So I experienced this problem twice in the last month trying to install Proxmox VE 5.0 or 5.1 onto a Supermicro x9SRL and a X9DRI-Ln4F, with some samsung 850 evo and pro ssds, and the 2 SATA 3.0 ports on the motherboards. both times merely changing the rootdelay did not solve the problem. I tried both long and short delays, anything 2-60.
The Proxmox VE website also has this recommendation on their website which I also tried to little sucess
edit /etc/default/zfs, set ZFS_INITRD_PRE_MOUNTROOT_SLEEP='4', and then issue a "update-initramfs -k 4.2.6-1-pve -u"
In the end the best solution I found was to change proxmox setting in the etc/default/Zfs to only import pools by-id instead of by name.
After doing that I would still get an rpool import error but it would automatically import by-id without user intervention and continue on.
I would also note that this problem only occurred when none of my pci express slots were populated with HBAs. Once I put the HBAs back in this problem reappeared. I thought it might be a renaming problem of the devices but I noticed that sometimes the same name ie. /dev/sda and /dev/sdb were kept and this problem still came up.
In the end I decided that this was not a satisfactory long term solution and thus I gave up on installing Proxmox 5 and went back to 4.4 on both of these boards. maybe supermicro X9 is too old for Proxmox 5.
