More super ubuntu tips: udev-based NIC naming & lvm silicon_medley_raid_member


February 10, 2015

I had a pretty booked weekend, but decided to see if I could slam an Intel Atom SoC-based board into my home server anyway. Plan of attack: take a snapshot of OS lvm volume in case things went south, mount snapshot to double-check it, shut down, swap out system boards, resume business as usual.

Actual result: Still on the old board, after taking the root volume snapshot, I was unable to mount the thing, because mount detected the volume as of type “silicon_medley_raid_member”  rather than the ext4 filesystem it was. I’d never seen that before, but mount -t ext4 fixed it; I examined the mounted snapshot which seemed fine and ignored that strange hiccup.

Upon power-up on the new board, the kernel failed to mount the real root filesystem, so the boot sequence halted before pivot_root with an error (mount: cannot mount /dev/mapper/ssd–vg-root on /root: no such device) and dumped me in busybox on the initramfs.

Now. given that a mount without explicit fs type had failed on the snapshot, I thought it possible that somehow the type bits of the root filesystem itself had gotten corrupted, and that was why the boot-time mount was failing as well. How to confirm and/or fix? Thanks to this blog post, I got answers to both. Saved me a bundle of trouble! Since it seems to always involve LVM, and neither I nor anyone else who has been bit by this has a clue what happened, I’m calling software bug.

With my machine booting again (35 watts lower!) I just had to tackle the NIC names, so the new NIC plugged to my lan was still eth0 etc, making all my configurations and firewall rules and such “just work.” I happily pulled up the MAC addresses with lshw -class network, and swapped out the old values with the new in /etc/udev/rules.d/70-persistent-net-rules. Aaand it didn’t work. So, what the heck.

Searching through dmesg, I discovered the igw driver sanely brought the NICs up as eth0 through eth3, and then udev came along and arbitrarily renamed them em1 through em4. No idea where it got that from. After quite a bit of prodding and searching, I found this suggestion which I am not sufficiently esteemed to upvote on that site. The default contents of the Ubuntu udev network interface rules file requires that the interface being renamed already be named eth*. That corresponds to


in the file. Seems udev selects em* as the name and sets it before it bothers to read the rules file, and thus it never renames them again to the desired values. Removing that constraint, I was again fully up and running.

I’m grateful these folks documented the respective magic incantations, leaving substantial portions of my weekend for more fun things. The whole thing was maybe 4 or 5 hours, not as bad as I’d feared. Next time I build up enthusiasm to break stuff, I’ll tackle the link-aggregating switch I picked up…