I decided to build an archive server for the purpose of backing up other fedora/centos desktops at the office.  I built a machine and have installed Centos 7.3 on it with all updates current.  I also purchased a 3.0 usb sata drive cabinet (Orico ORICO 9548U3-BK) and installed two 5T black WD drives.   There was no problem installing the usb cabinet or the drives.  I formatted each drive with xfs as /dev/sdc and /dev/sdd, and then combined them into a software mirrored raid with mdadm as /dev/md0.

I've always thought that the perceived wisdom is to not try and do software raid across USB - especially when both drives are at the other end of the same USB cable. Sure USB 3 is faster and there's a better chance it will appear to work at a reasonable speed, but it's not something I would contemplate.

Everything was working perfectly until I removed the terminal, keyboard and mouse and tried to reboot the machine.  It took a while to figure out, but when the mouse and keyboard were removed the boot process assigns the usb drives differently which makes /dev/md0 created by mdadm fail.

Which means that the drive letters are explicitly mentioned in /etc/mdadm.conf - you can change it to be wildcarded or leave mdadm to figure it all out itself.  See 'man mdadm.conf'.

My fstab file looks like :

/dev/mapper/centos_poar-root              /        xfs    defaults  0 0 UUID=f915a354-28bf-4110-bec9-3767ef1fe52c /boot    xfs    defaults  0 0 /dev/mapper/centos_poar-home              /home    xfs     defaults 0 0 /dev/mapper/centos_poar-u                 /u       xfs     defaults 0 0 /dev/mapper/centos_poar-swap              swap     swap    defaults 0 0 /dev/sda                                  /u0      btrfs   defaults 0 0 # entries below were combined into one mirrored raid system #/dev/sdc                                /u1      xfs     defaults 0 0 #/dev/sdd                                /u2      xfs     defaults 0 0 /dev/md0                                 /u1      xfs     defaults 0 0

Another likely issue is that you explicitly mention /dev/sda in the fstab - if the drives are re-ordered, then /dev/sda will not be what you think it is.  It's a much better idea to use UUIDs when mounting drives. You can find the UUID with

lsblk --fs /dev/sda

BTW, are you really using partitionless disks - is it really /dev/sda and not /dev/sda1 ?

This works perfectly when a usb mouse and a usb keyboard are attached, but when I remove the mouse and keyboard the system will not boot because the usb drives are relabeled as /dev/sda and /dev/sdb.

I would have thought that any SATA drives would have been processed before the USB drives - certainly it looks that way on my system. Try going through the output of dmesg to see if you can see what is really happening when in the boot sequence.

My thought is that if I could force the usb drives to be labeled as /dev/sdc and /dev/sdd whether the mouse and keyboard are attached or not, I might be able to fix the problem

It's much easier to make sure you don't explicitly use drive letters - because, as you've found out, they can change.  Use filesystem labels or UUIDs or disk IDs. The disk IDs can be found in /dev/disk/by-id and they should remain the same.

P.

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Pete,

Thank you for such excellent help; I appreciate your wisdom and challenges. The machine I am working on is still at the 'lab' stage so everything can be changed at this point. I don't have the benefit of much 'perceived wisdom' in that most of my knowledge comes from mistakes I have made or mistakes others on this list have made that Ican learn from. So thanks again.

Here are some of my thoughts... right or wrong.

#1. 3.0 USB drives I originally performed some testing on a small usb passport 3T drive and was amazed by the response and access times. I stumbled across the Orico cabinet during a NewEgg search, and decided to give it a try. I was even more impressed with the response and access times. I decided to attempt a mdadm raid on the the Orico and everything has worked better than expected except for the problems I have had with device assignments. I did have an electrical failure at my home related to weather of which the ups powered down after 30 minutes that caused one of the raid 5T drives to become corrupted. I reformatted the drive and rebuilt the raid with mdadm, and everything worked a lot better than I expected. I will only be using this system to store *.tar.gz backup files from other systems. What should cause me to fear the USB 3.0 connection.

#2. /etc/mdadm.conf The tutorials I read about how to use mdadm did not include mdadm.conf, and this was not created when I installed mdadm. I will take a look at the man pages.... thank you.

#3. UUID's Thanks for the suggestion to use names instead of device addresses and how to get the names with lsblk --fs /dev/sda. Using this should fix my problem of reassignment device names at boot time. Thanks much!!!

#4. Partitionless disks I orginally installed Centos 7.3 on an SSD, and then installed the hard drives subsequently. I noticed that I gained quite a bit of disc space by not using a partition. Since the files I will be storing on these discs are large and will only be accessed for archive purposes I wanted to use as much of the disk as possible. The original google searches I made related to using a partitionless disk did not give me compelling rational to use a partition, however I would surely like to hear from you and others about this.

Thanks again for your help, and instruction!!!!!

Greg