'PARTUUID' in '/etc/fstab' and (hardware) RAID0 don't play well together, do they?

Question

Fresh Arch Linux install on (hardware) RAID0 under 64-bit UEFI system with GPT partitions. Had to add

MODULES="ext4 dm_mod raid0"

HOOKS="base udev autodetect modconf block mdadm_udev filesystems keyboard fsck"

into /etc/mkinitcpio.conf so that partitions on RAID0 are recognized properly on boot. Otherwise,

ERROR: device 'UUID=<uuid>' not found. Skipping fsck.
ERROR: Unable to find root device 'UUID=<uuid>'.
...

would be issued.

There is one peculiarity however, and I don't know how to explain it. On the one hand, when /etc/fstab contains either /dev/* or UUID=* sources, Arch Linux boots normally. On the other hand, when it contains PARTUUID=* sources, a bunch of the corresponding Dependency failed errors (regarding mounting of those sources from /etc/fstab) happen on boot and it hangs.

Could you explain what's wrong about having PARTUUID=* in /etc/fstab in this case? Does that have something to do with RAID0?

---

$ cat /proc/mdstat
Personalities : [raid0] 
md126 : active raid0 sda[1] sdb[0]
      976768000 blocks super external:/md127/0 128k chunks

md127 : inactive sda[1](S) sdb[0](S)
      4904 blocks super external:imsm

unused devices: <none>

---

$ dmsetup table
No devices found

---

$ lsblk
NAME          MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda             8:0    0 465.8G  0 disk  
└─md126         9:126  0 931.5G  0 raid0 
  ├─md126p1   259:0    0     1G  0 md    /boot/efi
  ├─md126p2   259:1    0     1G  0 md    
  ├─md126p3   259:2    0     1G  0 md    
  ├─md126p4   259:3    0   256G  0 md    
  ├─md126p102 259:4    0    16G  0 md    [SWAP]
  ├─md126p103 259:5    0    16G  0 md    /
  ├─md126p104 259:6    0    16G  0 md    /var
  └─md126p105 259:7    0   256G  0 md    /home
sdb             8:16   0 465.8G  0 disk  
└─md126         9:126  0 931.5G  0 raid0 
  ├─md126p1   259:0    0     1G  0 md    /boot/efi
  ├─md126p2   259:1    0     1G  0 md    
  ├─md126p3   259:2    0     1G  0 md    
  ├─md126p4   259:3    0   256G  0 md    
  ├─md126p102 259:4    0    16G  0 md    [SWAP]
  ├─md126p103 259:5    0    16G  0 md    /
  ├─md126p104 259:6    0    16G  0 md    /var
  └─md126p105 259:7    0   256G  0 md    /home
sr0            11:0    1  1024M  0 rom

---

$ blkid
/dev/sda: TYPE="isw_raid_member"
/dev/sdb: TYPE="isw_raid_member"
/dev/md126p1: LABEL="EFI" UUID="722E-E4AB" TYPE="vfat" PARTLABEL="EFI system partition" PARTUUID="a8e94657-e6ea-4712-be06-ac9ffe6e2258"
/dev/md126p3: LABEL="Windows PE 5.0 (x64)" UUID="181C2F991C2F7144" TYPE="ntfs" PARTLABEL="Basic data partition" PARTUUID="15848c79-1456-418b-a243-830d0db894ce"
/dev/md126p4: LABEL="Windows 8.1 (x64)" UUID="AAB83149B83114F3" TYPE="ntfs" PARTLABEL="Basic data partition" PARTUUID="7d3a06f5-4c67-4299-80b0-029501e14f18"
/dev/md126p102: UUID="6a2d4998-3ac8-4135-9d72-47960b201d5d" TYPE="swap" PARTLABEL="Swap" PARTUUID="d418edd6-44eb-4058-921f-c68aa191c5ac"
/dev/md126p103: UUID="2c241730-a076-48d9-8d1f-6e10573a994f" TYPE="ext4" PARTLABEL="Arch Linux" PARTUUID="37200e1e-dea4-435a-a873-427e3ee8c494"
/dev/md126p104: UUID="8d4eff47-3a2b-46b4-9263-7bbf00d8d0db" TYPE="ext4" PARTLABEL="Variable" PARTUUID="cd15b1f0-e948-4975-9218-591efa5b9b95"
/dev/md126p105: UUID="e0b15e56-3846-4e75-96f8-4f75058b4a6b" TYPE="ext4" PARTLABEL="Home" PARTUUID="54e85323-522c-415a-b7bd-2eb83b6b4ee6"
/dev/md126: PTUUID="e4e1b9b8-c26f-416d-82d9-e9350d0b5ac2" PTTYPE="gpt"
/dev/md126p2: PARTLABEL="Microsoft reserved partition" PARTUUID="6e9264fd-da04-4966-b8e0-8f3124f47050"

Answer 1

Since it's now clear you're running software raid ("fake raid", where the firmware/BIOS also has a software RAID implementation to make booting Windows off of it easier—in this case, Intel Matrix Storage), you're probably seeing some bug in Arch's initramfs w/r/t partitioning md arrays.

True hardware raid is almost entirely transparent to the OS; e.g., you would see only one device, the RAID array, not one device per disk. A hardware RAID array looks just like a normal disk to the OS, at least once you've got the RAID driver installed (without it, the OS just doesn't see it at all).

For quite a while, you couldn't partition md arrays at all (it was common—still is—to use LVM on top of them, or to create multiple arrays); later, you could set up a partitionable one, but it wasn't the default; nowadays they can all be partitioned. But probably something still has an assumption about them not being partitionable, and is looking for that partuuid on a physical disk, not the RAID array.

Personally, I'd not worry about it and just use the UUID instead. Also, in general, for a Linux-only box, is usually better to not use the "fake raid" at all, and just use Linux mdraid directly with its native formats. With RAID-0, I'm sure you'll have a chance to rebuild the box soon enough...