINE_LINUX= to GRUB_CMDLINE_LINUX_DEFAULT= and add a new GRUB_CMDLINE_LINUX= below. The contents of this line will need to contain references to the PARTUUID of the partitions. This is where your ace ViM skills will come in handy:
GRUB_CMDLINE_LINUX="rootflags=device=/dev/disk/by-partuuid/0c6cc0a6-d19a-4884-938a-41516ebb4f74,device=/dev/disk/by-partuuid/a5516130-605f-4d70-83aa-5c159380df77,subvol=root rootfstype=btrfs"
And don’t forget to delete the reference lines at the end before you save.
grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg- reboot
- Enjoy a nice, fast btrfs powered raided NAS.
Better way to fix boot
As above, install, but don’t reboot. Get to console chroot and then;
rm /opt/rockstor/conf/64-btrfs.rulescp /usr/lib/udev/rules.d/64-btrfs.rules /etc/udev/rules.d/dracut -fv- reboot.