Hello, Gentoo. After having got the syslinux boot manager working well, I lost the root partition on my newer machine. I spent the entire evening yesterday trying to get it back again, with various expedients for recovering ext4 partitions from backup superblocks, and so on.
It wasn't until the middle of the night that it dawned on me what had happened, and I immediately got up and had it fixed within twenty minutes. The cause was me booting up the machine with a rescue disk. This assembled my RAID partitions /dev/md127 and /dev/md126 reversed, but also wrote those wrong identifiers, 126 and 127, into the "preferred minor" field of the partitions' super blocks. In essence, they got swapped. Hence trying to boot up into my normal system, /dev/md126, the root partition, was an unformatted empty space on the SSD. I don't blame the rescue disk for this occurrence. For some reason, when the kernel assembles /dev/md devices, it only seems to pay attention to the "preferred minor" fields when they are wrong. :-( mdadm appears to write the "preferred minor" fields at random when assembling the RAID arrays. I don't think it should, unless explicitly asked. There is an argument to mdadm which specifies the writing of these fields. In fact I used this to effect a repair, ironically enough, from the rescue disk booted with the option to suppress the automatic assembly of the arrays. Just for the record, all my RAID arrays have metadata version 0.90, the (old fashioned) one that allows auto-assembly by the kernel without the need of an initramfs. The moral of the story: if your system uses software RAID, be careful indeed before you boot up with a rescue disk. -- Alan Mackenzie (Nuremberg, Germany).
