I'm setting up a new Ubuntu server and ran into two issues. First, I created a RAID 1 array from two 8 TB drives with mdadm, but it does not come back after reboot. Mounting it manually fails because SSH reports that /dev/md0 does not exist, while the server console says it cannot look up the block device. The guide I followed had me add mdadm information, update /etc/fstab, and run update-initramfs, but the array still is not available. Running cat /proc/mdstat only shows the RAID personalities and no active arrays. I also tried generating the mdadm configuration with mdadm --detail --scan rather than typing the entry manually. Separately, the Pi-hole installation script appears to hang before the installer interface opens. Any suggestions for diagnosing either problem?
3 Answers
The Pi-hole problem is probably independent of the RAID issue. Run the installer from a terminal with a stable network connection and watch for an error rather than relying on a window appearing. Check that the system has working DNS, is fully updated, and meets Pi-hole's supported OS requirements. If the script appears frozen, interrupt it and rerun it with shell tracing or inspect the last command it printed; avoid downloading and executing an unknown replacement script just to get past the hang.
The empty /proc/mdstat output means the RAID array is not currently assembled, so this is happening before the mount step. Check whether the disks and any RAID metadata are visible with lsblk and sudo mdadm --examine /dev/sdX /dev/sdY, replacing the device names as appropriate. You can then try sudo mdadm --assemble --scan and check /proc/mdstat again. If it assembles successfully, confirm the actual array name and filesystem before updating fstab; mounting by the filesystem UUID is usually safer than assuming the device will always be /dev/md0.
The mdadm configuration generated by mdadm --detail --scan should normally be valid, but it only helps if the array metadata is present and the initramfs contains the needed mdadm setup. First verify the array can be assembled manually and inspect the output from mdadm --assemble --scan. Also check lsblk -f, the exact contents of /etc/mdadm/mdadm.conf, and whether fstab refers to a filesystem that actually exists. Do not recreate the array or run commands that write metadata until you have confirmed the data and array state, since that can destroy recoverable information.
Also make sure you are examining the whole disks or the correct partitions. If the array was created from /dev/sda1 and /dev/sdb1, checking /dev/sda and /dev/sdb will give misleading results.