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Originally md device files were /dev/md[[:digit:]]+. Now /dev/md/[[:digit:]]+ is preferred but does not work on at least Ubuntu 12.04 Precise.
Andrey reports that having auto-generated /dev/md/* names in mdadm.conf breaks GRUB or initramfs in recent (Trusty and Jessie) systems. TODO: clarify (including adding related bug references) and determine whether our policy should be to defend against such auto-generated names or whether this is a transient situation which will be bug-passed.
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