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RAIDs (Redundant arrays of independent disks) allow systems to keep runing even if some parts fail. With hardware raids you just plug more then one disk side by side. A buzzer or email watchdog will notify you if a disk fails and you have to plug a new (spare) one to up the redundancy again.

Unfortunately ubuntu's md (software) raid setup seems to suffer from a little incompleteness.

The mdadm assembling has been moved to the hotplug system (udev rules).

However:

• The proper mdadm --incremental option does not work in initramfs (not creating device nodes) 251663

• The proper command to start only selected hotplugable raids degraded (i.e. rootfs after timeout from initramfs) is not available. 251646

• The proper "mdadm --incremental --scan --run" command to start all remaining hotplugable raids degraded (after system has been up for a say 20 sec timeout) does not start anything. 244808

• Using the legacy method to start an array degraded breaks later --incremental (re)additions.

• mdadm still reads a static /etc/mdadm/mdadm.conf containing UUIDs (does not just go assembling matching superblocks) and refuses to assemble other arrays hotpluged into the system. (breaks any actual md hotpluging) 252345

• The initramfs boot process is not (a state machine) capable of assembling the root setup from devices appearing in any order and degrading necessary raids after some time.

  • 251164 boot impossible due to missing initramfs failure hook integration

  • 136252 mdadm, initramfs missing ARRAY lines

  • 247153 encrypted root initialisation races/fails on hotplug devices (does not wait)

  • 488317 installed system fails to boot with degraded raid holding cryptdisk

• For degraded regular (non-rootfs) arrays there isn't any init script to start/run them at all. 259145 non-root raids fail to run degraded on boot

When fixing ReliableRaid issues be aware of 244810 inconsistency with the --no-degraded option.